RMD 23-03: Revisions to the categorization of potato wart (Synchytrium endobioticum) primary contact and other contact fields

Preface

As described by the International Plant Protection Convention (IPPC), Pest Risk Analysis (PRA) includes 3 stages: initiation, pest risk assessment, and pest risk management. Initiating the PRA process involves identifying pests and pathways of concern and defining the PRA area. Pest risk assessment provides the scientific basis for the overall management of risk. Pest risk management is the process of identifying and evaluating potential mitigation measures which may be applied to reduce the identified pest risk to acceptable levels and selecting appropriate measures.

This Risk Management Document (RMD) records the pest risk management process for the identified issue. It is consistent with the principles, terminology, and guidelines provided in the IPPC standards for PRA.

On this page

• Executive summary
• Purpose
• Scope
• Definitions
• Background
• Risk management considerations
• Risk management options
  • Category C fields
  • Category D fields
• Risk management proposals
  • Category C
  • Category D
• References

Executive summary

Synchytrium endobioticum is a fungal pathogen and causal agent of potato wart (PW) or potato canker disease. Potato (Solanum tuberosum) is the only known cultivated host. This quarantine pest originated in South America and has been reported in Newfoundland and Labrador (NL) and Prince Edward Island (PEI) in Canada and most of Europe, as well as many other countries around the world.

Since 2000, PW in PEI has been managed through regulatory programs. In 2005, the Potato Wart Domestic Long Term Management Plan (PWDLTMP) was implemented (updated in 2009) to provide the minimum sampling, testing and surveillance activities required for regulated parcels of land and the criteria for regulating new detections. The PWDLTMP follows a risk based field system and categorizes fields into Categories A (index), B (adjacent), C (primary contact) and D (other contact) and imposes varying levels of restrictions based on risk. The PWDLTMP was put in place to help contain, control and prevent the spread of PW within and out of PEI. The field categories, as defined in the PWDLTMP, form the restricted area as referenced in the Potato Wart Order (PWO), which in part restricts the movement of regulated things from restricted areas in PEI.

There have been additional PW detections since that time and as of October 2023 there were 37 positive fields and over 17,300 ha (42,600 acres) of land under various levels of restriction due to association with those PW positive fields. In 2020, following detections of PW on a PEI seed potato farm, the CFIA initiated a program review to update its risk mitigation strategies including pest risk assessment and pathway risk assessment which will contribute to the development of a new National PW response plan. The risk posed by Category C and Category D fields has been the subject of discussion.

In 2022, an International Advisory Panel (IAP) recommended splitting Category C fields into s different field categories based on the nature of contact of the field with the Index field. A PWDLTMP Working Group (WG), which had been established by the CFIA, recommended the same measure for Category D fields without specifying the criteria for the splitting the s risk categories.

This RMD examines the pros and cons of the recommended measure and other alternative options. The following options were assessed:

1. Maintaining status quo for both Category C and D fields
2. Subdividing Category C fields based on the nature of exposure to potato wart
3. Subdividing Category D fields
4. Elevating fields considered as higher risk Category D to Category C

The CFIA's current proposed approach to address the identified risks is to maintain the current classification for Category C fields while elevating fields considered as higher risk Category D to Category C. All input received during the comment period will be considered in the development of the final decision.

Purpose

The purpose of this RMD is to assess and review options for possible changes to the criteria for Category C (primary contact) and D (other contact) field classifications in the PWDLTMP and the risk factors that can lead to potential spread of PW. The final outcome of this RMD is to support the development of a new national-level potato wart response plan to replace the existing PWDLTMP.

Scope

This RMD pertains to the CFIA's management program for PW in Canada. It outlines the background and scientific rationale for Category C and D field categories as described in the PWDLTMP to help contain, control and prevent the spread of PW in PEI. It also provides a risk-based assessment of these 2 field categories to consider potential changes to further address the risk presented by PW.

Definitions

Definitions of terms used in this document can be found in the Plant Health Glossary of Terms, Potato Wart Domestic Long-Term Management Plan, and Potato Wart Order.

Additionally, for the purpose of this risk management document, the following terms are defined as follows:

Field

A distinct plot of land distinguished from neighbouring plots of land by physical boundaries. Physical boundaries include a tree line, fence line, permanent waterway, ditch, road or permanent lane, berm, permanent irrigation track, transition to a non-agricultural area (fallow, lawn, wetland), and a real property boundary.

Susceptible varieties

Potato varieties which have been reviewed by the NPPO and are considered to be susceptible to infection by specified pathotypes of S. endobioticum, using criteria defined by the NPPO. Note: There are different levels of susceptibility.

