Recommendations of the International Advisory Panel on Potato Wart Disease management on Prince Edward Island 2022

Acknowledgements

The visit of the International Advisory Panel to Prince Edward Island (PEI) between the 12th and 20th August 2022 required a great deal of organisation and preparation and the Panel would like to acknowledge the efforts and efficiency of so many in Canada and PEI.

List of abbreviations

AAFC: Agriculture and Agri-Food Canada
CFIA: Canadian Food Inspection Agency
CPM: Commission on Phytosanitary Measures
EFSA: European Food Safety Authority
EPPO: European and Mediterranean Plant Protection Organisation
ETI: Effector-triggered immunity
GIS: Geographic Information System
IAP: International Advisory Panel
IPPC: International Plant Protection Convention
ISPM: International Standards for Phytosanitary Measures
KASP: Kompetitive allele specific PCR
NAPPO: North American Plant Protection Organisation
NL: Newfoundland and Labrador
NLR: Nucleotide-binding domain leucine-rich
NPPO: National Plant Protection Organisation
PCN: Potato Cyst Nematode
PCR: Polymerase Chain Reaction
PEI: Prince Edward Island
PFA: Pest free area
PFPP: Pest free place of production
PFPS: Pest free production site
QRL: Quantitative resistance locus
RNA: Ribonucleic Acid
RSPM: Regional Standards for Phytosanitary Measures
SASA: Science & Advice for Scottish Agriculture
USDA: United States Department of Agriculture

List of figures

Figure 1: Map of Index fields, provided by CFIA to IAP
Figure 2: Schematic presentation of the current management measures for Index fields according to the 'PW Domestic Long Term Management Plan'
Figure 3: Schematic presentation of the current management measures for Adjacent fields according to the 'PW Domestic Long Term Management Plan'
Figure 4: Schematic presentation of the current management measures for Adjacent fields not being used for potatoes according to the 'PW Domestic Long Term Management Plan'
Figure 5: Schematic presentation of the current management measures for Primary Contact fields according to the 'PW Domestic Long Term Management Plan'
Figure 6: Schematic presentation of the current management measures for Primary Contact fields not being used for potatoes according to the 'PW Domestic Long Term Management Plan'
Figure 7: Schematic presentation of the current management measures for Other Contact fields according to the 'PW Domestic Long Term Management Plan'
Figure 8: Schematic presentation of the current management measures for new fields, or portion of fields, entering potato production according to the 'PW Domestic Long Term Management Plan'
Figure 9: Schematic illustration of the current situation on PEI (top) and possible classifications of fields depending on the dissemination of resting spores in the Index field. Middle: Fields with dispersed infestation. Bottom: Localised infestation in the Index field.
Figure 10: Schematic illustration of the regulations for Index, Adjacent, Primary Contact and Other Contact fields, as recommended by the IAP
Figure 11: Resting spores of S. endobioticum in various stages of plasmolysis (A, B) and of a broken resting spore (C) compared to a viable resting spore (D) (courtesy SASA, included in EPPO PM 7/28(2))
Figure 12: More typical symptoms expression (left) vs inconspicuous symptoms after a dry period (right). Other pictures can be found on the EPPO Global Database
Figure 13: Map of PEI with restricted field categories, provided by CFIA to IAP
Figure 14: An example of a risk matrix categorising risk of disease spread for a number of scenarios as Very high (red), High (red), Medium (brown), Low (brown) or Very low (green).
Figure 15: Map of Regulated Fields due to movement of seed potatoes, provided by CFIA to IAP

List of tables

Table 1: Development of cultivation of certified seed potatoes of resistant varieties compared to the cultivation of Russet Burbank and the overall cultivation of certified seed potatoes on PEI over the last nine years
Table 2: Hectares and lots cultivated with the resistant varieties Goldrush and Prospect on PEI for the years 2020 to 2022
Table 3: Risk categories used in the assessment regarding the risk of dissemination of resting spores or infested potatoes to parts of PEI, which might be considered as PFA
Table 4: Overview on recommendations from the IAP on management of Potato Wart Disease on PEI

Foreword

In July 2022, the Canadian Food Inspection Agency (CFIA) requested support from international experts to obtain independent expert scientific advice on the management and control of Potato Wart Disease on Prince Edward Island (PEI). The panel comprised members with significant technical expertise in their respective fields, including experience in the application of relevant international phytosanitary standards to address Potato Wart Disease.

The scope of this work was focussed on providing recommendations regarding the designation of PEI as a pest free area for Potato Wart Disease in areas where the pest is not known to occur.

The planned activities and scope of work included:

Provision of relevant information and guidance to panel members by a Canadian resource group (identified below);
Travel to PEI in the summer of 2022 to obtain direct knowledge of potato cultivation in PEI, and to understand how the pest has been managed since it was first detected.
Provision of recommendations in the form of a detailed report (to be provided to the Canadian resource group), regarding any immediate opportunities to establish specific pest designations provided for in IPPC guidance (i.e. pest free areas, pest free places, pest free production sites) for potato wart in parts of PEI, and/or identify the work and information required in order to do so in the future; and
A request for panel members to remain available for consulting purposes following completion of this work.

A potential second phase of work for the panel was to conduct a review of broader potato wart pest status of PEI in consideration of current and previous potato wart control measures, with the intent of providing additional recommendations such as maintaining pest designations and possible amendments to the Potato Wart Domestic Long Term Management Plan sampling and confirmatory processes.

A Canadian Resource Group was formed to: brief the panel on the scope of their work; furnish initial reference information; respond to any questions that may arise at any time during the panel's work; provide oversight of the panel's work; review draft reports as appropriate; and generally monitor progress of work.

One United States Department of Agriculture expert was invited to serve in an observer role associated with the international panel.

The International Advisory Panel was formed at the end of July 2022 consisting of the following members:

Dr. Wellcome Ho, Principal Scientist, Ministry for Primary Industries (New Zealand)
Dr. Iain Kirkwood, Technical Manager, Potatoes New Zealand Inc. (New Zealand)
Rodney Martin, Head of Potato Plant Health and Seed Potato Certification Inspectorate, Department of Agriculture, Environment and Rural Affairs (DAERA) (Northern Ireland)
Dr. Silke Steinmöller, Senior Scientist, JKI Institute for National and International Plant Health (Germany)
Dr. Theo van der Lee, Senior Scientist Molecular Phytopathology, Wageningen University and Research (The Netherlands)

The IAP visited PEI from August 14th to August 20th 2022 to obtain a comprehensive overview of the current situation, participating in field visits and discussions with different stakeholders and researchers.

Supplementary documents were made available for the IAP in advance to prepare for the visit. Additional information was provided following the visit.

The IAP collected and assessed all the information provided and their general conclusions are summarised in this report. This report therefore reflects the independent expert opinion of the panel members on the situation regarding Potato Wart Disease on PEI, current management practices and possible improvements, as well as an assessment regarding potential designation of any pest free status for Potato Wart Disease on PEI.

December 2022

Executive Summary

Potato Wart Disease, caused by the soil born fungus Synchytrium endobioticum has a world-wide quarantine status. Potato Wart Disease was first detected in Prince Edward Island in 2000, with recurring detection of additional infected fields every few years, including 2021 and 2022. An International Advisory Panel (IAP) was tasked to provide recommendations with respect to specify the International Plant Protection Convention (IPPC) pest designations (e.g. pest free areas, pest free places, pest free production sites) for potato wart on PEI.

The IAP visited PEI on 14-20 August 2022 to understand how Potato Wart Disease was managed on PEI and met with various stakeholders and researchers. Given the limited presence of the pathogen, the IAP considers most part of PEI outside the Regulated Fields as a PFA for S. endobioticum. A number of measures are recommended for the maintenance of the PFA status:

Two biosecurity control areas should be set up around two regions (coded by the letter H and the cluster A, C, D, K, M, and N) where potato wart infestations were found because the risk of further spread of the pathogen in these areas is considered to be medium to high. The exact boundary of the PFA around these areas should take into account risk factors such as previous land use, land ownership, and historical disposal of potato waste, and should be set by the CFIA in conjunction with stakeholders such as the potato industry on PEI.
The industry should move to growing only resistant varieties in the control areas as soon as possible.
All machinery should be cleaned to a high standard before leaving Regulated Fields or the control areas. The CFIA should verify the freedom from soil for equipment after the cleaning process.
No seed potatoes produced in the control areas should be planted in the PFA.
Movement of unprocessed potatoes from the control areas into the PFA should be assessed for risk and if needed mitigations measures should be introduced.
A diagnostic and surveillance programme should be set up to provide continuous monitoring of the PFA including tare soil testing.

In addition a list of recommendations were provided to improve the management of Potato Wart Disease and these can be summarised as:

Modification in control measures on restricted fields, such as no potatoes (or other plants for replanting) for a minimum of 20 years in Index fields.
Buffer zones should be reviewed and may extend further than the current definition of adjacent fields. Only resistant varieties should be grown in Buffer Zones.
Improvement in surveillance methods, such as adjusting intensity of sampling according to the category of field, the end use of the crop, or the destination of the crop; and inspection of cull bins for infected tubers with inconspicuous symptoms.
Improvement in detection methods, such as implementation of tare soil testing in surveillance programme; and incorporation of molecular testing along with microscopic examination for resting spores in soil samples.
Improvement in detection sensitivity, such as making more of images available that represent different levels of disease expressions; and use of 3-D printed infected tubers to test detection sensitivity.
Improvement in tracing system, such as incorporation of surveillance and diagnostic data into the existing Geographic Information System (GIS) to enable thorough risk assessment of infected fields and efficient back-tracing.
Identifying plant resistant varieties for disease management. This includes the standardized screening of potato varieties and breeding material for potato wart resistance, generation of lists of potato varieties with effective resistance and recommendations on the use of resistant potato varieties for control strategies.
Improvement in waste management, such as cease using digestate from anaerobic digestions or other waste from processing on agricultural fields.
Improvement in communications and partnership, such as setting up a multi-actor communication platform for the agencies, stakeholders and industry to discuss issues and explore opportunities.

Introduction

Potato Wart Disease is a soilborne disease, caused by the obligate biotrophic fungus Synchytrium endobioticum. The fungus is listed in most areas of the world as a quarantine pest, whose introduction and spread are prohibited. Its current distribution is mostly restricted to cooler climates, including many European countries as well as parts of Russia and Turkey. The known occurrence of the pest on other continents is limited and includes New Zealand, parts of China and India, South Africa, Tunisia, Peru, the Falkland Islands and Bolivia. In North America, S. endobioticum is only known to occur in Newfoundland and Labrador and on Prince Edward Island (PEI).

PEI has a very short history of dealing with Potato Wart Disease compared to other regions. Potato Wart Disease was reported in Canada as early as 1909 in the region of Newfoundland and Labrador (NL) but the first report on Potato Wart Disease in PEI was in the year 2000 on a single field. The precise event that introduced Potato Wart Disease into PEI is unknown as there were several possibilities of introduction from areas where Potato Wart Disease occurred. Due to proximity and historical trade, commerce, travel and immigration between Newfoundland and PEI, there is a higher probability of potato wart exposure from Newfoundland compared to other sources. It is likely that several places on PEI (and other provinces in eastern Canada) may have had potential exposure to Potato Wart Disease and that it could have become established in some kitchen gardens. Potato Wart Disease has often been detected in kitchen gardens in Europe and on Newfoundland. A few different potato wart sites in PEI can be connected to the location of a former kitchen garden or potato storage facility, noted as possible sources of infection in some cases.

Recent molecular genetic studies and comparative genomics show that all isolates originating from Canada (NL and PEI) are extremely similar (Vossenberg et al. 2022). Although different pathotypes were identified (pathotypes 2 and 6 in NL and pathotype 6 and 8 on PEI), they are genetically closely linked and can be traced to the same origin. This supports the assumption that Potato Wart Disease may have been introduced to PEI in previous years from NL e.g. by potatoes that had been planted in home kitchen gardens. Commercial production fields were apparently not affected as no infections were reported, even though surveillance for potato wart was conducted periodically in eastern Canada for many decades. A seed potato certification program has been in place in Canada since the early 1900s and official programs were started in 1918 in New Brunswick and PEI. In the early 1990s a cost recovery program which included seed potato field inspections was initiated by the Government of Canada. This may have led to a shift in the percentage of the annual potato crop that was certified as seed potatoes in some provinces, including PEI. In 1992 PEI growers entered 23,681 Ha for seed certification. With time, some potato farms moved away from seed potato production and specialised in the production of table stock or processing potatoes. By the time potato wart was detected in 2000, the area entered for seed certification had been reduced to 7,098 Ha. In 2022 the area entered for seed potato certification was 5,580 Ha. However, it is not unusual that potatoes entered for seed potato certification are sold as table stock or less commonly as potatoes for processing. There is a high density of potato farms in PEI, especially in the central production area, but there are no designated or protected areas for exclusive seed production. Today, the majority of potato farms in PEI continue to produce some acres of seed potatoes, generally for their own use. These farms will often produce potatoes for human consumption and seed potatoes within the same production system. In 2022, 142 seed potato "farm units" were registered with the CFIA in PEI via the CFIA National Seed Potato Certification program. It is possible for a single grower to enter multiple 'farm units' when biosecurity measures are in place to separate the 'units'.

The change from seed potato production to the cultivation of processing potatoes was supported by the expansion of processing facilities located on PEI. An already existing processing plant was acquired by Cavendish in 1980, a second processing facility was constructed by Cavendish in 1996. In the past there was another processing plant (McCains French fry facility) in Borden PEI which was in operation for approximately 20 years, closing in October 2014. In Souris there is a dehydrated potato products facility (granules, flakes etc), AgraWest Foods Ltd. This facility has been in operation since 1997. There are also a significant number of potato packing facilities located in different areas of PEI (approximately 55 facilities).

