RMD 23-02: Seed potato production in fields associated with potato wart, Synchytrium endobioticum

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
  • Field category A
  • Field category B
  • Field category C
  • Field category D
• Risk management proposals

Executive summary

In 2000, Synchytrium endobioticum, the causal agent of potato wart (PW) was discovered in the province of Prince Edward Island (PEI). There have been additional detections since that time and as of October 2023 there are 37 positive fields and over 17,300 ha (42,600 acres) of land are under various levels of restriction due to association with those positive fields. Seed potato production is one of the highest-risk activities for the spread of PW as both the tubers and the associated soil could contain resting spores and the tubers are intended for planting, thus supporting the spread and establishment of the pest in new areas.

As part of an initiative by the Canadian Food Inspection Agency (CFIA) to review the mitigation of risks associated with potato wart in Canada and PEI, a new national-level potato wart response plan is under development. This will include biosecurity and traceability requirements for users of associated fields (Category A, B, C, and D under the Potato Wart Domestic Long Term Management Plant [PWDLTMP]). Specific restrictions relating to potato production in PW associated fields are also being re-evaluated, including conditions to be met before seed potato production is permitted.

This RMD examines phytosanitary options and risk factors for the production of potatoes in PW associated fields and movement of seed potatoes from PW associated fields. Interprovincial movement requirements are not addressed in this RMD.

3 options based on the field categories (A, B, C and D) as described in the Potato Wart Domestic Long-term Management Plan were reviewed:

• maintaining the status quo
• prohibiting seed potato production in all PW associated field categories until they are no longer subject to phytosanitary restrictions
• limiting seed potato production in Category D fields to use in PW associated fields only.

The CFIA's current proposed approach to address the identified risks is to prohibit seed potato production in all PW associated fields. 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 present options for seed potato production in PW associated fields, both under the current Potato Wart Order (PWO) in PEI and in any fields that may become PW associated fields elsewhere in Canada. The finalised version of this RMD will support the development of a new national-level potato wart response plan to replace the existing PWDLTMP.

Scope

For the purposes of this document, the term "restricted area", as defined by the PWO, includes all fields currently categorized under the PWDLTMP (Categories A, B, C and D). The RMD does not include the province of Newfoundland and Labrador (NL) which is regulated for PW under the Plant Protection Regulations, Schedule II. It applies to all other PW associated fields in the rest of Canada.

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.

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. The highest risk for spreading PW within Canada is human-mediated spread through the movement of field-grown seed potatoes and the soil associated with them. Several Category A (index) fields in PEI are believed to have been infested via the movement of seed potatoes, illustrating the potential risk associated with seed potato production.

The PWDLTMP was originally designed to respond to detections of PW in PEI. It contains processes to assess each field category type and to support the eventual release of fields from some regulatory requirements including those related to the production of seed potatoes. The future national-level potato wart response plan is expected to maintain the field classifications used in the PWDLTMP but the specific restrictions related to seed potato production are under review and are the subject of this RMD.

Under the PWDLTMP, Category A (index) fields are eligible for full release from the specified requirements after a period of 27-30 years (estimated using standard crop rotation practices). However, as recent science has indicated resting spores are able to survive for 40 years or more in field soil and low population levels are very difficult to detect with current testing protocols, permitting seed potato production in Category A fields may pose a significant risk.

Category B fields are adjacent to Category A fields and are primarily at risk due to localized spread of spores (either natural or human-mediated). Under the PWDLTMP, seed potato production in Category B fields is prohibited through the issuance of a regulatory notice until the associated Category A field was no longer subject to the PWDLTMP.

Category C fields have a history of direct contact with a Category A field, via movement of equipment, soil, or seed potatoes. The PWDLTMP describes a series of soil-sampling and surveillance steps that allow for the gradual easing of restrictions on these fields through amendments of regulatory notices until seed potato production is eventually permitted. Due to the relatively high risk associated with these fields and a number of potential pathways for pest spread, prohibition on seed potato production until the field is no longer subject to the specific restrictions remains an important consideration in the options presented below.

Category D (other contact) fields have a history of shared equipment with Category A fields but only indirectly (for example, equipment moved to a Category C field before moving to the Category D field), and are characterized in the PWDLTMP as having low risk and are therefore subject to a reduced level of restriction. However, several current Category A fields in PEI were classified as Category D fields prior to confirmation of infestation with PW. This has highlighted 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. This concern was also described in the 2022 report from the International Advisory Panel, and is central to one of the recommendations of the 2022-23 PWDLTMP Working Group. Changes to the phytosanitary restrictions for Category D fields may be required to mitigate this risk.

The restrictions for the different field categories will be reviewed during the development of the future national-level potato wart response plan. The length of time restrictions will remain in place will be dependent on the crop rotation used by the grower. In PEI, the provincial Agricultural Crop Rotation Act (PDF) and associated regulations limit the planting of potato crops to one in every 3 years, although growers can apply for an exemption permitting 2 crops in every 5 years. Other provinces may have different requirements or no requirements, which would affect the time required for fields outside of PEI to be released from restrictions.