Background

Synchytrium endobioticum is a soil-borne fungal pathogen that infects susceptible potato varieties and causes potato wart disease. It is a regulated pest for Canada and all its major trading partners. In Canada, PW is known to occur in NL and as of October 2023, has been confirmed in 37 fields in PEI. S. endobioticum resting spores are persistent in soil and able to survive extreme environmental conditions in a dormant state for more than 40 years. S. endobioticum requires cool, moist conditions during the growing season, in particular in the spring, and generally does not survive in areas with low annual precipitation. Climate suitability modeling indicates that most of Canada's potato-producing regions are a good climatic match for this pathogen and as a result the probability of establishment of new introductions in other regions of Canada, is rated high. There are no effective chemical treatments available to control PW and there are currently only a limited number of known PW resistant varieties (Goldrush and Prospect) of commercial importance in the North American context.

In November 2021, the CFIA completed a PRA which included information on the probability of establishment and spread of PW, including a list of potential risk factors (Table 1) that have likely played a role in PW spread within PEI. The PRA concluded that the probability of spread of PW by natural means is low, while the highest risk pathways for spreading PW within Canada are all human-mediated, for example, through the movement of field-grown seed potatoes and soil associated with these tubers, the movement of PW resting spores between fields in tare soil or with the soil adhering to agricultural equipment, and through the application of organic waste materials from potato packing and processing operations or manure (originating from livestock fed infected potato tubers) onto agricultural land. The PRA also provided a basis to examine the effectiveness of the PWDLTMP and response to PW in PEI. The CFIA implemented the Potato Wart Order on November 21, 2021 in PEI, to help contain, control and prevent the spread of the pest in PEI and to other areas of Canada.

In Summer 2022, the CFIA established an IAP of independent experts with experience and expertise in the biology or regulation of S. endobioticum, to assess and review the situation in PEI and to provide a report of expert opinions and recommendations. The report was completed in December 2022 and provided recommendations to support a different approach to the management of PW disease in PEI, along with considerations for establishment of a pest free area(s).

To support the review of the PWDLTMP, the CFIA established a technical working group of provincial and potato industry representatives, including the Governments of PEI and Ontario, Canadian Potato Council and PEI Potato Board. The PWDLTMP WG provided a final report of recommendations in April 2023.

The PWDLTMP was originally designed to respond to the situation in PEI. It contains processes to assess each field category and to support the eventual release of fields from specific requirements including those related to the production of seed potatoes. The new national-level potato wart response plan is expected to maintain the field classification system of the PWDLTMP, including Categories C and D.

Risk management considerations

The Potato Wart Domestic Long-Term Management Plan (PWDLTMP) – March 5, 2009

The current PWDLTMP is a risk-based regulatory management strategy that identifies 4 basic field categories. Fields identified as Category A (Index), Category B (Adjacent), Category C (Primary Contact) and Category D (Other Contact) present varying degrees of plant health risk based on several risk factors (Figure 1). The degree or extent of restrictions placed on an individual field is related to the level of risk based on its relationship and proximity to the associated Category A field.

Description for photo – Figure 1: Restricted field categorizations

An index field is represented in a box in red. The index field includes a picture of a plant in the top left corner. An arrow starts at this plant in the top left corner and follows a path to a tractor next to an orange box below the index field. The orange box represents primary contact fields where seed potatoes, tare soil, potato waste or equipment moved directly from the index field during the last 10 years.

To the right of the orange box, is a tractor and an arrow that points to a yellow box. The yellow box represents other contact fields where equipment moved after first moving to the primary contact field in orange. On the left side of the index field, there a white box that represents a field that is not regulated as it is more than 15 metres from the index field.

Above the index field are two boxes in blue that represent two adjacent fields that are less than 15 metres from the index field in red.

Source: Report – Recommendations of the International Advisory Panel, December 2022.

When PW is detected in a field (resting spores or tubers with warts), an investigation is immediately initiated, including trace-back and trace-forward activities to determine the source of infection and potential movement of PW from the Category A field. During this process, fields are assessed for risk and categorized accordingly. Each field is soil sampled using a grid pattern and samples are submitted to the Charlottetown laboratory for analysis for S. endobioticum resting spores.

Over time, each field must undergo the assessment steps for its field categorization as identified in the PWDLTMP to determine if PW is present in the field. As a field moves through the steps in the plan, restrictions are eased and when all steps are completed the field will be free from restrictions under the PWDLTMP.

Category A, B, and C fields are considered higher risk fields in the PWDLTMP and are currently placed under regulatory notice subjecting them to specific quarantine and movement restrictions. The PWDLTMP does not require specific restrictions on Category D fields as they are considered low risk due to their secondary relationship to the Category A fields.

The United States Federal Order (DA-2022-14 (PDF)) recognizes the current risk-based field categories A, B, C, and D.