Overall the area of land used for potato production has decreased, from the peak year of potato production in PEI in 1999, with 46.000 Ha planted and 44.500 Ha harvested to approximately 35.000 Ha annually planted with potatoes since 2010. About 62% of the potato production in PEI occurs in Prince county, 22% in Queens county and 16% in Kings county. The most significant volume of potato production occurs in the "Central production area" of PEI, the area between Charlottetown and Summerside.

The first identification of potato wart in 2000 resulted in a sequence of regulatory policies for Potato Wart Disease:

2000 CFIA investigation response
2000 Operational Workplan
2001 CFIA's establishment of a pest free area
2001 CFIA's pest management plan for potato wart
2001 Potato Wart three Conducive Years Operational Work plans (versions July 2001, November 2002 and January 2003)
2005 Potato Wart Domestic Long Term Management Plan (updated in 2009)

During the period 2001 to 2004 and in addition to the soil sampling activities under the Potato Wart Three Conducive Years Operational Work plan, the CFIA conducted an Island wide visual surveillance program (post-harvest field walking) of fields under potato production at the time. During the same period a tare soil sampling program (1 Kg of soil per seed lot) was also undertaken on seed potato farms. These efforts provided a baseline of surveillance for the island, although potato wart has since been detected at additional locations.

The detections found since 2000 were traced back and grouped, with five distinct clusters of outbreaks that were not knowingly linked through Index fields to each other. The possible source of infection for each of the clusters is often not known, neither is the timing of introduction, so according to present knowledge based on a 10-year trace-back investigation approach, there does not appear to be a common source of infection between the clusters. It is possible that with an extension of the investigations beyond the 10 years, with reliable records of past land use – especially in the commercial sector (processing and table stock), linkages between clusters could appear e.g. in form of common land use due to land rental, leasing, sharing or trading. Since 2006 the number of potato farms on PEI has decreased from 330 farms to 175 farms in 2021, which has led to the exchange of fields between landowners, farmers and corporations: about 96% of these farms are family farms. The Lands Protection Act imposed limitations on ownership of agricultural land. Individuals were limited to 1,000 acres and corporations limited to 3,000 acres. If potato growers require more acres to meet production goals, they might need to lease, rent, trade or share fields. This may increase the number of potato growers using the same or common fields with time.

The pest status for S. endobioticum on PEI was defined as: Pest present, not widely distributed and under official control, as there were no signs of an extended spread of the pest based on inspections and soil testing. As a consequence the rest of PEI was considered a pest free area (PFA) for S. endobioticum in line with the international standards ISPM 4 (Requirements for the establishment of pest free areas) and ISPM 8 (Determination of pest status in an area), in the versions in force at that time. General surveillance activities were carried out in the pest free area, including graded tuber inspections as part of the CFIA national seed potato certification program, finished product inspections for grade in the frame of the DFIA food program, or graded tuber inspections for export requirements. Additional general surveillance activities were conducted by the Province, e.g. inspections of ungraded tubers for the crop insurance program or visual inspections due to industry commercial farming practices and processor activities. Between 2004 and 2014, soil sampling activities for Potato Wart Disease were limited to the activities under the management plan on restricted fields only. No soil sampling activities occurred on unrestricted fields prior to 2015, after the baseline survey between 2001 and 2004. In 2015 the USDA published a Federal Order establishing requirements for potatoes originating from the Province of PEI (unrestricted areas), including a mandatory soil test (conducted by the CFIA) for seed potato fields following harvest of the crop designated for export. Sampling varied from 1,417 to 284 samples per year between 2014 and 2021 crop years, depending on US trade volumes and US regulatory requirements. Some fields were tested more than once during this period as they were designated for US export within a 3-year crop rotation cycle. The sampling rates were conducted in accordance with the definition of "method B" of the Canada-United States PCN guidelines. In the frame of the surveillance and testing activities further outbreaks were detected over the following years. The US soil testing requirements resulted in the detection of potato wart in two unrestricted seed potato fields in 2020. During the inspection of the 2020 crops produced in those fields no symptomatic tubers were found, which is understandable, as only low levels of infection were detected in the soil samples. The source of the infection remains undetermined as no linkages to homestead locations or potato storage areas were identified. In the same year the CFIA established a national soil-sampling program which included PEI in 2020 but not in 2022. For PEI, 178 soil samples were collected in 2020 from 8 unrestricted fields.

In the years 2021 and 2022 fields were found to be infested though these fields had been sampled and inspected for some years and thought to be free from Potato Wart Disease; this raised doubts on the efficacy of the management measures in place. Potato wart was detected in four fields on three separate farms in two consecutive crop years. These detections directly followed the two detections of potato wart on unrestricted fields in a seed farm in 2020. Later finds occurred through soil sampling in the frame of the trace-out investigations in February and July of 2022.

With the Potato Wart Order from November 2021, the Canadian Food Inspection Agency (CFIA) considered PEI as a place infested with Potato Wart Disease.

Once the Ministerial Order was in place the CFIA removed the PFA status from the unrestricted fields in PEI. The Ministerial Order enabled the establishment of a contiguous regulated area, and management and mitigation activities for potato wart on unrestricted fields in PEI. The management plan remains in place for the restricted fields within PEI.

An international panel of independent experts was established to review pest status designations, provide independent observations, and identify prospective options regarding the establishment of the specific pest status for S. endobioticum on PEI.

The observations, conclusions and recommendation from the International Advisory Panel (IAP) are summarised in the following sections.

Management of Potato Wart Disease on PEI

Comprehensive information was provided to the IAP concerning the situation regarding Potato Wart Disease on PEI; including a map with the rough location of Index fields on the Island, their affiliation to defined clusters and the years of detection (Fig. 1).

Picture - Figure 1: Map of Index fields, provided by CFIA to IAP. Description follows.

Currently there are 35 Index fields and, so far, no Index fields have been descheduled. An additional piece of land was used for disposal of tare soil and was found positive for Potato Wart Disease when tested, demonstrating that tare soil presents a pathway for resting spore movement. As the land was not suitable for agricultural production it is not listed as an Index field. A notice remains on the piece of land to contain Potato Wart Disease and prevent soil movement. This area can still be used for disposal of tare soil.

The 35 Index fields were grouped into five clusters of linked infections, according to the information available in the course of the follow-up for 10 years. One cluster comprising nine fields was identified in the region around Summerside, where potato wart has been consistently detected over the past 22 years and most recently in 2021 and 2022. This was the first cluster that was identified and has had outbreaks over an extended period (locations A, C, D, K, M, N on the map with nine fields). This contrasts to the other clusters where Potato Wart Disease was identified in a narrow time window either in the same or in subsequent years.

At a second cluster (location B on the map with 8 fields), four Index fields were detected on a mainly processing farm operation in 2002. The farm also produced seed potatoes at the time of the detections. Symptomatic tubers were found by the CFIA in two fields as part of the island-wide surveillance activities. The other two Index fields were found via soil sampling during the resulting investigation on this farm. The origin of these detections has not been determined and the timing of introduction is not known. It is likely to pre-date the 2000 detection because of the high level of infestation observed in some fields. The farm was known to have sold table stock and seed outside of the 10 year trace back window. A 0.5 km buffer zone was implemented, which was removed in spring 2003. Four additional Index fields were detected in 2004 (one by soil sampling and three by symptomatic tubers), of which three fields were in potato production and one was not. The grower elected to stop using the Index fields for potato production by 2004 but the fields continue to be used for other agricultural crops. The grower does not grow potatoes on his own farm. Other fields that were rented by the grower in the past and as part of the investigation may still be in potato production. No potato wart has been found around location B in the last 20 years. Bioassays have been conducted on the fields and most would theoretically be eligible for the production of resistant potato varieties under the long-term management plan. Although cluster B has not been related to another cluster via Index fields, it is connected to other investigations through common field use, both Primary Contact and Other Contact fields. Regarding all fields regulated under the 'Potato Wart Domestic Long Term Management Plan', 48 fields were restricted comprising 479 Ha.

A third cluster (locations G, E, F on the map) was found in Malpeque Bay with symptomatic tubers found in one field; subsequent linked fields were detected by identification of resting spores during soil analysis. The detection in 2012 (location G), via the submission of symptomatic tubers, was on a farm that had a history of producing table stock and seed potatoes. Some seed potatoes were sold within the 10-year investigation period and resulted in the detection of additional Index fields. The investigation identified prior long-distance movement of seed potatoes from the Index field at location G, to the fields at locations E and F (both in Prince County) where potato wart was subsequently detected in 2013 and 2014 on two separate farms as a result of soil sampling. One farm produced table stock and seed potatoes and the other farm produced processing and seed potatoes. In total three potato farms were implicated in these investigations, of which all had a history of producing seed potatoes. Regarding all fields regulated under the 'Potato Wart Domestic Long Term Management Plan', 273 fields were restricted or 2,517 Ha.

The fourth cluster included a seed tuber field in 2014 (H with eight fields) on a farm that had a history of producing table stock, processing and seed potatoes. The field itself was not entered for seed potato certification in the year of detection but had produced seed potatoes historically including during the 10-year trace-back period. The investigation identified the long-distance movement of seed potatoes from the Index field at location H in Queens County (8 Index fields) to fields at location I (Prince County) where potato wart was detected in 2014 and 2016 and at location J (Kings county) where potato wart was detected in 2016, via soil sampling. One farm produced processing potatoes and the other farm was a beef farm that rented land to potato growers. The potato grower had a history of rental land use. The movement of equipment from the beef farm Index field by the potato grower has created two Primary Contact fields on another farm. 140 Primary Contact fields were created due to the movement of seed potatoes across PEI, of which three have become Index fields. One Primary Contact field was created in the province of Ontario, due to the movement and planting of seed potatoes from one of the Index fields. The beef farm with the Index field could rent fields to other producers for agriculture production.

The Index field continues to be used for production of agricultural products such as soybean and corn but not for grazing cattle.

The fifth cluster (L) represents a single location of two fields, which was identified based on the presence of resting spores in soil samples for exporting purposes. One field had a history of soil testing (2016) as it was designated for US export. The second field was new to export. All fields under farm ownership were unrestricted at the time of detection. Two samples were positive for potato wart by morphological identification of spores and confirmatory PCR testing. Subsequent resampling of the soil did not result in the detection of additional spores. This farm specialises in the production of seed potatoes but also produces table stock. It is a large farm with a history of active export to the US, other provinces in Canada and in PEI. The detections were at low levels and the source of infection is not known. At the time of writing the investigation has not been linked to other investigations. One Primary Contact field was created in the province of Ontario due to the movement and planting of seed potatoes from one of the Index fields. Primary Contact fields were also created in the US due to the movement and planting of seed potatoes from one of the Index fields.

As of July 2022, over 40,000 acres in PEI are restricted for potato wart, accounting for approximately 16% of the farmed potato acres in PEI.

The identification of Index fields in the period 2000 to 2014 was mainly through findings of infested potato tubers via inspections. In recent years the detection of spores in soil became more prominent because of the comprehensive sampling schemes implemented on Regulated Fields.

Management measures implemented on Regulated Fields on PEI are part of the 'Potato Wart (PW) Domestic Long Term Management Plan'. The plan defines the different categories of Regulated Fields according to their risk of infestation with resting spores of S. endobioticum and outlines specifications on cultivation requirements, soil management, and testing and surveillance activities on these fields. As the assessment of these measures form an important part of this report, the specifications of the 'PW Domestic Long Term Management Plan' are outlined below as understood by the IAP (Fig. 2 to 8).

Category A: Index fields

Defined as: fields determined to be infested with S. endobioticum, the pathogen causing Potato Wart. They represent the highest risk regarding the dissemination of resting spores either by movement of tubers, plant material or soil, or by the movement of contaminated equipment.

Picture - Figure 2: Schematic presentation of the current management measures for Index fields according to the 'PW Domestic Long Term Management Plan' Description follows.

Category B: Adjacent fields to the Index field

Fields or tracts of land bordering an infested field and not separated by any major highway or any major water course, or forested area, or nonagricultural area of more than 15 meters wide, which acts as a physical barrier for the spread of spores into other agricultural fields. These fields may have been contaminated with resting spores originating from Index fields, for example via equipment, wind, animals, water run-off, etc.

Picture - Figure 3: Schematic presentation of the current management measures for Adjacent fields according to the 'PW Domestic Long Term Management Plan' Description follows.

Different provisions exist for Adjacent fields which have not been used for potato production for at least 20 years and/or are not intended to be used for potato growing, as the risk of infestations associated with these fields is considered to be low. However, the immediate proximity to an Index field is perceived as some risk.

Picture - Figure 4: Schematic presentation of the current management measures for Adjacent fields not being used for potatoes according to the 'PW Domestic Long Term Management Plan' Description follows.

Category C: Primary Contact fields

Fields that may have had soil (e.g. tare soil) or potatoes transferred to them from an Index field, or had common equipment moved to them directly after use in an Index field, so that resting spores may have been spread on these fields.

Picture - Figure 5: Schematic presentation of the current management measures for Primary Contact fields according to the 'PW Domestic Long Term Management Plan' Description follows.

Again, provisions for Primary Contact fields where no potatoes have been grown for at least 20 years and/or which are not intended for potato growing in the future are similar to the provisions for Adjacent fields not cultivated with potatoes.

Picture - Figure 6: Schematic presentation of the current management measures for Primary Contact fields not being used for potatoes according to the 'PW Domestic Long Term Management Plan' Description follows.

Category D: Other Contact fields

Fields where equipment has been shared with the Index field, after use in a Primary Contact field. These fields represent the lowest risk regarding dissemination of resting spores. However, dissemination of resting spores cannot be completely excluded.

Picture - Figure 7: Schematic presentation of the current management measures for Other Contact fields according to the 'PW Domestic Long Term Management Plan' Description follows.

Category E: New fields or portion of fields entering potato production

Land going into seed potato production for the first time which was not part of the Island-wide 2001 to 2004 survey, or potato land where homesteads have recently been incorporated into potato production fields (seed and commercial) but does not include "new land" going into commercial production.

Picture - Figure 8: Schematic presentation of the current management measures for new fields, or portion of fields, entering potato production according to the 'PW Domestic Long Term Management Plan' Description follows.