Risk management considerations

The objective of this risk management document is to identify options for seed potato production in fields that are classified as Category A, B, C, and D under the current PWDLTMP. The options presented consider a number of existing programs, guidelines, and expert scientific advice as well as recommendations from several groups that were convened to provide advice to CFIA on PW management. All of the options that permit seed potato production assume soil sampling with no detection of PW would already have taken place in the field as part of the initial PW investigations. The procedures for releasing fields from the PWDLTMP restrictions are currently under review; for the purposes of this document the specific requirements for release are not considered, only whether a field has met all requirements to be released from restrictions under the future national-level potato wart response plan. 

Applicable legislation, programs, guidelines, assessments, and recommendations were reviewed, including:

1. The PWDLTMP (March 5, 2009) placed the following restrictions on the production of seed potatoes:
   • Category A: seed potato production prohibited until restrictions on the field are lifted (minimum 27 years from date of detection in PEI; may vary based on provincial crop rotation regulations).
     
   • Category B: seed potato production prohibited until restrictions on the associated Category A field are lifted (minimum 27 years from date of detection as noted in Category A).
     
   • Category C: seed potato production prohibited until the field has undergone soil sampling (8m x 8m grid) and post-harvest surveillance of 4 susceptible potato crops (minimum 10 years from date of detection depending on provincial crop rotation regulations) with no PW detected.
     
   • Category D: no prohibition on seed potato production after field has undergone soil sampling (8m x 8m grid) with no PW detected.
2. The U.S. Federal Order DA-2022-14 (PDF) for entry requirements for importation of potatoes from PEI of April 1, 2022, prohibits the entry of field grown seed potatoes for planting from PEI due to the presence of PW in the province. It also restricts entry of potatoes for human consumption that were produced from seed potatoes grown in restricted fields (defined as index (A) fields, adjacent (B) fields, primary contact (C) fields and other contact (D) fields).

3. The phytosanitary risk associated with potato wart is described in CFIA's Plant Health Risk Assessment:

   "The movement of potatoes, soil and other regulated things with associated soil, such as farm machinery, presents a risk for the spread of potato wart 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."

4. The International Advisory Panel (IAP) on Potato Wart Disease management on Prince Edward Island (December 2022) makes recommendations under the "Conclusions and recommendations on management of potato wart disease on PEI" section. The IAP discussed the use of resistant potato varieties and prohibition of potatoes in fields where potato wart has been found:
   • Category A: prohibition of potato production on infested fields for a minimum of 20 years is a key measure used for control of potato wart disease. Seed potato production is prohibited until restrictions on the field are lifted (minimum 30 years from date of detection: 20 year prohibition on all potato production, followed by ten years with no spore detection before restrictions can be fully removed).
   • Category B: potato production limited to resistant varieties for processing or human consumption until restrictions on the associated Category A field are lifted (minimum 30 years from date of detection).
     
   • Category C: fields subject to additional regulatory controls for a minimum of 2 or 4 crops of resistant variety potatoes for processing use (depending on type of association with Category A field) plus grid or tare soil sampling after each of those potato crops with no PW detected.
     
   • Category D: fields subject to additional regulatory controls for a minimum of 1 crop of resistant variety potatoes for processing use plus grid or tare soil sampling after that potato crop with no PW detected.

   Furthermore, the IAP report suggested that "biosecurity control areas" should be established around specific clusters of PW detections, and that no seed potatoes produced in the "biosecurity control areas" should be planted in the pest free area that they envision could be established outside of the control areas (with the exception of seed potatoes produced in a Pest Free Place of Production).

5. The PWDLTMP Working Group Final Report (April 2023) did not make a recommendation on seed potato production in PW associated fields, as consensus was not reached. Some members supported a complete prohibition while others suggested that seed production could be permitted in some Category D fields if they were deemed "low-risk".
6. The Canada and United States Guidelines on Surveillance and Phytosanitary Actions for the Potato Cyst Nematodes Globodera rostochiensis and Globodera pallida (17 April, 2014), Section 9, restrict seed potato production indefinitely in infested fields, even after release from regulatory controls.
7. Potato production is prohibited indefinitely in Canadian fields where potato rot nematode, Ditylenchus destructor, has been detected (CFIA 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; Appendix 3.).

Risk management options

Field category A

Analysis of current program

Under the PWDLTMP, seed potato production in Category A fields is prohibited until the field has met all of the requirements for release from specified restrictions. This process would take a minimum of 27 years in PEI and is based on an ongoing series of assessments (soil testing, greenhouse bioassays, and field inspection activities). This process permits future seed production in previously infested fields.

Allowing the eventual production of seed potatoes in fields that were infested with PW (Category A) poses a significant risk, considering the pest can remain dormant for more than 40 years, and in the event that a low-level PW population was not detected during the assessment processes for lifting of specific restrictions. This would allow the highest-risk pathway (seed potato production) to occur in the highest-risk fields (Category A).

The approach in the PWDLTMP is not consistent with the Canada-United States guidelines on the management of the potato cyst nematodes (PCN), a similarly long-lived soil borne quarantine pest of potatoes. Under these guidelines, the planting of seed potatoes in previously infested fields is prohibited indefinitely.  Similarly, the planting of host crops is prohibited indefinitely to mitigate risks associated with potato rot nematode in Canada.