This RMD reviews Category C and D fields, including their risk-based definitions and considers the recommendations from both the IAP (December 2022) and PWDLTMP WG (April 2023). The specific details of Category C and D fields are described in several section of the PWDLTMP, in general:

• Category C fields have a history of direct contact with a Category A field.
  • The risk associated with these fields is based on the possible transfer of soil from the Category A field via movement of equipment or planting of seed potatoes produced in a Category A field.
  • Category C fields are subject to one soil test during the investigation of the associated Category A field in addition to visual surveillance of 4 crops following production of PW susceptible cultivars. Biosecurity controls including freedom from soil and processing end use for potatoes produced  are required but may be lifted using the field assessment process identified in the PWDLTMP.
  • Category C fields have been involved in 14 investigations where they have been re-categorised as Category A fields following the detection of PW in the field. 3 of these detections were in subsequent years after the initial investigations.
• Category D fields are not restricted under the PWDLTMP, due to their distant association with a Category A field and would only be subject to specific restrictions if PW was detected.
  • The risk associated with these fields is based on the movement of common equipment with soil from a Category C field, thus indirectly from the associated Category A field.
  • Category D fields are subject to one soil test during the investigation of the associated Category A field and visual surveillance of 5 crops of PW susceptible cultivars. Biosecurity controls and freedom from soil requirements are not required but good biosecurity practices are always recommended.
  • Category D fields have been involved in 16 investigations where they have been re-categorised as Category A fields following the detection of PW in the field. 9 of these detections were in subsequent years after the initial investigations.

Risk factors and pathways for potato wart spread

The long-lived resting spores of S. endobioticum are the characteristic survival and dispersal form of the fungus. The spread of the fungus occurs primarily through the movement of infested soil and by infected seed tubers (Franc 2007). Other potential risk factors for PW spread are identified in Table 1.

The CFIA's Plant Health Risk Assessment^Footnote 1 identified a number of risk factors that may play a role in the spread of PW from an index field to other areas.

The likelihood that PW becomes established in a field (in other words, infects a potato tuber and multiplies), after it is spread, is dependent on a series of different factors, such as the number of spores that were brought into the field, the age of the spores when they first entered the field, the number of locations where spores were deposited within the field, etc. The exact factors that contribute to a pest becoming established in a field after it was introduced are unique to each individual field.

Subcategorization of Category C and D fields requires ranking of risk factors in terms of their contribution to pest entry and establishment. However, none of the risk factors can be quantified.

CFIA experience in managing potato wart in PEI

Since the initial detection of PW in 2000, a number of restricted fields were released from restrictions under the PWDLTMP. However, some of these fields, mainly situated in the Kensington area, have returned to their former restricted status due to associations with new PW investigations. The majority of PW detections have occurred in commercial fields located in Queens and Prince counties. The detections in 2012, 2014 and 2020 were associated with fields that produced seed potatoes including the detection in Kings County.

Historically, PW investigations have been triggered by the discovery of PW symptomatic tubers during harvest, which is followed by intensive soil sampling and testing on the associated fields which may then result in the detection of additional PW positive fields through soil analysis. The regulatory response required to delimit and contain the investigations initiated between 2020 to 2023 (8 new detections from Fall 2020 to Spring 2023) significantly increased the overall size of the restricted fields across PEI and further supported the in-depth program review.

Table 2 identifies the total number of restricted fields, as of October 2023, associated with detections of PW in PEI, totaling 1,362 restricted fields or 17,306 hectares (42,764 acres).

Table 2: Number and total size of the potato wart restricted area by field category.

Source: CFIA investigations 2000 to 2023.

The phytosanitary risk associated with potato wart is described in CFIA's Plant Health Risk Assessment (Request 2021-051)

The movement of potatoes, soil and other regulated things with associated soil, such as farm machinery, presents a risk for the spread of PW and this risk means that limiting the movement of these items from restricted areas to other areas is a high priority. Seed potato tubers pose the highest risk of spread of the pathogen, since the resting spores (either within the tuber or in associated soil) have immediate access to host tissue for invasion.

Recommendations of the International Advisory Panel on Potato Wart disease management on Prince Edwards Island (2022)

The IAP report provided a number of recommendations, including:

• Creation of 2 Category C field types based on exposure risk factors (primary contact field – equipment and primary contact field – seed/ tare soil)

Potato Wart Domestic Long-term Management Plan Working Group (PWDLTMP WG) Final Report (April 2023)

The WG provided a number of recommendations including:

• Conducting a comprehensive risk analysis on Category D fields with a view to sub-divide these fields into "higher risk" and "lower risk" subcategories.
• Subdividing Category C fields, in consideration of the IAP recommendation.