In the past 10 years, approximately 11,000 acres of land has been used for seed potatoes for the first time. CFIA collected information for new commercial potato land if it could be associated with a recent homestead.

In addition to the provisions of the 'PW Domestic Long Term Management Plan', a 'Ministerial Order' was put in place November 2021 which identifies unrestricted land and restricted areas. Off island movement of domestic potatoes from the unrestricted areas requires written authorisation. Exemptions are possible. e.g. for potatoes for consumption produced in the unrestricted area moved under normal industry standards (washed and free from soil, treated with a sprout inhibitor and moved directly to the retail market, the consumer or a facility for processing, repacking or packaging). The Ministerial Order restricts the off-island movement of soil and other restricted things (e.g. nursery stock, root crops, turf) from all of PEI. Exemptions are in place e.g., for machinery, implements and conveyances when they have only been used outside a restricted area, and most of the plant debris and soil have been removed prior to movement so that only a negligible quantity remains; or for potato nuclear stock. Written authorisation is also required for the movement of regulated things out of a restricted area and to or from one restricted area to another.

Surveillance activities form an important part of the management measures for Potato Wart Disease on PEI. Soil grid sampling, bioassays and the inspection of tubers for visual symptoms are the primary means of identification and quantification of Potato Wart infested fields and tuber lots. These activities on Regulated Fields are complemented by general surveillance activities on potatoes in the unrestricted areas conducted by the Province, e.g. inspections of ungraded tubers for the crop insurance program or visual inspections due to industry commercial farming practices and processor activities.

Definition of fields as Index fields is based on detection of either symptoms on tubers or resting spores in the soil. Soil grids applied for direct examination for resting spores under the microscope are either by 8 m x 8 m grids (Adjacent fields, Primary Contact fields); or by 4 m x 4 m grids (Index fields, Primary Contact fields with no potato cultivation for 20 years, first 15 m of an Adjacent field) on Regulated Fields. All soil samples are examined at the Central Laboratory in Charlottetown, following a strict protocol on handling of samples and determination of the presence of resting spores. However differentiation between viable and non-viable resting spores, as recommended in the EPPO PM Diagnostic Protocol for Synchytrium endobioticum (EPPO 2017b), is not conducted on PEI.

Descheduling of Regulated Fields is mainly based on successive cultivation of susceptible varieties. The number of crops that require to be cultivated under conducive conditions and found to be free from Potato Wart Disease vary between the categories of fields. The 'PW Domestic Long Term Management Plan' does not outline any requirements on the cropping frequency for potatoes. The 'Agricultural Crop Rotation Act', residing with the Province of PEI and introduced in 2015, regulates that potatoes should not be planted on any area of land greater than 1.0 hectares at any time for more than one calendar year in any three consecutive calendar years. Growers can apply to the Province of PEI for permission to plant two potato crops in a five-year rotation. As of 2020, 934 exemptions have been granted to 82 farmers. The province of PEI does not track the total hectarage under exemption management plans.

Potatoes produced on Regulated Fields fall under restrictions and may be used for processing with mitigation measures in place or after a number of crops with no findings of Potato Wart Disease as table stock with the requirement of phytosanitary inspections. On Index fields soil sampling is performed every 5 years for a minimum of 15 years. If spore counts of every sample are less than five spores per gram and the bioassay is negative the grower can plant resistant varieties for use only in PEI at facilities, under a compliance agreement. Bioassays are not conducted until the spore counts are below five resting spores per gram.

Plant resistance has been tested on a field plot in Avondale for the breeding program developed by Agriculture and Agri-Food Canada (AAFC). The Avondale site was established prior to 1976 and maintained S. endobioticum isolates with pathotypes 2, 6, and 8 inoculated using soil from an earlier research program.

The AAFC Potato Research Centre develops new cultivars and technologies for the production, handling, and management of potatoes and maintains a national repository of potato gene resources. New varieties produced from the breeding programmes are evaluated at Avondale and tested up to seven growing seasons with increasing tuber numbers. At the end of the testing period those varieties exhibiting no potato wart field symptoms are considered to have demonstrated field resistance to pathotypes 2, 6(O1) and 8(F1). During this time approximately 100 plants of each variety have been individually evaluated for field resistance to potato wart, assessed using visual symptoms (present or absent). Seedlings which exhibit suitable characteristics in both field and commercial testing are then put forward for registration under the Seeds Act Variety Registration System. From varieties listed as resistant for pathotypes 6 and 8, only four (Goldrush, Frontier Russet, Hilite Russet and Prospect) are currently in commercial production on PEI. The industry also has available some varieties of European origin that are rated as resistant to pathotype 6. In 2022, 16 varieties were in a field test in Newfoundland with pathotype 6 being identified as being present in the field. Ratings were not finished at the time of the report.

A total of 60% of the acreage of potatoes is currently grown with susceptible varieties, 20% is grown with resistant varieties and the remaining 20% is grown with varieties where the information on resistance is not known. In particular, the popular variety Russet Burbank is very susceptible to pathotypes 6 and 8.

With regard to seed potato production, in 2022 a total of 506 Ha was cultivated with resistant varieties on PEI, which corresponds to approximately 9% of the overall hectarage planted with seed potatoes. The variety Goldrush was cultivated on 209 Ha, Prospect on 235 Ha. Table 1 shows the development of cultivation of certified seed potatoes of resistant varieties compared to the cultivation of Russet Burbank and the overall cultivation of certified seed potatoes on PEI over the last nine years.

Table 1: Development of cultivation of certified seed potatoes of resistant varieties compared to the cultivation of Russet Burbank and the overall cultivation of certified seed potatoes on PEI over the last nine years
Year	Hectares certified seed potatoes of resistant varieties PEI	Hectares certified of Russet Burbank PEI	Hectares certified seed potatoes PEI
2013	1230.51	1427.069	7139.905
2014	1343.535	1299.241	7320.489
2015	1228.905	823.325	6730.226
2016	1175.89	903.305	6474.255
2017	1008.71	918.933	6736.628
2018	819.134	830.395	6665.315
2019	764.49	814.892	7014.404
2020	614.786	670.675	7085.102
2021	621.892	531.337	7253.854
2022	506.175	426.245	5767.467

Currently there is more interest in growing resistant varieties but the number of suitable varieties is still limited as white flesh potatoes are preferred by the market and most of the resistance has been developed in cream potato varieties which are preferred in Europe. One of the most promising resistant potato varieties is Goldrush. Table 2 shows the hectares grown with Goldrush and Prospect on PEI over the least three years.

Table 2: Hectares and lots cultivated with the resistant varieties Goldrush and Prospect on PEI for the years 2020 to 2022
Variety	2020 Lots	2020 Hectares	2021 Lots	2021 Hectares	2022 Lots	2022 Hectares
GOLDRUSH	36	274.558	36	272.829	26	188.722
PROSPECT	19	337.96	23	268.818	24	235.07
Total		614.786		621.892		506.175

Based on the molecular analysis potato variety Goldrush contains the Sen 3 resistance gene that would provide broad-spectrum resistance to wart disease including the pathotype 6 found on PEI. For the other varieties little information was available for the IAP.

If growers have to fulfill the given numbers of susceptible variety rotations on their Regulated Fields they can select a variety based on the susceptibility test performed at Central Laboratory in Charlottetown. The laboratory tests a variety (3 tubers only) against a specific "Index field" derived isolate for Potato Wart Disease symptom expression.

Soil from Index, Adjacent or Primary Contact fields must be returned to the respective fields and all equipment must be free from soil when leaving these fields until the restrictions are lifted according to the 'Potato Wart Domestic Long Term Management Plan'. After that, movement of tare soil is addressed through best management practices.

According to the 'Ministerial Order' movement of potato plants from Regulated Fields is subject to authorisation by an inspector. This includes discarded potatoes as well as other plant residues which could therefore be regarded as regulated. Directive 'D-96-05: Phytosanitary Requirements for the Importation and Domestic Movement of Non-Propagative Potatoes (Solanum tuberosum) and Related Potato Articles, Including Associated Soil' generally specifies the necessity of movement certificates for movement of regulated articles in Canada. In addition to potatoes produced in a regulated area, this Directive also regulates potato waste (culls, peels, etc.) and by-products (wash water, flume water and other effluents). D-96-05 and supporting QSM-09 (Quality Management System Requirements for Facilities Receiving and Handling Regulated Non-Propagative Potatoes and Related Potato Articles, Including Associated Soil) provide guidance on compliance agreements.

A compliance agreement, approved by the CFIA, is required for a facility to manage potatoes harvested from an Index field, along with Adjacent and Primary Contact fields, until the cleaning and disinfectant requirements are lifted. If potatoes are washed in the facility under a compliance agreement, the risk of potato wart would be mitigated and the final processing facility may not require a compliance agreement. Processing plants and packaging facilities in PEI which do not handle regulated potatoes do not require a compliance agreement.

A compliance agreement covers all activities related to the disposal of material and waste from the approved facility including all kinds of waste which are subject to CFIA approval. Disposal on agricultural land is prohibited. So-called high-risk activities may be permitted if the phytosanitary risk is mitigated to a level approved by the CFIA. These risk activities include feeding to cattle, spreading on fields, using wash water to irrigate fields, composting etc.

All regulated articles and by-products leaving an approved facility for disposal must be directed to a site that has been pre-approved by CFIA. Approved facilities must keep all records that pertain to the handling of regulated articles under a CA for a minimum period of ten years. Regulated articles must be completely segregated from the rest of the material handled in the facility.

As stated in the 'PW Domestic Long Term Management Plan', potatoes cultivated on Index fields must be washed and processed in an approved facility under a compliance agreement (PEI Potato Solutions, RWL Holding). This ensures that there is no risk of dissemination of resting spores as all waste will be disposed of on a specific site approved by CFIA which is not publicly accessible. Potatoes from Adjacent or Primary Contact fields require handling under a compliance agreement until they have met the requirements of 1) negative soil results and 2) post-harvest surveillance of a susceptible variety in which no potato wart has been detected.

Waste from potato processing from non-Regulated Fields is not specifically regulated but conditions of the Ministerial Order (November 2021) must be met for the movement of regulated things, such as fresh potatoes, off PEI. Written authorisation is required for regulated things from restricted areas of PEI as well as from unrestricted areas (when specific exemptions do not apply). This may include a compliance agreement to ensure the regulated things, such as fresh potatoes, do not present a risk of spreading potato wart.

The biggest processing plant on PEI, Cavendish Farms, a major producer of French fries, presented information on their waste management. This included a compliance agreement with CFIA which was approved in 2022 requiring all waste streams to be documented ensuring accountability. But compliance agreements have also been in place at the facility in the past for various reasons as a means to dispose of regulated materials. Upon arrival, dirt, stones and other organics are removed. The raw material passes through a steam peeler at a pressure between 17 to 19 bar, at a temperature between 200 to 260 °C with an exposure time between one second to a maximum of 17 seconds. Since 2009, a mesophilic anaerobic digestion facility has been in use for processing of non-saleable potato material with a retention time of approximately 60 days at temperatures between 37 – 40 °C and a pH of 4 – 5. Prior to its installation waste was mainly distributed to farmers and used as feed for livestock. Digestate from the facility is spread on fields owned by the company. Wastewater is treated in a company owned facility, distributed on farm owned wetlands and into salt water after final treatment.

No information on waste treatment was provided for the producer of dehydrated products (AgraWest Foods Ltd) or for the other packaging facilities located on PEI.

Conclusions and recommendations on management of Potato Wart Disease on PEI

As stated in the Terms of Reference for the International Advisory Panel, the scope of work was to focus on providing recommendations with respect to specific International Plant Protection Convention pest designations for potato wart that may be applied in parts of PEI. The initial scope was to focus on recommendations regarding any immediate opportunities to establish specific pest designations for potato wart on parts of PEI. This was followed by a review of broader potato wart pest status of PEI, in consideration of current and previous potato wart control measures, with the intent of providing additional recommendations such as maintaining pest designations and possible amendments to the 'Potato Wart Domestic Long Term Management Plan' sampling and confirmatory processes. The IAP is of the opinion that this forms an essential part of the assessment on a possible PFA status of PEI and decided to include relevant recommendations on the current management in this report. Certain aspects were considered of major importance for managing the disease and determination of PFA status. The assessment and recommendations of these aspects are presented in the following section. All recommendations of the IAP on management of Potato Wart Disease are also listed in Table 4 in Annex I.

Management measures on Regulated Fields

Generally potatoes are considered the only cultivated host for S. endobioticum, even though some wild hosts have been described as becoming infected in Mexico and other solanaceous plants, e.g. tomatoes, could be artificially inoculated (EPPO 2022). The persistence and longevity of the resting spores is the most remarkable feature of the pest and it must be assumed that a certain percentage of resting spores will remain viable in the soil for an unknown period, even though the viability of these spores will be reduced over time. Cultivation of potatoes in fields found infested or considered possibly infested may have a significant impact on the time that needs to pass until fields can be securely descheduled. Therefore, prohibition of potato cultivation on infested fields for a long period is a key measure used for control of Potato Wart Disease. On PEI cultivation of susceptible potato varieties is required for descheduling as defined in the 'PW Domestic Long Term Management Plan'.

The European and Mediterranean Plant Protection Organisation (EPPO) recommends descheduling of previously infested plots only after 20 years, following either two bioassays with negative results or one bioassay with negative results combined with direct examination of soil for the presence of viable resting spores with no findings (EPPO 2017a). Conditions for containment of the pest include prohibition of potato cultivation or other plants for replanting for the period the plot is infested (EPPO 2017c). As a general principle the plot should have been cultivated during the period of scheduling; it should not have been under permanent grassland. The new Implementing Regulation (EU) 2022/1195 (ANONYMOUS 2022) published by the European Commission in July 2022 is largely aligned with the recommendations from EPPO. Potato cultivation and cultivation of other plants for planting on productions sites defined to be infested with S. endobioticum is prohibited for a minimum of 20 years.