Therefore maintaining the current approach is not being considered as an option.

Alternative option – Seed potato production prohibited indefinitely in Category A fields

Following release of all other restrictions referenced in the PWDLTMP, a prohibition on the production of seed potatoes would remain in place indefinitely for Category A fields.

Pros

• Category A fields, which pose the highest risk due to having a detected population of PW, will be prohibited from producing seed potatoes indefinitely to provide assurances that risk related to the seed potato pathway is permanently addressed.
• Addresses concerns related to the challenges observed in using survey methods with limited sensitivity to detect low levels of microscopic resting spores.
• Consistent with approaches for fields infested with other soil-borne pests of potatoes including PCN and potato rot nematode.

Cons

• Does not offer any possibility of the complete release from restrictions for owners/users of Category A fields.
• Requires permanent monitoring of Category A fields by CFIA.

Field category B

Analysis of current program

Under the PWDLTMP, Category B fields undergo investigation sampling and ongoing visual surveillance following susceptible potato crops to reduce specific restrictions, but a prohibition on seed potato production remains in place until the associated Category A field is released from the PWDLTMP. This approach prohibits seed potato production in Category B fields until completion of required steps for both the Category B field and the associated Category A field. It also reduces the risk of spread if an undetected low-level population of PW is present in the Category B field. 

Noting that the final release from specific restrictions of a Category B field is dependent on the associated Category A field, only the owner of a Category A field has control over the timeline for the assessment activities and potential release. If the fields are owned by 2 different farm units, a Category B field may present a lower risk for PW, as this field would not have shared farming equipment, seed potatoes, or storage facilities.

Alternative option – Seed potato production prohibited until the field is released from specific restrictions (not tied to status of Category A field)

A series of sampling and visual surveillance assessments (during the investigation and subsequent potato crop production) would be conducted on a Category B field and if all results are satisfactory, the field could be released from specific restrictions if a minimum 15 m buffer is established between the Category A and Category B field. This release would not be linked to the status of the associated Category A field. Seed potato production would continue to be prohibited within 15 m of a Category A field.

Pros

• Meets the required 15 m buffer requirement to separate a Category A field from non-categorized fields.
• Prohibits seed potato production in Category B fields until completion of required steps for the field.
• Uncouples the status of Category B fields from that of the associated Category A field.
• Allows owners or users of Category B fields to manage their land and plan for eventual release from specific restrictions.

Cons

• Some natural spread is possible from a Category A field into Category B fields, particularly if there is soil erosion (run-off) from the Category A field. Category B fields may contain undetectable low-level populations of PW and permitting seed potato production may result in increased risk of spread.
• Does not align with PCN guidelines, where release of adjacent fields from regulatory controls is contingent on not detected bioassay results from the corresponding infested field.

Field category C

Analysis of current program

Under the PWDLTMP, Category C fields can follow a series of sampling and post-harvest visual surveillance assessments leading to release from the specific restrictions under the PWDLTMP. Currently seed production is permitted after a minimum of 4 susceptible potato crops with no evidence of the presence of PW (minimum 10 years from date of detection depending on provincial crop rotation regulations).

The approach does not align with the Canada-United States guidelines on potato cyst nematode, which permit limited production of seed potatoes in exposed fields for use only within the regulated area.

Category C fields are a high risk field category and as of October 2023, on 9 separate occasions PW has been detected in fields that were Category C fields at the time of their detection.

Therefore, other options are not being considered at this time.

Field category D

Analysis of current program

Under the PWDLTMP, there are no specific restrictions placed on Category D fields. Seed potato production is permitted following "not detected" results from the investigation soil sampling and testing.

As no regulatory notices are placed on these fields, in practice there are no restrictions on potato production for all end uses in the Category D fields for domestic use when the conditions of the PWO are met. This approach allows Category D, the largest category of fields (in both number and area), to produce potatoes for all end uses, including seed potatoes.

While Category D fields should have a low probability of harbouring PW based on their indirect connection to a Category A field, as of October 2023 10 Category D fields have been found positive for potato wart through either soil testing results or detection of infested tubers. This indicates that at least some Category D fields are a higher risk than originally thought.

The United States Federal Order of April 1, 2022, prohibits import of any potatoes for human consumption grown from seed potatoes produced in a PW associated field in PEI. Any seed potatoes produced in Category D fields require strict controls and rigorous traceability to ensure that their progeny is not used to produce potatoes for human consumption that enter the U.S. market.

Therefore maintaining the status quo will not be considered as an option.

Risk management proposals

The proposals presented below are based on the objective of eventual eradication of PW in areas of Canada other than NL. Pest eradication cannot be achieved unless both pest reproduction and spread are prevented. In the absence of treatment methods that effectively devitalize resting spores in the field, the only known phytosanitary measure which can prevent pest reproduction is the prohibition of potato crops on known infested fields for the full lifespan of PW resting spores. It is therefore considered prudent to prohibit seed potato production in PW associated fields until a comprehensive evaluation of their status has been completed as supported by specific restrictions on field use.

Current CFIA proposed options