Risk management options

This RMD focuses on the pros and cons of the suggested subcategorization of Categories C and D fields from the perspective of mitigating the risk of PW spread. The impact of any potential options on the resource that would be required from the operational aspect have also been taken into account where applicable.

Category C fields

Analysis of current program

Under the current PWDLTMP, all Category C fields are linked to an associated Category A field, where PW has been confirmed. This relationship is determined during the investigation.

Category C fields present a significant associated phytosanitary risk as there is evidence that soil (for example, tare soil) was transferred from the Category A field, common equipment sharing occurred between the Category A and C field, or propagative material produced in the Category A field was subsequently planted in the Category C field.

Category C fields remain the highest risk field category and have been confirmed to be infested with PW 14 different times; of which 7 Category C fields were potentially exposed to PW due to transfer of seed and 6 Category C fields were potentially exposed to PW due to sharing of equipment and one field potentially exposed as a result of both seed transfer and equipment sharing. As of October 2023, there were 333 Category C fields, 2 of which are in Ontario and the remainder in PEI, representing 3,613 hectares (8,928 acres).

Category C fields are under restrictions that include tare soil and seed potato planting restrictions until the field has been fully released from restrictions under the PWLTDMP.

Alternative option: Subdividing Category C fields based on the nature of exposure to potato wart

Based on the recommendations in the IAP report, 2 different requirements could be established for Category C fields.  One set of requirements for fields exposed to PW through the planting of seed potatoes and /or spreading of soil (tare) and a second set for fields that only shared equipment directly after being used in a Category A field.

Based on this designation, fields exposed by only sharing equipment would be subjected to fewer restrictions while the current restrictions will continue to be applied to the Category C fields related to seed use and/or soil.

Pros:

• Allows a faster path for relaxing restrictions for Category C fields related to only shared equipment. Fields released from restrictions will be free to produce crops with relaxed restrictions and reduce costs of complying with PW related restrictions.
• Opportunity to shift resources towards higher risk fields and other priority activities.

Cons:

• In PEI, Category C fields have tested positive for PW, following exposure to PW due to use of seed potatoes, movement of tare soil and/ or through sharing of equipment.  Each pathway has proven to be a concern for the spread of PW.
• The risk of PW spread via equipment movement from a Category A field onto a Category C field could be underestimated resulting in erroneous classification in the low risk Category C sub-category.
• Any changes to the classification system will require consultation with trading partners and may result in more scrutiny of the existing field-based classification system employed in PEI.

Category D fields

Analysis of current program

Under the PWDLTMP, Category D fields are not restricted due to their distant and indirect linkage to their associated Category A field. They are only subject to additional restrictions if PW is detected in investigation soil samples or if a symptomatic tuber is found during surveillance activities and determined through laboratory analysis to be PW. The field category has been considered low risk and exempt from additional restrictions, such as freedom from soil.

As of October 2023, there were 941 Category D fields in PEI, representing 12,174 hectares (30,083 acres). The current requirements of investigation soil sampling and post harvest surveillance activities on 4 crops of susceptible varieties on all Category D fields requires significant resources to administer due to the large number of implicated fields.

However, 10 current Category A fields in PEI that were formerly classified as Category D fields can be linked back directly or indirectly to the original 2000 investigation. This highlights concerns that there may be an elevated risk associated with at least some Category D fields, due to factors that are not considered in the PWDLTMP field categorization scheme. This concern was also described in the 2022 IAP report, and is central to the review of this field category.

Previous land use, land ownership, tare soil management, historical disposal of potato waste on agricultural fields, shared farm equipment and relationships to multiple investigations may have resulted in the inadvertent spread of PW. The potential risk for pest spread from category D fields, in the event that they are harbouring undetected PW infestations, may not be adequately addressed in the current PWDLTMP – specifically the lack of restrictions on soil movement, freedom from soil requirements and seed potato production.

Risk management proposals

The objective of this RMD is to propose options for a more rigorous process of field risk assessment to help further reduce the potential risk of spread of PW. Given the level of risk each field category represents, the merits of further subcategorization should be evaluated separately. Additional considerations are presented below.

Similarly, maintaining the status quo without strengthening restrictions ignores the risk posed by some Category D fields. The PWDLTMP requires only monitoring and post harvest surveillance and tuber inspection for Category D fields following a not detected soil analysis result in the investigation. The production of one or 2 crops is considered insufficient for the purposes of detecting any population of resting spores that may be present within the field prior to lifting all restrictions on the field. A single post-harvest field inspection of tubers on a susceptible crop is unlikely to detect the pathogen, if a low level infestation is present, as the pest may not have reached detectable levels.

Following receipt of comments and feedback from stakeholders, the specific details of the final decision will be defined in the future Potato Wart National Response Plan, replacing the current PWDLTMP.