Cultivation of resistant potato varieties after five years on Index fields is currently possible under the 'PW Domestic Long Term Management Plan'. This poses two risks regarding Potato Wart Disease. Firstly, resting spores may spread due to soil movement associated with potato handling and logistics. Even with a high standard of equipment cleaning and post-harvest management of potatoes from infested fields, it is unlikely that all soil will be removed. The dissemination of low numbers of spores on other fields could result in new infections. As currently mostly susceptible varieties are cultivated on PEI, such low numbers of spores might multiply over the years. It is important to note that a minor infection can remain undetected as the number of spores in the soil may be low and the distribution in the field may be patchy. Also, low spore concentrations or non-conducive weather conditions can result in low disease expression on tubers, in which case potato warts could be too small to detect. It may take several crop cycles before such introductions of low numbers of spores are discovered. However, during this period there can be further dissemination to other fields. Secondly, after the short period of five years the potentially large numbers of viable spores could increase the risk of break-down of resistance and could result in new pathotypes which would render the resistance genes ineffective (Van de Vossenburg et al. 2022).

Currently cultivation of susceptible varieties on Index fields is not only permitted after 20 years, but also encouraged as it is mandatory for descheduling of Index fields. The permission for the cultivation of susceptible varieties is given when less than five spores per gram of soil are found in each sample and all bioassays are negative. EPPO proposes cultivation of susceptible varieties after 20 years on an infested field only if two bioassays give negative results or if one bioassay is negative and zero spores can be found during soil sampling with three samples per hectare, consisting of 60 subsamples per sample. It is noted that the soil sampling scheme applied on Index fields on PEI results in a higher number of samples per hectare. However, permitting cultivation of susceptible varieties when spores can still to be detected in the soil poses a risk and the required number of susceptible crops needed for descheduling might result in an increase of the infestation level in the field.

Similar concerns apply to the mandatory cultivation of susceptible varieties on the Adjacent, Primary Contact or Other Contact fields combined with post-harvest field and tuber inspections. In effect this is a multi-year field bioassay to ensure a field is free from Potato Wart Disease. This practice does not consider that symptom development on tubers might vary and that potato warts can fall off the tubers during harvest activities. Additionally, only a portion of tubers from each field can be subject to direct inspections. Thus, infestations might go unnoticed for a number of crop cycles.

No buffer zones are set up around Index fields under the current management, even though Adjacent fields may act as a buffer zone. However, the current definition of Adjacent fields does not adequately reflect the risk of spread of resting spores from the Index field. Information provided to the IAP indicates that minor barriers or smaller roads are considered as a reason to exclude neighbouring fields from regulation. Additionally, under the 'PW Domestic Long Term Management Plan' the cultivation of susceptible varieties is also mandatory for Adjacent fields in order to lift restrictions, e.g. on soil movement. EPPO Standard PM 9/5(2) 'National regulatory control systems Synchytrium endobioticum' (EPPO 2017c) requires the setting up of a buffer zone which ensures the protection of the areas around an infested production site. EPPO Standard PM 5/10 (1) 'Guidelines on the design and implementation of a buffer zone' (EPPO 2021) states that the appropriate size and likely effectiveness of a buffer zone is dependent in particular on the biology of the pest, the size and the density of the pest population, the ease of detection, the types of habitat in the buffer zone and the control measures applied in the buffer zone and in the infested area. The new EU Commission Implementing Regulation (EU) 2022/1195 (ANONYMOUS 2022) states that the buffer zone size should be based on sound scientific principles, the biology of the pest, the level of infestation, the distribution and frequency of cultivation of potatoes, the environment and geographical conditions, as well as the specific risk of spread of resting spores. In a buffer zone, only resistant ware potatoes maybe cultivated until the infested field can be descheduled in order to prevent the increase of undetected levels of infestation.

Recommendations

Taking into consideration that PEI is the only area in North America where findings of Potato Wart Disease have caused major disruption to export markets, especially to the USA, the IAP recommends that the management of Potato Wart Disease should be stricter than the provisions of EPPO PM 9/5(2). It is therefore recommended that no potatoes or other plants for replanting or root crops should be grown on Index fields for a minimum of 20 years. Cultivation of other crops is acceptable. After 20 years the fields should be resampled. If no resting spores can be found under the microscope and all bioassays are negative, resistant processing potatoes could be produced with mitigation measures in place combined with resampling and tuber inspections after every crop. Restrictions on soil movement and soil freedom for equipment should apply for the whole time the field is designated as an Index field. After further ten years provided that no spores are found in the samples, all restrictions on the field can be lifted. However, if the following crop is a susceptible variety the crop should be inspected preferably during harvest as per the EPPO standard.

A safety zone is defined as an area with the highest likelihood of direct dissemination of resting spores, for instance by water or wind, even though soil testing results are negative. This safety zone should extend to 15 meters and be set up around the infested field/area for the whole time the Index field is under regulation. No potatoes should be cultivated in the Safety Zone and it should be surrounded by a barrier, e.g. a fence, to prevent movement of equipment, soil or animals to and from the infected area. For the adjacent field a safety zone should be made based on a risk assessment regarding the likelihood of dissemination of resting spores (water, wind).

In addition a buffer zone, defined as an area with no infestations detected but has a higher risk of contamination, should be set up for the whole time the Index field is under regulation, using the guidelines provided by EPPO in PM 5/10 (1), including the Adjacent fields. In this context Index fields should be reviewed to determine if more Adjacent fields should be included in the buffer zone, than under the current definition. Soil sampling schemes in a 4x4 m grid, as currently prescribed for the first 15 m of Adjacent fields not used for potato production, should be part of the set up of the buffer zones to ensure that fields included in the buffer zone are free from S. endobioticum. Only potatoes of a resistant variety for processing end use or table stock with mitigation measures in place should be grown in the buffer zone. Mitigation measures include handling of potatoes in facilities under compliance agreement, treatment of table stock by washing and treatment with sprout inhibitors and waste disposal. Restrictions on soil movement and soil freedom for equipment should apply for a minimum of 20 years in the buffer zone.

It is recommended that a delimiting survey should be applied on large Index fields on an intensive grid to determine the actual dimensions of the infestation. In case of localised infestations (hotspots), it should be possible to classify the rest of the field as safety zone (15 meter) and buffer zone (Fig.9).

This should also be reflected in the classification of Adjacent, Primary Contactor Other Contact fields.

Picture - Schematic illustration of the current situation on PEI. Description follows.

For Primary Contact fields, the IAP recommends that the requirements be different for the following types of field:

In fields scheduled due to the planting of seed potatoes or disposal of tare soil from an Index field, a minimum of four crops of resistant potato varieties for processing end use or table stock with mitigation measures in place should be required before descheduling.
In fields scheduled due to shared equipment with an Index field, a minimum of two crops of resistant potato varieties for processing end use with mitigation measures in place should be cultivated before descheduling.

The IAP considers the risk connected with Other Contact fields to be low and therefore recommends one crop of a resistant variety for processing end use or table stock with mitigation measures in place should be required before descheduling.

All potato crops produced on Regulated Fields should be subject to adequate surveillance measures, e.g. grid soil and tare soil testing and tuber inspections after harvest. Restrictions on soil movement and requirement of soil freedom for equipment should apply for Primary Contact and Other Contact fields for the whole time the field is under regulation. Tubers of the first crops of susceptible varieties produced on any regulated field should be inspected, preferably during harvest. Figure 10 gives a schematic overview of the proposed restrictions on Regulated Fields.

Picture - Figure 10: Schematic illustration of the regulations for Index, Adjacent, Primary Contact and Other Contact fields, as recommended by the IAP Description follows.

Resistance testing

In the Pest Categorisation on S. endobioticum conducted by the European Food Safety Authority (EFSA 2018) the use of different methods to determine resistance in potato varieties in the Member States of the EU is mentioned as a factor in reducing the effectiveness of the phytosanitary measures applied for control of the pest. BAAYEN et al (2005) showed that varieties that responded in the executed laboratory tests with formation of small warts will remain free or virtually free from infection under field conditions.

The authors concluded that plants of such cultivars will generate too few and too small warts, and hence too few sporangia to support infection of neighbouring plants of the same crop or of plants of a consecutive crop of the same cultivar. Nevertheless, field tests are not included in the EPPO Standard PM 3/88(1) 'Testing of potato varieties to assess resistance to Synchytrium endobioticum' (EPPO 2020). Even though molecular analyses are being developed to facilitate assessment of resistance, they are not considered by this standard.

A resistant variety is defined by EPPO as one which reacts to the pathogenic agent in such a way that there is no danger of secondary infection (i.e. no resting spores are produced). As a response to a pathogen, plants can react with a hypersensitive response in the form of a cell death triggered by the NLR (nucleotide-binding domain leucine-rich containing protein family). This effector-triggered immunity (ETI) is highly efficient in restricting disease progress, forming an effective resistance against a pathogen. Quantitative resistance locus (QRL) analysis identified genomic regions providing resistance to (and combinations of) the pathotypes of major importance (Obidiegwu et al., 2015; Prodhomme et al., 2020a). In these mapping studies, major- and minor-effect QRLs could be distinguished. The major QRLs are referred to as Sen genes (from S. endobioticum), followed by a sequential number. The identification of the first S. endobioticum effector protein, AvrSen1, indicates that the major wart disease resistance genes are based on the highly efficient ETI. The Sen1 gene is widespread in potato breeding germplasm (Prodhomme et al., 2020a, 2020b) and was already present in old varieties such as Pink Fir Apple (1850), Champion (1876), and Belle de Fontenay (1885), which suggests Sen1 was present in the European breeding material before the first introduction of S. endobioticum in Europe (Prodhomme et al., 2020b). Resistance to the higher pathotypes is required on PEI as pathotypes 6 and 8 are found on the Island. Resistance to higher pathotypes is observed less frequently in potato-breeding germplasm but resistance to pathotype 6 and 8 is widely available. For instance, Sen2 was identified in a complex diploid potato hybrid with multiple wild species origins and provides a broad spectrum of resistance to pathotypes 1(D1), 2(Ch1), 2(G1), 3(M1), 6(O1), 8(F1), 18(T1), and 39(P1). Sen3 also provides broad spectrum resistance to pathotypes 1(D1), 2(G1), 6(O1), and 18(T1) and resides at the same locus as Sen1, on the upper arm of chromosome 11 (Bartkiewicz et al., 2018; Obidiegwu et al., 2015; Prodhomme et al., 2019). This gene is mainly found in German and Polish potato varieties and seems to be present in the resistant variety Goldrush that is popular on PEI. Sen4 was reported to provide resistance to pathotypes 2(G1), 6(O1), and 18(T1) and is mainly found in Dutch starch varieties as well as in the German variety Panda. Sen5 is very rare in the potato breeding material, having been identified in only three commercial Dutch starch cultivars so far. Additional resistances present in potato breeding germplasm could not be positioned on the potato genetic linkage map and consequently no markers for these genes are available.

The use of resistant varieties in combination with strict phytosanitary measures was very successful in the last century, wart disease diminished and consequently breeding for potato wart resistance lost priority until the discovery of new pathotypes that caused disease in previously resistant varieties. This resistance breakdown subsequently classified the avirulent pathotype 1 (D1) from the virulent pathotypes such as 2, 6 and 18. Other potato varieties negative for all the Sen genes described so far, but resistant to one or several pathotypes have been reported (Prodhomme et al., 2020a). These indicate that additional resistance sources already present in the commercial gene pool can be further characterised and exploited. In many cases, the resistance for wart disease under field conditions is stable. Nevertheless, it is generally acknowledged that the stacking of multiple ETI receptors in crop varieties will enhance durability of resistance (Luo et al., 2021). In the process of selection of effective wart disease resistance in breeding programmes, major-and minor-effect QRLs have been stacked, indicating this may be a requirement for the effective control of the pathogen (Prodhomme et al., 2020a).

Recommendations

The IAP recommends that resistance testing should be determined according EPPO Standard PM 3/88(1) 'Testing of potato varieties to assess resistance to Synchytrium endobioticum' (EPPO 2020). In addition, the IAP recommends listing potato varieties grown on PEI with their respective resistance level for the pathotypes 6 and 8 and define a clear cut-off of the required resistance levels in the different situations/conditions. Finally, the IAP recommends that isolates from all Index fields should be characterised for their pathotype, if new pathotypes are observed the list should be updated. As determining the resistance in bioassays by microscopic observations is labour intensive, time consuming, and costly, it is recommended that the full spectrum of potato varieties that would be suitable for PEI and breeding material that could first be used is screened for the Sen genes using molecular markers. Several markers are publicly available as Kompetitive Allele Specific PCR (KASP) markers that allow reliable screening for tetraploid plants and diploid material. Flanking KASP markers can be used to determine haplotype blocks in the potato genome on which resistance genes are present. An additional benefit of the molecular markers is that different Sen resistance genes are screened and selected. This information can be used to select varieties with multiple resistance genes. Alternatively different types of resistance genes can be used alternating in the field. Both strategies could enhance the durability of the resistance. The potato variety Goldrush seems to have the Sen 3 resistance gene which could provide broad spectrum resistance to all isolates currently identified on PEI. However, Sen 3 seems to be the only major resistance gene in Goldrush. Therefore, it is advisable to test the efficacy of this resistance gene on the isolates collected from PEI and to see if additional resistance genes can be stacked in future breeding programmes. This will potentially increase the durability of resistance. For other potato varieties with Sen 3 or other resistance genes a similar approach is suggested.

In parallel, the resistant varieties and breeding material can be used for a quick scan of susceptibility using a laboratory test by inoculating the potato tuber either by the Glynne–Lemmerzahl and/or the Spieckermann method as recommended in the EPPO Standard PM 3/88(1) (EPPO 2020). These bioassays would confirm the prediction based on the Sen markers, which is important as the pathotype 6 or pathotype 8 isolates from PEI could be distinct from their European counterparts (although they are genetically very similar). In these bioassays additional potato wart resistances could be detected that are not yet mapped and for which Sen gene markers are not available. Isolates of S. endobioticum from PEI should be stored in living collections for future reference for resistance testing.

After this first susceptibility test more elaborate laboratory testing according to EPPO Standard PM 3/88(1) 'Testing of potato varieties to assess resistance to Synchytrium endobioticum' (EPPO 2020) should be used in which more information could be obtained on the resistance level. This is important as partial resistance can result in loss of efficacy of the resistance genes in field situations. For these bioassays, it is important to use a representative set of the isolates from PEI.

The IAP recommends the use of only resistant cultivars in buffer zones and Primary Contact or Other Contact fields for the management of Potato Wart Disease. Eventually resistant varieties are recommended for all commercial potato fields at least in high-risk areas, as this could be regarded as an additional safety precaution for risk mitigation and disease management.

Surveillance via soil testing and field and tuber inspections

Management of Potato Wart Disease on PEI relies strongly on soil testing in combination with bioassays and field and tuber inspections. Both methods have drawbacks and limitations. Detection of the pest within a given field is dependent on distribution of resting spores in combination with sampling intensity, which influences the chance of collecting soil samples which contain sufficient resting spores to allow for detection and quantification. Distribution of resting spores in the field might be aggregated in single spots in the field, especially in cases of low infestation. Hampson and Coombes (1996) examined the spatial distribution of resting spores in the soil. They concluded that no particular pattern of aggregation emerged; the aggregates assumed random patterns of distribution. This makes soil sampling for the detection of the pest difficult, particularly when inoculum levels are very low. The probability of hitting spots with a higher spore concentration in a given grid is not very high, except in cases where the spore concentration is very high and many spores have already been distributed over the whole field. In addition, tuber inspections will only be successful when infested tubers are included in the samples and symptoms expression might vary, depending on a number of factors, including the inoculum level in the field (Baayen et al. 2005), and inconspicuous symptoms may be overlooked. Additionally, warts tend to break off easily when tubers are handled, therefore remaining in the fields when potatoes are harvested and making detection even more difficult.

Recommendations

The IAP recommends soil testing of all Index fields in a compressive and GIS based structure. Also recommended is the structuring all data and associated meta-data in such a GIS based information system that would provide a clear overview on locations and links. Potentially, also information on tracing of equipment could be included in this system. Such information is important for phytopathogenic forensics and risk calculations. Although the current Survey123 program provides locations it does not contain important information on the spread of the infection in the field. An example of a GIS based system which enables tracking of all this information is provided as a supplementary document. Furthermore, the IAP is of the opinion that testing of tare or piler soil could be a useful addition to surveillance programs and could serve to fill a number of different functions within the PEI Potato Wart Management program. Application of tare soil testing in the surveillance of Regulated Fields would potentially decrease the amount of soil to be tested. Moreover, the probability of occurrence of resting spores in soil attached to tubers is expected to be higher, increasing the efficiency of the sampling. Depending on the location of sampling the collected tare soil could represent a good mixture of soil from the whole field, enhancing the probability to find spores even when the infestation levels are low. This targeting of the sample will result in an increased confidence in detection. For example, tare soil testing has been used to identify a number of Potato Cyst Nematode (PCN) outbreaks in Australia. If integrated in seed certification schemes, in addition to the currently conducted visual inspections, tare soil sampling could increase the level of confidence that seed potatoes are free from Potato Wart Disease. It could also be applied in phytosanitary inspections to increase the level of security when issuing phytosanitary certificates.

A drawback of using tare soil is that the testing can only occur following the growth of the crop and therefore cannot be used to determine the suitability of whether a field should or should not be used to grow potatoes, e.g. for seed potato production. Also, unless there is a complete clean down of the trucks between loads (which is not generally practical) the use of tare soil can never be 100% reliable as regards its source. There is always the risk that the soil from a previous load could contaminate the sample.

Additionally, post-harvest visual inspections on tubers should focus on sampling points with the highest likelihood to detecting infected tubers. Observing the line while passing over a roller table during any of the post-harvest handling operations would be the ideal method of inspection. However, given the uneven spatial distribution of the pathogen within the field this necessitates an inspector standing on the line for the entire time, which in most cases is not practical. A potential alternative would be to run the cull bins over a roller table (separate from the main line) and inspect those tubers, as they are continually removed by staff on the line because they have identified an issue. If the issue is obviously Potato Wart Disease, this would be identified by the staff as such; however, many of the early stages of infection may be removed but not identified as wart as the symptoms can be confused with other diseases. This method may be a more cost-effective method to survey Regulated Fields.

The IAP considers that the differentiation of viable and non-viable spores would be useful to integrate into the surveillance process. Findings of only very low numbers of resting spores in the soil can be relics of earlier infestations. If these spores are clearly non-viable and additionally bioassays give negative results, only low risk is connected with these fields and their classification as Index fields should be reconsidered. Application of long-term management measures on such fields as Index fields is unjustified and may reduce cooperation from potato growers. Nevertheless, there still is some risk regarding the dissemination of viable resting spores, therefore some precautionary measures should apply. The IAP recommends that such fields are classified in the same way as Primary Contact fields, and the requirement to grow two crops of a resistant variety for processing end-use with mitigation measures and restrictions on soil movement and requirement of freedom from soil for equipment should be applied. The EPPO diagnostic protocol for S. endobioticum (EPPO 2017b) gives some guidance on the differentiation between viable and non-viable resting spores. However, it is generally agreed that resting spores with incomplete, heterogeneous contents may be difficult to identify as dead or alive and that in case of doubt resting spores should be considered viable (Fig.11).

New molecular methods are being developed based on RNA detection that so far show a good correlation between RNA presence and viability. The validation of the method is complex as it is currently benchmarked by bioassay evaluation. Protocols are available, and the panel suggest using these where appropriate. Unfortunately, with low numbers of resting spores the RNA based detection may not be sensitive enough. As a consequence, the method could classify larger amounts of spores as viable when RNA is detected eliminating the need for microscopic observations. In case of negative results, microscopic determination is still necessary to verify especially when low numbers of spores are available.

Under the current management program, detection of Potato Wart Disease relies heavily on visual inspection, e.g. during certification or insurance inspections or while sorting and handling. The IAP believes that more information on the various expressions of symptoms, especially the more inconspicuous ones, could be useful. A wider range of pictures should be made available that represent different disease expressions (Fig. 12). Real size 3D-models could be prepared, similar in weight, size and shape to real potatoes, complete with the range of disease expressions typical of the disease. These could potentially also be used to see which symptoms can be detected in the different visual observations that are currently conducted on PEI. This exercise could raise awareness and improve the search profile. This raised awareness will probably increase the number of notifications of Potato Wart Disease but also will lead to more false positives. It is recommended that CFIA should set up a verification system to quickly evaluate and, if possible, confirm such notifications.

Picture - Figure 12: Potato wart. Description follows.

Waste management

The current management strategies to handle waste from Regulated Fields are quite comprehensive, but former use of waste on agricultural fields may have contributed to the spread of resting spores to new fields. Prior to the detection of potato wart in 2000, waste disposal at processing facilities was not required as PEI was considered free from potato wart. The first Operational work plan of December 13, 2000 provides an example of waste management requirements. It documents a procedure for waste, cull and peel disposal for bulk potatoes leaving PEI and destined for use as French fries and associated fried potato products. At the time the potential risk associated with peels, processing line rejects, raw culls, and dirt sludge were considered remote, but they did present a potential pathway for dissemination of infective spores. The Operational work plan recognised several mitigation methods including:

Deep burial of cull potatoes, rejects and all soil residues at an approved landfill
Blanching partially processed tubers at 160-170° F for 15 minutes followed by deep frying at 375-380° F for 60 seconds;
Steam peeling of soil free tubers at a minimum temperature of 360F at 200 PSI for a minimum of 10-15 seconds followed by the separation and removal of steam peels;
Composting, reflecting the temperature and time requirement standard of Zakopal 1970.

Over time, four processing plants were located on PEI. McCains, which closed in 2014, two Cavendish plants and AgraWest Foods Ltd, a facility for dehydrated potato products. In addition to the processing facilities there are a large number of packing facilities which create a waste stream related to the washing of potatoes and disposal of organic materials.

Even though Cavendish Farms has had a comprehensive waste management strategy in place for some years, it is likely that contaminated waste may have been disposed of on farmland in earlier years. Given the persistence of resting spores and the fact that, especially for processing uses, mostly susceptible varieties are cultivated it is likely that some recent findings of Potato Wart Disease are the result of such waste disposal from many years ago.

Use of potato associated wastes on arable land must be considered a very high risk if S. endobioticum is present for dissemination of resting spores. This is also valid for waste that was subject to any form of subsequent treatment, such as composting, pasteurisation, anaerobic digestion etc. as inactivation of resting spores by these treatments has not been proven. Schleusner et al. (2019) showed that resting spores of S. endobioticum not only survived mesophilic anaerobic digestion but also subsequent storage of the digestate for at least 4 weeks. Therefore, use of agricultural fields for the disposal of biowaste, which includes waste from potato processing industries, should be considered a high risk for the dissemination of resting spores. In Germany this fact is reflected in the legislation as use of sewage sludge from municipalities on agricultural fields is prohibited if it might contain waste from potato processing industries. The German Fertiliser Ordinance postulates that substances cannot be classified as fertiliser if they contain among other things resting spores of S. endobioticum.

It must therefore be concluded that there is a certain risk connected with disposal of any waste on agricultural land, even after treatments like composting, heating or anaerobic digestion. This is especially true for all areas where processing and packing facilities were located and where waste in any form is used on farmland or for feeding of cattle.

Recommendations

The IAP recommends that facilities under a compliance agreement for the washing, and possibly those for processing of potatoes should have the highest standard of sanitation in place. This refers not only to the facility itself and the handling and disposal of waste, but also to the whole area where the facility is located. Resting spores can be easily disseminated via dust from uploading or through potatoes that fall from trucks. If arrival areas for trucks consist of concrete, cleaning of these areas would be simplified. Additionally flat-water basins for tyres positioned at the entrance of the arrival area would reduce the risk of dissemination of resting spores by dust adhering to the tyres. Similarly, water basins for footwear should be positioned at all spots where people leave the facility to avoid dissemination of resting spores by shoes.

As an additional security measure waste from any processing or handling of potatoes (washing and packing) like bio-sludge from anaerobic digestion or other bio-solids, compost or liquids from washing or processing should not be used on agricultural land as it poses a risk of dissemination of resting spores, if waste from infested potatoes is included. It is also recommended that extra scrutiny be put on fields on which waste from potato processing has been disposed of in former years, e.g. by comprehensive post-harvest inspections. This refers especially to processors or packaging facilities which do not have compliance agreements for waste disposal in place.

Communication and future research needs and possibilities

An aspect that caught the attention of the IAP was the lack of involvement and understanding of some of the affected parties of restrictions and other management measures which apply as, for example, growers were in doubt that positives resulting from the detection of resting spores in the soil should be considered as true cases of the disease. However the IAP is of the opinion that the testing and identifications schemes of the CFIA are correct and that identification of viable resting spores should lead to determination of a field as an Index field. Therefore the IAP considers this as problem of communication between CFIA and the growers.

The IAP was made aware of the challenges for the agencies and industry in agreeing on the management of Potato Wart Disease in PEI, including different interpretations of test results were very low numbers of spores were found; descheduling processes for Primary and Other Contact fields; the export ban on seed potatoes currently in place. This is not a surprise, considering the complexity of the findings. Potato Wart Disease is one of the most, if not the most, difficult diseases to eradicate and control. Significant biosecurity measures must be implemented for control which affect the potato industry and local communities. However, the governmental agencies, industry partners, service providers, science community, and local communities are all essential stakeholders in managing Potato Wart Disease on PEI. Effective communication and engagement across all parties is fundamental to the success of such a major biosecurity response through providing accurate and timely information, promoting partnerships, supporting new initiatives, influencing behaviour change, and building trust.

Recommendations

The IAP is of the opinion that the setup of a communication system for open discussion, e.g. by a task force consisting of representatives of all affected parties, could enhance the success of the management measures for control of Potato Wart Disease on PEI. Similar communication systems have been successfully applied in other states; examples for such systems from New Zealand and Australia are provided in Annex II of this document.

In addition, the IAP sees the need and the possibilities for various research activities that could be conducted on PEI or NL. The results of these research activities could provide valuable insights into potato wart infestations and their spread and may help to develop more efficient control measures. Some important aspects are already under analysis by the CFIA, such as investigating the number of susceptible potato crops needed to elevate infestation levels to a detectable threshold or questioning whether the current surveillance and survey methods are sufficient to detect low-level populations.

The topic of fast and sensitive survey methods was considered particularly important by the IAP. A key element in the control of Potato Wart Disease is the efficacy and speed in which the disease can be detected, as the timely implementation of control measures can reduce the risk of spreading the disease and limit the impact on potato production and trade. Methods of detection include visual inspection of tubers at harvest, resting spore detection in soil from grid sampling or from tare soil sampling. In addition, bioassays can be performed in which soil is used in inoculation experiments and the inoculated tubers are monitored for wart formation. Furthermore, molecular methods are available for detection, identification and quantification. Depending on the application, the season, and the distribution of the disease, these methods can vary in their suitability and efficacy. A comparison of methods in terms of performance characteristics such as specificity, sensitivity, robustness, throughput and cost could be organised and help in selecting the most suitable procedure and protocols. They could also assist in understanding the epidemiological aspects of the disease, e.g. history of infestations in an area, disease expression in certain fields and dissemination over short and longer distances. For example, if resting spores are not able to infect potatoes in a bioassay, this could indicate that the infestations in the field have occurred many years ago. The combination of the bioassay with microscopic analysis of the spores and molecular testing for RNA could provide further information about the viability of the resting spores, leading to a more accurate risk assessment for the field.

An important aspect regarding the efficacy of a survey is the matrix of samples analysed. Soil is mostly sampled in the field by a predefined grid. Tare soil collected from harvested potatoes has been in close contact with the host and is therefore more likely to be contaminated with resting spores. Additionally, potatoes are harvested from every location of the field therefore a tare soil sample is likely to be more representative than fixed sampling points in the field. This would make tare soil a more suitable substrate for detection. Potential complications with tare soil testing are: (i) samples can only be taken after harvest, (ii) exact positioning of the sample location could be difficult, (iii) samples may be less consistent as the amount of soil attached to the tubers may depend on soil moisture and organic matter, (iv) limited amounts of tare soil might make bioassays more difficult. In the opinion of the IAP the sensitivity of tare soil testing should be determined compared to grid sampling, in a research trial as well as testing symptoms expressions in bioassays conducted with tare soil. It is likely that an optimal survey for S. endobioticum might require a combination of soil sampling in a certain grid and tare soil testing.

Another important aspect concerns the back-tracing of the initial introduction of the infestation into a field. The detailed mapping of the positive sampling sites and the number of spores found in an Index field could be used to determine the site of the original infection, the most likely way of introduction (tubers, soil, manure or equipment) or the mechanism of local spread (along the river or flooded area, along the track of equipment, cattle feed area). This information would provide important insight into the local epidemiology and spread of the disease in an area. It could also provide information on the time that may have passed since the first introduction, helping to define the Primary Contact and Other Contact fields. Details of sample collection, type of tests conducted (morphology/real-time PCR/bioassay), and test results (e.g. location of positive and negative samples, number of viable and non-viable spores, positive and negative bioassays) should be incorporated into the existing digital map system to visualise the disease in the fields. This will enable thorough risk assessment of infected fields, efficient and thorough back-tracing, and informed decisions made on setting safety and buffer zones. In this context, further studies on the spread of resting spores by wind and water would also be significant. Some research on wind dispersal has been conducted by HAMPSON (1996) in Newfoundland. The author concluded that wind dispersal of resting spores is possible and that vehicles could present an impact surface and therefore also contribute to dissemination of resting spores over longer distances. Further research could be useful to determine the range of distances over which resting spores might be spread via wind or waterways, as well as the importance of flooding regarding the dissemination of resting spores in an area. Sampling strategies are required to concentrate the spores to a particular fraction, which would potentially allow monitoring of water and air samples. Validation of the procedures could possibly be performed in NL where wart disease occurs more frequently. Strategies could then be applied on PEI on Index field locations. The results could be very useful to determine associated fields that might lay in some distance to the Index field.

Another important research area is the investigation into possible inactivation of resting spores in the fields and in soil. Currently, the management of Potato Wart Disease worldwide usually relies on ensuring that potatoes are not grown in infested fields and the cultivation of resistant potato varieties in surrounding areas. But this presents a very passive way of control of a disease and therefore many years might pass before a field can be descheduled. Additionally, soil from infested fields must either stay on these fields or must be disposed of on other designated areas to limit the risk of spread of resting spores to other fields. Measures to either inactivate resting spores or to enforce germination of resting spores at times where no host is in the field could reduce this time significantly. Some research was conducted in this area in the middle of the last century (an overview is given in Langerfeld, 1984); but even today, no procedure has been developed for either inactivating resting spores or enhancing germination at a favorable time in the soil. The areas in NL which are infested with Potato Wart Disease offer great opportunity for research trials in this area. Additionally, they offer the opportunity for testing and validation of new surveillance methods, like the use of sniffer dogs to detect infested potatoes in the field, or the use of drones with hyperspectral cameras. Such methods could contribute to specific surveys for Potato Wart Disease in areas where the pest is not known to occur, saving personnel resources in terms of both inspectors and laboratory staff.

Conclusions on a possible Pest Free Area status for S. endobioticum of PEI

The Terms of Reference for the International Advisory Panel state that the scope of work should focus on providing recommendations with respect to specific International Plant Protection Convention pest designations for potato wart that may be applied in parts of PEI. In order to fulfil this task, the IAP has examined and assessed all information provided to them during their visit on PEI and afterwards. The thoughts and recommendations regarding the pest status of S. endobioticum on PEI and the possible determination of parts of PEI as PFA are summarised in this section.

According to ISPM 8 (IPPC Secretariat. 2021) the pest status of an area should be based on pest records or other sources. Pest records can be the result of general or specific surveillance, defined in ISPM 6 (IPPC Secretariat 2018) as:

General surveillance: a process whereby information on pests of concern in an area is gathered from various sources. Sources may include national or local government bodies, research institutions, universities, museums, scientific societies (including those of independent specialists), producers, consultants, the general public, scientific and trade journals, unpublished data, and the websites of other NPPOs or international organisations (e.g. the IPPC, regional plant protection organisations, the Convention on Biological Diversity).

Specific surveillance: a process whereby information on pests of concern in an area is obtained by the NPPO over a defined period. NPPOs actively gather specific pest-related data. Specific surveillance includes surveys that are conducted to determine the characteristics of a pest population or to determine which species are present or absent in an area.

Both kinds of surveillance were conducted on PEI in form of visual inspections of tubers and soil sampling, e.g. the island wide visual surveillance program from 2001 to 2004, all visual inspections on tubers for seed certification or insurance or the Canada wide surveillance, which included PEI in 2020. Findings of potato wart were recorded and traced back for a period of ten years. Comprehensive information is available on the Regulated Fields and the surveillance activities conducted on these fields over the years. Of course, it must be taken into account that the information that can be obtained through the surveys is limited, since detection of the pest via visual inspection or soil sampling is difficult and depends on infestation levels in the field, dissemination of the pest in the field, symptom expression and sampling design.

Even though it is possible that not all infestations of potato wart in the fields have been detected at present, the dissemination of the pest is limited and S. endobioticum has today been detected on only 35 fields, corresponding to 0.6 % of all land grown with potatoes on PEI. These 35 Index fields were grouped in five clusters of linked infections (see Fig 1). Additionally, one piece of land used for disposal of tare soil, has been found infested. This piece of land is not regulated as an Index field, as it is not suitable for agricultural production. Connected to the Index fields are a large number of fields under regulation as Adjacent, Primary Contact or Other Contact fields. According to the CFIA in 2022, over 16,000 hectares in PEI are restricted for potato wart, accounting for approximately 16% of the land farmed with potato in PEI (Fig. 13).

Regarding the current total number of Index fields on PEI, the IAP concludes that the pest status for S. endobioticum should be defined as 'Pest present, not widely distributed and under official control', according to ISPM 8. Official control measures currently taken for eradication of the pest in the Index fields and prevention of further dissemination are described in the section on 'Management of Potato Wart Disease on PEI', of this report. Additional control measures are proposed by the IAP and described in the section on 'Conclusions and recommendations on management of Potato Wart Disease on PEI'.

In a similar measure, the Regional Standards for Phytosanitary Measures (RSPM) 3 'Movement of Potatoes into a NAPPO Member Country' of the North American Plant Protection Organisation (NAPPO) defines the status for S. endobioticum for Canada as P2 (present: only in some areas) and P9 (present: subject to official control) (NAPPO, 2011).

Picture - Figure 13: Map of PEI with restricted field categories, provided by CFIA to IAP. Description follows.

The IAP also considered whether parts of PEI could be considered as PFA or if the definition of pest free places of production or (PFPP) even pest free production sites (PFPS) would be more appropriate. The International Standard for Phytosanitary Measures (ISPM) 5 'Glossary of phytosanitary terms' (IPPC Secretariat 2022) defines a PFA as 'an area in which a specific pest is absent as demonstrated by scientific evidence and in which, where appropriate, this condition is being officially maintained [ISPM 2, 1995; revised CPM, 2015]'.

According to ISPM 5 a PFPP is a 'Place of production in which a specific pest is absent as demonstrated by scientific evidence and in which, where appropriate, this condition is being officially maintained for a defined period' [ISPM 10, 1999; revised CPM, 2015]. A place of production is defined as 'Any premises or collection of fields operated as a single production or farming unit' [FAO, 1990; revised CEPM, 1999; CPM, 2015]. As the smallest possible unit, the PFPS is defined in ISPM 5 as 'A production site in which a specific pest is absent, as demonstrated by scientific evidence, and in which, where appropriate, this condition is being officially maintained for a defined period' [ISPM 10, 1999; revised CPM, 2015]. The production site is defined as 'A defined part of a place of production, that is managed as a separate unit for phytosanitary purposes' [CPM, 2015].

The establishment and use of a PFA, PFPP or PFPS by a national plant protection organisation (NPPO) provides for the export of plants, plant products and other regulated articles from one country to another country without the need for application of additional phytosanitary measures when certain requirements are met.

The concept of the PFPP or PFPS is distinct from that of the pest free area. Even though the objective is the same, the implementation is different. A PFA is much larger than a place of production, includes many places of production and may extend to a whole country or parts of several countries. It may be isolated by a natural barrier or an appropriate usually large buffer zone. A PFPP or PFPS may be situated in an area where the pest concerned is prevalent and is isolated, if at all, by creating a buffer zone in its immediate vicinity. A PFA is generally maintained over many years without interruption, whereas the status of a PFPP or PFPS may be maintained for only one or a few growing seasons. A PFA is managed as a whole, by the NPPO of the exporting country. A PFPP or PFPS is managed individually by the producer, under the supervision and responsibility of the NPPO. If the pest is found in a PFA, the status of the whole area is called into question. If it is found in a PFPP or PFPS, that place loses its status but other places of production or production sites in the area operating the same system are not directly affected. The choice of PFPP or PFA as a management option will depend on the actual distribution of the pest concerned in the exporting country, on the characteristics of the pest and on administrative considerations. Both systems can offer the required assurance of pest freedom: the PFA mainly assures this by the common application of measures to an area covering many places of production; the PFPP or PFPS mainly assures this by the fact that management procedures, surveys and inspections are applied specifically and intensively to it.

With the exception of the fields under regulation according to the 'Potato Wart Domestic Long Term Management Plan', the whole of PEI was considered a PFA before the Ministerial Order went into force in November 2021 and changed this status by regarding the whole of PEI as a place infested with S. endobioticum.

Regarding the distribution of S. endobioticum on PEI with limited distribution and under official control, which is reflected in the pest status proposed by the IAP, the IAP considers that a PFA status for parts of PEI is acceptable.

The current version of ISPM 4 'Establishment of pest free areas' (IPPC Secretariat, 2017) proposes three different types of PFAs:

an entire country;
an uninfected part of a country in which a limited infested area is present; and
an uninfected part of a country situated within a generally infested area.

The PFA status to be considered for PEI would refer to point b): an uninfected part of a country in which a limited infestation area is present.

Considerations on establishment of PFAs must take into account the biology of the pest (e.g. its survival potential, availability of host plants, rate of reproduction) its current distribution, the possible ways of dissemination, the surveillance methods available and the management practices applied to control the pest and to exclude introduction into new areas.

The risk connected with S. endobioticum is mainly based on the persistence of the resting spores in the soil and the fact that no reliable eradication measures are known to clear infested fields in a manageable time. Solanum tuberosum is so far the only cultivated host and the pest is not able to move over long distances out of its own, but is mainly distributed to other places via human activities. The availability of host plants and the suitability of climate will not be a limiting factor for dissemination of potato wart on PEI, as potatoes are the main crop cultivated on the island and the climate is mild, with spring temperatures between 8 to 20 °C. A limiting factor could be the humidity in spring, which is of major importance for the infestation of the tubers in the field. However, general information available show that it is likely that sufficient humidity will be available in spring. Additionally, irrigation can also support infestation of tubers in spring. Since S. endobioticum has limited possibilities for dispersal restricted to wind or water, dispersal it mostly is dispersed by human activities, like soil transport with resting spores (e.g. via machinery) to other fields, waste disposal from infested tubers to agricultural land or movement of infested seed potatoes. While movement of resting spores via wind or water might be difficult to control and must be therefore considered accordingly when setting up the buffer zone, human activity-based dispersal can be efficiently controlled by different management measures.

ISPM 4 defines three main components in establishing and maintaining a PFA. These are:

systems to establish freedom
phytosanitary measures to maintain freedom
checks to verify freedom has been maintained.

The systems to establish freedom are based on surveillance activities. As stated, regarding the pest status of S. endobioticum on PEI, general and specific surveys are conducted on PEI. Even though it is possible that not all infestations of potato wart on PEI have been detected today, it is considered that most infested fields are under regulation and that pest freedom for the rest of PEI is established.

After ensuring that the pest does not occur in those parts of a country that is considered to be the PFA, a system to maintain pest freedom should be set up. In case of S. endobioticum, this maintenance relies mainly on the prevention of dissemination of resting spores or infested potatoes from infested areas into the PFA. The assessment of the risk of such dissemination was therefore part of the considerations of the IAP. The risk was assessed under the assumption that the management measures proposed in this report in the section on 'Conclusions and recommendations on management of Potato Wart Disease on PEI' should be implemented. The risk categories summarised in table 3 are used in accordance with EPPO guidelines on pest risk assessment (EPPO, 2011).

Table 3: Risk categories used in the assessment regarding the risk of dissemination of resting spores or infested potatoes to parts of PEI, which might be considered as PFA
Risk	Description
Very low	The probability of dissemination of resting spores or infested potatoes is very unlikely
low	The probability of dissemination of resting spores or infested potatoes is unlikely
medium	The probability of dissemination of resting spores or infested potatoes is moderately likely
high	The probability of dissemination of resting spores or infested potatoes is likely
very high	The probability of dissemination of resting spores or infested potatoes is very likely

Risk assessments are subject to a certain degree of uncertainty and depend on the information available. Detailed information was provided to the IAP and some of which was classified as confidential. Therefore, considerations on this information in this report are generalised.

The IAP see merit in the use of a risk matrix in order to visualise, assess and compare the different levels of risk of disease dissemination of potato wart, under different scenarios. Each new scenario could be inserted into the matrix as an Index, Adjacent, Primary Contact, or Other Contact field. These can then be tracked over time and reassessed / repositioned following soil tests, bioassays, or the growth of a susceptible or resistant variety. The risk associated with each scenario would initially need to be assessed by a committee made up of the relevant plant pathology experts (CFIA) and grower industry representatives. Such a system may also help provide a clearer pathway to descheduling a particular field. As the information within the matrix builds up and new pathways are identified, the various risk categorisations can be adjusted allowing a more refined assessment to be made.

An example of a risk matrix is provided below (Fig. 14), the assessments are those of the IAP given the information that was received. However, it should be noted that this example does not cover all aspects that should be considered for a risk assessment but is mainly based on spore counts in Index fields as these are used as threshold for permission for cultivation of resistant varieties on Index fields.

For risk assessment on the likelihood of distribution of resting spores onto other fields, the number of spores found in the samples can be considered, as high spore counts in a number of samples indicate a severe infestation in a field which mostly is present for some years. Therefore, the likelihood of distribution is very high. However, it is more complex to conclude from low spore counts in the samples the possible risk of dissemination. Detection of the pest within a given field is dependent on sampling intensity and the distribution of the resting spores in the field, which both influence the chance to collect soil subsamples which give a representative impression of the infestation status of the field. It is less likely to hit a small number of disease foci in a field with a given sampling grid, and the infestation level and the risk of dissemination might be underrated.

Additionally, the efficacy of the soil extraction method for recovering resting spores from the samples influences the detection and quantification of the spores in the samples. Spore extraction efficiency of the method used by the CFIA is assessed to be ~40%. Variations between soil subsamples, due to chaotic distribution of spores within the field, is more pronounced and problematic for evaluation of spore density in soil when soil spore concentrations are low. Studies demonstrated that extracted soil spore concentrations can vary by one or two orders of magnitude from the expected value (Van Leewen et. al 2005). Therefore, a comprehensive risk assessment should also consider other aspects that might influence the dissemination of resting spores, such as the biology of the pest, cultural practices, possible pathways for spread, including movement of seed potatoes, disposal of tare soil and equipment, management and disposal of on-farm waste material (culls, tare soil, wash water from private packing facilities), management and disposal of industrial waste material, and history of land use and ownership.

Index (A)
Scenario	Risk
Dispersed high soil count bioassay	Very high risk
Hotspot high soil count positive bioassay	Very high risk / high risk
5 years post detection >5 spores/g	Very high risk / high risk
5 years post detection <5 spores/g	High risk
10 years post detection >5 spores/g	High risk
10 years post detection <5 spores/g	Medium risk
20 years post detection <5 spores/g viable	Medium risk
20 years post detection <5 spores/g non-viable	Low risk
25 years post detection 0 counts resistant varieties only	Very low risk

Adjacent (B)
Scenario	Risk
Post detection planted with host	Medium risk
Post detection planted with non host	Medium risk
5 years post index detection negative soil test	Medium risk/ low risk
10 years post index detection no host planted	Low risk
10 years post index detection susceptible variety planted	Low risk
10 years post index detection resistant variety planted	Very low risk

Primary Contact (C)
Scenario	Risk
Post index detection - seed movement	Very high risk / high risk
Post index detection - machinery movement	High risk
5 years post index detection - seed - post susceptible variety	Medium risk / low risk
5 years post index detection - machinary - post susceptible variety	Low risk
5 years post index detection - machinary - post resistant variety	Very low risk

Other Contact (D)
Scenario	Risk
Post index detection with host	Medium risk
Post index detection without host	Medium risk/ low risk
5 years post index detection - no hosts planted	Low risk
5 years post index detection - post susceptible variety	Very low risk
5 years post index detection - post resistant variety	Very low risk

Figure 14: An example of a risk matrix categorising risk of disease spread for a number of scenarios as Very high (red), High (red), Medium (brown), Low (brown) or Very low (green).

A significant number of Index, Adjacent and especially Primary Contact fields has been scheduled due to the planting of seed potatoes from Index fields (Fig. 15). Planting of infested seed potatoes is one of the most efficient ways to disseminate resting spores of Potato Wart Disease over long distances. Detection of such infestations might not be easy, especially when the number of infested tubers planted in a field is very low. Depending on the varieties used and frequency of potatoes in the crop rotation cycle, upbuilding of an infestation level detectable during a survey can take a number of crop cycles. Currently the number of Index fields resulting from risk exposure as a Primary Contact field due to planting of seed potatoes, seems to be low. However, it is possible that potato wart may still be detected in some of these fields at a later stage when infestation levels have built up. This should be considered in a risk assessment for setting up of boundaries of pest free areas.

Another important aspect to be considered is the decrease in the number of farms. As the Lands Protection Act limits the ownership of land on PEI, potato growers must rent, trade, or share fields to meet production goals. This has led to an increase of the exchange of fields between landowners, farmers and corporations, potentially increasing the risk of dissemination of potato wart to more fields. In addition, tracing back of possibly infested fields is more difficult if no comprehensive data on the operators of every field exists. In most cases the date of introduction of potato wart in a field is more complicated to identify when a number of growers may have used the field. Under the CFIA's Seed potato certification program the previous two agricultural crops must be identified, but no such provision exists for production of commercial potato crops.

Picture - Figure 15: Map of Regulated Fields due to movement of seed potatoes, provided by CFIA to IAP. Description follows.

For the assessment regarding definition of most parts of PEI as PFA, the clusters should be considered separately. A description of the fields regulated in the different clusters is included in the section 'Management of Potato Wart Disease on PEI' of this report.

The largest number of Regulated Fields is located in the area between Charlottetown and Summerside, which is also the central production area of PEI. Only six Index fields are located outside this area, belonging to two different clusters. Fields under the letter E and F are connected to Index fields under the letter G (close to the central production area) via movement of seed potatoes. The fields under the letter I and J are connected to Index fields under the letter H, also via movement of seed potatoes. A number of Adjacent and especially of Primary Contact and Other contact fields have been determined outside the central production area, mostly in connection to the Index fields under the letter E, F and I. This includes fields scheduled as Primary Contact fields due to the movement of seed potatoes. The investigations leading to the determination of the Regulated Fields were conducted between 2013 (letters E and F) and 2014/2016 (letters I and J). This means that these fields are under regulation for six to nine years without further detection of the pest.

Regulated Fields located in the central production area between Charlottetown and Summerside are allocated to all five clusters and comprise identifications over a period of more than 20 years with a corresponding high number of Adjacent, Primary Contact and Other Contact fields. So far, no connections between the Regulated Fields allocated to the different clusters were detected in the 10 years trace-back activities. However, we do not exclude that connections from earlier years exist due to shared land, which would have been detected if the trace-back system went further back, provided the information is available.

Due to the information available, the IAP assessed the risk of further dissemination of potato wart with the Index fields in the different clusters. It is noted that this assessment is not comprehensive and does not replace a detailed risk assessment for each cluster and the connected Regulated Fields.

For fields clustered under letter B, the risk regarding further dissemination is considered to be very low due to the period of time since the detection and the fact that no other Index fields have been detected since and that the owner does not grow any potatoes on the Index fields.

With regard to the two fields comprised under letter L and also forming a unique cluster, information provided to the IAP states that the findings from export samples could not be confirmed in the intensive follow-up soil tests. Therefore, the IAP also considers the risk connected to these fields to be very low.

Additionally, the IAP considers the fields under the letters E, F and G, which do belong to a separate cluster, as low risk regarding further spread of resting spores of S. endobioticum because of the low numbers of positive soil samples and the low spore counts associated with these fields.

The field under letter J is connected to the Index fields under letter H via planting of seed potatoes. It is a single field under regulation since 2015, with limited numbers of positives and no subsequent findings. The IAP therefore considers the risk of dissemination of Potato Wart Disease from this field as low.

The two fields under letter I, also connected to the Index fields under letter H via planting of seed potatoes, differ in the intensity of infestation but with a limited number of positive samples found during the last sampling period. The risk associated with these fields regarding the further dissemination of Potato Wart Disease is considered to be low by the IAP.

Regarding all fields under the letters H, A, C, D, K, M and N, the IAP is of the opinion that a higher risk is connected with these clusters due to the infestation levels found on some of the fields or the incidence of infestations in the area.

For letter H it must be considered that these fields belonged to a seed potato producer and that planting of infested tubers poses a high risk of spreading the disease. Even though the number of fields connected to this letter is not very high, the severe level of infestation in the fields first found under these letters imply that the infestation likely occurred many years prior to its detection (potentially more than the 10 years of the current trace-back). Furthermore, most of the fields regulated due to the movement and planting of seed potatoes are related to the investigation on fields under letter H. Although no subsequent findings were reported other fields could have been infected. The IAP believe the risk of further dissemination of Potato Wart Disease from fields should be regarded as low to medium.

Infestations with S. endobioticum in the cluster comprising the letters A, C, D, K, M and N were detected every few years, including the years 2018, 2021 and 2022, with infested fields being mostly connected to earlier findings as Primary Contact or Other contact fields. However, the infestation does not necessarily have to be a result of dissemination via equipment. Information provided by Cavendish Farms states that digestate from the anaerobic digester is used on farm-owned fields. In earlier years other residues were distributed on fields used for potato production. Waste derived from processing of infested potatoes, including digestate, can contain viable resting spores of S. endobioticum as none of the processes can guarantee the full inactivation of the resting spores. Therefore, use of such waste on farmland should be regarded as one of the highest risks regarding dissemination of the resting spores. Waste management with respect to biosecurity has significantly improved: in 2009 waste was no longer fed to livestock, in 2013 tare soil and rock was disposed of on a CFIA controlled site, and management was further improved in 2022 when all waste streams were tracked, documented and reported, resulting in a lower risk of dissemination of resting spores in this area. Nevertheless, this previous risk to which the fields were exposed cannot be ignored and the disseminations from the past may not have been detected. Infestations with very low numbers of resting spores may take many crop cycles with susceptible potato varieties to build up inoculum in the field to a level that might be detected in soil tests or via tuber inspections. Partly as a consequence of the mandatory growing of susceptible potato varieties for a number of crop cycles on Adjacent, Primary Contact and Other Contact fields, often susceptible varieties were also grown in this area. The IAP considers it likely that other infested fields exist and will be detected in the following years. Until detection of these infestations and implementation of further management measures, the risk of dissemination of Potato Wart Disease from fields in this area is considered by the IAP to be medium to high.

The IAP is of the opinion that, given the currently very limited distribution of S. endobioticum, the predominant part of PEI outside of the Regulated Fields, could be considered as PFA for S. endobioticum due to the low risk of further spread of potato wart in this area. The central production area is considered to be of higher risk and a part of that area, including fields under letter H and fields in the area around cluster A, C, D, K, M and N should be excluded from the PFA status as the IAP considers the risk of further spread of Potato Wart Disease in this area to be medium to high.

Phytosanitary measures to maintain the PFA status should include the precautionary measures proposed by the IAP in this report for the management of Regulated Fields. In addition, two biosecurity control areas should be set up around H and the cluster A, C, D, K, M, and N where additional restrictions and control measures should apply:

All machinery should be cleaned to a high standard before leaving Regulated Fields or the control areas. The CFIA should verify the freedom from soil for equipment after the cleaning process.
No seed potatoes produced in the control areas should be planted in the PFA
Movement of unprocessed potatoes from the control areas into the PFA should be assessed for risk and if needed, mitigation measures should be introduced.
Additional measures should apply for the area containing fields combined under the letters A, C, D, K, M and N,
1. No seed potatoes should be produced for planting outside this area.
2. The sole use of resistant varieties for both seed and ware production in this area should be implemented as soon as possible.
3. Table potatoes could be produced for export, if washed and treated with sprout inhibitor.
4. No potato waste from processing should be used on any farmland but go to a specific waste disposal site.
5. The digestate from the bio-digester could be used on land that is never to be used for potato production (e.g. permanent grassland).
6. Additional precautionary measures (restrictions on machinery movement, tare soil testing, grid sampling and bioassays) should be introduced in these areas.

The IAP is aware that the proposed biosecurity measures will affect a number of growers who may not be connected with any outbreaks of potato wart. Therefore, there should be consideration given to implementation of PFPP or PFPS in the biosecurity control areas on PEI. As an additional precautionary measure, PFPPs or PFPSs could also be established for seed potato production for export in areas close to Regulated Fields on other parts of PEI if the growers are connected to Regulated Fields.

The exact boundary of the PFA should be set by the CFIA in conjunction with the industry on PEI. This should take into account risk factors such as previous land use, land ownership, and historical disposal of potato waste. For example, if a grower within the restricted area also grows potatoes in the pest free area there must be a restriction on movement of potato material, or equipment. Ownership of land, renting and trading should be registered to facilitate trace-back in case of outbreaks of S. endobioticum. This refers mainly to operators with fields in and outside of the PFA.

Regular surveillance activities should be implemented to check that the PFA status is maintained as defined in ISPM 4 and to assure customers of seed and ware from this area. The IAP believe that tare soil testing could be a very useful measure in this respect as it would allow for regular sampling of larger areas, depending on the sampling spot, without increasing the effort for the laboratory too much. Statistically based surveys, as described by EFSA (2020) can help to decide on appropriate survey designs.

In general, the IAP proposes that a strict protocol for definition and maintenance of the PFA on PEI is set up by CFIA in discussion with stakeholders on PEI as well as with relevant exporting countries similar to Annex I of RSPM 3 (NAPPO 2011) on 'Establishment, maintenance and recognition of production areas free of potato cyst nematodes (Globodera rostochiensis and G. pallida) in NAPPO member countries.

References

Anonymous, 2022. Commission Implementing Regulation (EU) 2022/1195 of 11 July 2022 establishing measures to eradicate and prevent the spread of Synchytrium endobioticum (Schilbersky) Percival. Official Journal of the European Union, L 185/65

Baayen RP, Bonthius H, Withagen JCM, Wander JGN, Lamers JL, Meffert JP, Cochius G, van Leeuwen GCM, Henricks H, Heerink BGJ, van den Boogert PHJF, van de Griend P and Bosch RA, 2005. Resistance of potato cultivars to Synchytrium endobioticum in field and laboratory tests, risk of secondary infection, and implications for phytosanitary regulations. Bulletin OEPP/EPPO Bulletin, 35, 9–23.

Bartkiewicz, A, Chilla, F, Terefe-Ayana, D, Lubeck, J, Strahwald, J, Tacke, E, Hoffebert, HR, Flath, K, Linde, M, Debener, T, 2018. Improved genetic resolution for linkage mapping of resistance to potato wart in monoparental dihaploids with potential diagnostic value in tetraploid potato varieties. Theoretical and Applied Genetics, 131, 2555–2566.

EFSA PLH Panel (EFSA Panel on Plant Health), Jeger M, Bragard C, Caffier D, Candresse T, Chatzivassiliou E, Dehnen-Schmutz K, Gilioli G, Grégoire J-C, Jaques Miret JA, Macleod A, Navajas Navarro M, Niere B, Parnell S, Potting R, Rafoss T, Urek G, van Bruggen A, van der Werf W, West J, Winter S, Vloutoglou I, Bottex B and Rossi V, 2018. Scientific Opinion on the pest categorisation of Synchytrium endobioticum. EFSA Journal 2018, 16(7):5352, 37 pp. https://doi.org/10.2903/j.efsa.2018.5352

EFSA (European Food Safety Authority), Lazaro, E, Parnell, S, Vicent Civera, A, Schans, J, Schenk, M, Cortinas Abrahantes, J, Zancanaro, G and VOS, S, 2020. General guidelines for statistically sound and risk-based surveys of plant pests. EFSA supporting publication 2020: 17( 9): EN-1919. 65 pp. doi: 10.2903/sp.efsa.2020.EN-1919.

EPPO, 2011. PM 5/3 (5) Guidelines on Pest Risk Analysis. Published by the European and Mediterranean Plant Protection Organization, available online (PDF - 360 kb)

EPPO, 2017a. PM 3/59 (3) Synchytrium endobioticum: descheduling of previously infested plots. Bulletin OEPP/EPPO Bulletin, 41, 385-388.

EPPO, 2017b. PM 7/28 (2) Synchytrium endobioticum. Bulletin OEPP/EPPO Bulletin, 47, 420-440.

EPPO, 2017c. PM 9/5(2) National regulatory control systems Synchytrium endobioticum. Bulletin OEPP/EPPO Bulletin, 47 (3), 511–512.

EPPO, 2020. PM 3/88(1) Testing of potato varieties to assess resistance to Synchytrium endobioticum. Bulletin OEPP/EPPO Bulletin, 50 (3), 364 – 371.

EPPO, 2021. PM 5/10 (1) Guidelines on the design and implementation of a buffer zone. EPPO Bull, 51: 438-450. https://doi.org/10.1111/epp.12777.

EPPO, 2022. Synchytrium endobioticum. EPPO datasheets on pests recommended for regulation. Available online.

Hampson MC, 1996. A Qualitative Assessment of Wind Dispersal of Resting Spores of Synchytrium endobioticum, the Causal Agent of Wart Disease of Potato. Plant Disease, 80, 779-782.

Hampson, MC and Coombes JW, 1996. Spatial distribution of Synchytrium endobioticum, the cause of potato wart, in field soil. Plant Disease, 80, 1006–1010.

IPPC Secretariat. 2018. Surveillance. International Standard for Phytosanitary Measures No. 6 Rome. FAO on behalf of the Secretariat of the International Plant Protection Convention.

IPPC Secretariat. 2021. Determination of pest status in an area. International Standard for Phytosanitary Measures No. 8. Rome. FAO on behalf of the Secretariat of the International Plant Protection Convention.

IPPC Secretariat, 2022. Glossary of phytosanitary terms. International Standard for Phytosanitary Measures No. 5. Rome. FAO on behalf of the Secretariat of the International Plant Protection Convention.

Laidlaw WMR, 1985. A method for the detection of resting sporangia of the Potato Wart Disease (Synchytrium endobioticum) in the soil of old outbreak sites. Potato Research, 28, 223–232.

Langerfeld, E, 1984. Synchytrium endobioticum (Schilb.) Perc. Zusammenfassende Darstellung des Erregers des Kartoffelkrebses anhand von Literaturberichten [Summarized presentation of the pathogen of Potato Wart Disease on the basis of literature reports]. Mitteilungen aus der Biologischen Bundesanstalt für Land- und Forstwirtschaft Berlin-Dahlem, 219, 142pp

NAPPO, 2011. North American Plant Protection Organisation (NAPPO) Regional Standard for Phytosanitary Measures (RSPM) 3 Movement of Potatoes into a NAPPO Member Country.

Obidiegwu, JE., Santetomo, R, Flath, K, Tacke, E, Hofferbert, HR., Hofman, A Walkemeier, B, Gebhardt, C, 2015. Genomic architecture of potato resistance to Synchytrium endobioticum disentangled using SSR markers and the 8.3k SolCAP SNP genotyping array. BMC Genetics, 16, 38.

Prodhomme, C, Esselink, D, Borm, T, Visser, RGF., van Eck, HJ & Vossen, JH, 2019. Comparative subsequence sets analysis (CoSSA) is a robust approach to identify haplotype specific SNPs; mapping and pedigree analysis of a Potato Wart Disease resistance gene Sen3. Plant Methods, 15, 60.

Prodhomme, C, van Arkel, G, Plich, J, Tammes, JE, Rijk, J, van Eck, HJ, Visser, RGF, Vossen, JK, 2020a. A hitchhiker's guide to the Potato Wart Disease resistance galaxy. Theoretical and Applied Genetics, 133, 3419–3439.

Prodhomme, C, Vos, PG, Paulo, MJ, Tammes, JE, Visser, RGF, Vossen, JH, van Eck, HJ, 2020b. Distribution of P1(D1) wart disease resistance in potato germplasm and GWAS identification of haplotype-specific SNP markers. Theoretical and Applied Genetics, 133, 1859–1871.

Przetakiewicz J, 2015. The viability of winter sporangia of Synchytrium endobioticum (Schilb.) Perc. from Poland. American Journal of Potato Research, 92, 704–708.

Schleusner, Y, Muller, P, Heiermann, M, Buttner, C, 2019. Synchytrium endobioticum – risk from biogas plants? Bulletin OEPP/EPPO Bulletin 49 (1), 92 – 103.

van de Vossenberg, BTLH, Prodhomme, C, Vossen, JH, van der Lee, TAJ, 2022. Synchytrium endobioticum, the Potato Wart Disease pathogen. Molecular Plant Pathology, 23, 461 – 474.

van Leeuwen, GCM, Wander, JGN, Lamers, J, Meffert, JP, van den Boogert, PHJF, Baayen, RP, 2005. Direct examination of soil for sporangia of Synchytrium endobioticum using chloroform, calcium chloride and zinc sulphate as extraction reagents. Bulletin OEPP/EPPO Bulletin, 35(1), 25-31.

Zakopal, J. (1970): Neue Möglichkeiten zur Inaktivierung der Dauersporangien des Kartoffelkrebserregers Synchytrium endobioticum (Schilb.) Perc. Zentralblatt für Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene, Abt. II 125, S. 505 – 514.

Annex I

Table 4: Overview on recommendations from the IAP on management of Potato Wart Disease on PEI

Measures on fields

Index fields

No potatoes or other plants for replanting for a minimum of 20 years.
No other root crops due to the increased risk of soil staying attached to the crops.
Cultivation of other crops, e.g. cereals, maize, soya should be possible, if soil is removed from equipment and a high hygiene status is maintained.
Resampling after 20 years at an intensive rate.
If no resting spores are found and all bioassays give negative results, resistant ware potatoes might be grown
Tare soil should be examined after each potato crop
Restrictions on soil movement and requirement that equipment be soil-free remain in place
Resampling after further 10 years. If no resting spores are found and all bioassays give negative results, all restrictions could be lifted.
First crop of a susceptible variety should be inspected, preferably during harvest and tare soil should be tested
A safety zone of uninfected land should be put around infested land for the time the Index field is under regulation and surrounded by a fence or other barrier to restrict movement of equipment or soil and to prevent animals from passing through
A buffer zone should be set up around the infested part of the field and the safety zone for the time the Index field is under regulation to ensure protection of the surrounding area
Soil sampling comparable to the first 15 m of Adjacent fields without potatoes under current regulations should be used for setting up the buffer zone
Only resistant potatoes for processing with mitigation measures in place should be grown in the buffer zones. Tare soil should be tested after each resistant potato crop
Large Index fields should be surveyed with an intensive grid to delimit the actual dimensions of the infestation
In case of localised infestations, the Index field could be split, the uninfected area (outside the safety zone) should be included in the buffer zone This should be reflected in the classification of Adjacent, Primary Contact or Other Contact fields
Risk assessment should be used to determine locations where Potato Wart Disease could be present.

Adjacent fields

Will be part of the buffer zone with the corresponding regulations.

Primary contact fields

Should be differentiated between Primary Contact fields due to planting of seed potatoes from infested fields or shared equipment

Seed

Only resistant potatoes for processing with mitigation measures allowed for example for a minimum of 4 crops together with tare soil testing

Equipment

Only resistant potatoes for processing with mitigation measures for example for a minimum of 2 crops and tare soil testing
Only resistant varieties for processing end use for at least 1 crop cycle and tare soil testing

Other Contact fields

Only resistant varieties for processing end use for at least 1 crop cycle and tare soil testing

Surveillance methods

Implementation of tare soil testing as surveillance method
Collection should take place as close to the harvest operation as possible, e.g. when downloading bulk trucks or below a dirt separator on the piler
Intensity of sampling should depend on the category of field, the end use of the crop or the destination of the crop
Officials should inspect cull bins regularly to detect more inconspicuous symptoms of Potato Wart Disease
Microscopic examination for resting spores in the soil should be combined with molecular testing
The viability status of resting spores should be determined whenever possible, with documentation of microscopic images
Only viable resting spores should be considered as infestations and imply further actions

Detection sensitivity

Making more of images available that represent different disease expressions
Real size 3-D models similar in size, weight and shape to real potatoes complete with a broad range of disease expressions could be produced and used to test which symptoms would be detected in the different visual inspections
As more false positive notifications are to be expected as consequence to awareness raising, CFIA should be prepared for increased effort to verify notifications

Resistant varieties

Screening of all potato varieties and breeding material for SEN genes using molecular markers
Additionally, use of resistant varieties and breeding material for a quick scan of susceptibility by Glynne-Lemmerzahl or Spieckermann method
Subsequently a bioassay with representative sets of isolates from PEI should be performed
Isolates should be stored in living collections for future reference for resistance testing

Waste management

Highest sanitation standards for facilities under compliance agreement, including outside areas
No use of digestate from anaerobic digestions or other waste from processing on agricultural fields
Fields on which waste from potato processing has been used in former years should be under specific observation when potatoes are grown, e.g. by comprehensive post-harvest inspections

Communications and partnership

Set up a communication system by a task force incorporating all the different parties to discuss issues to provide a two-way communication for the agencies to explain what is non-negotiable due to legislation or current circumstances, and the industry to provide feedback and explore alternatives

Future research

Back-tracing of possible ways of infestations in the field from various sources
Research on tare soil testing - symptoms expression, verification of sensitivity
Possibilities to inactivate resting spores in the soil
Monitoring of water and wind dispersal of resting spores
Testing of new methods for surveillance in the field or in the soil

Annex II

Examples for comprehensive communication schemes in New Zealand and Australia

In New Zealand, primary sectors and the Ministry for Primary Industries (MPI, the NPPO in New Zealand) have the opportunity to form partnerships under the Government Industry Agreement for Biosecurity Readiness and Response (GIA) for improving New Zealand's biosecurity.

A primary sector group can sign up to a GIA if they consider there are significant biosecurity concerns which can be better addressed through a formal partnership with MPI to reduce and manage the impact on production and trade for their members. This a formal means to generate better biosecurity through:

Improved engagement across the biosecurity system where MPI, industry signatories and other stakeholders know what they can expect from the wider biosecurity system, and how they can become actively involved in the management of associated risk.
Operational agreements for better biosecurity readiness – joint decisions on readiness priorities and agreed cost share.
Response in partnership – joint decision making with MPI in response and agreed cost share.

The costs shared by each sector are calculated based on the relative proportion of benefit they receive from the readiness and response activities. Funding was provided to help industry to transition into the cost-sharing environment.

The potato industry in New Zealand has been through several joint responses with MPI addressing a number of biosecurity incursions and both sides have benefited from the experience. Potatoes New Zealand are the main industry body and have a much greater understanding of and connection with the industry than MPI and were able to geo-locate fields and communicate with growers and landowners more easily. Conversely, MPI had a greater understanding of the trade implications and communication with trading partners. By this 50:50 partnership the industry felt better involved in the decisions. Sector groups who do not sign up may be kept informed on biosecurity activities, have less influence in setting biosecurity agenda, and still need to pay for readiness and response activities from which they have benefited.

More details about GIA in New Zealand can be found at these websites:

A similar partnership was available in Australia and details can be found at these websites:

Date modified:: 2023-01-16

Recommendations of the International Advisory Panel on Potato Wart Disease management on Prince Edward Island 2022

On this page

List of abbreviations

List of figures

List of tables

Foreword

Executive Summary

Introduction

Management of Potato Wart Disease on PEI

Category A: Index fields

Category B: Adjacent fields to the Index field

Category C: Primary Contact fields

Category D: Other Contact fields

Category E: New fields or portion of fields entering potato production

Conclusions and recommendations on management of Potato Wart Disease on PEI

Management measures on Regulated Fields

Recommendations

Resistance testing

Recommendations

Surveillance via soil testing and field and tuber inspections

Recommendations

Waste management

Recommendations

Communication and future research needs and possibilities

Recommendations

Conclusions on a possible Pest Free Area status for S. endobioticum of PEI

References

Annex I

Measures on fields

Index fields

Adjacent fields

Primary contact fields

Seed

Equipment

Other Contact fields

Surveillance methods

Detection sensitivity

Resistant varieties

Waste management

Communications and partnership

Future research

Annex II

Examples for comprehensive communication schemes in New Zealand and Australia