Clarification of detection threshold and draft proposals for introduction of "viability confirmation" status of index fields and "field of concern" field classification

On this page

• Clarification of the threshold for detection
• Introduction of "viability confirmation" status for index fields
• Fields of concern

Clarification of the threshold for detection

The current threshold for declaring a detection of Synchytrium endobioticum, the causal agent of potato wart, in soil is confirmation of a minimum of two spores in a single soil sample.

• Spores must be confirmed via both microscopic examination of morphology and molecular detection of DNA (real-time TaqMan polymerase chain reaction [PCR]).

The CFIA has reviewed the threshold for detection and has determined that the detection of two spores, confirmed both morphologically and molecularly, from a single field meets the detection threshold, regardless of whether both spores were detected in a single soil sample (or even in a single sampling event).

Considerations

• Confirmation of multiple spores in a field is evidence of S. endobioticum presence.
• Two spores each confirmed by two separate methods demonstrates replication of results for the field.
• The requirement for two spores in one sample is arbitrary since the definition of a sample and the area the sample represents can vary depending on the sampling method.
  • Grid soil is collected from multiple sub-samples and amalgamated into a single sample that represents a specific area of the field but the area it represents can vary significantly depending on the grid size used, the soil type and sampling conditions.
  • Tare soil is collected as it falls from tubers that were harvested from a field and sub-sampled so that a single sample represents a portion of the harvested lot. The entire amount of soil collected from the lot is considered as a single sample as it represents the entire field, and no single sub-sample can be linked to a specific area of the field.
• Single spores in multiple soil samples indicate distribution of the pathogen in the field.

CFIA decision

Two or more spores of S. endobioticum collected from a single field and confirmed by two methods of analysis (morphological and molecular) will replace the current "detection" threshold that requires two or more spores be confirmed in a single soil sample.

Introduction of "viability confirmation" status for index fields

Proposed naming convention

"Index – viability confirmed" will be used for fields that meet the above definition of "detected" (two or more spores in the field) and where symptomatic tubers have been observed either directly from the field or in laboratory bioassays for fields in the associated potato wart "cluster".

"Index – viability not confirmed" will be used for fields that meet the above definition of "detected" (two or more spores in the field) but where there is no confirmed symptomatic tubers or evidence of spore increase for fields in the associated potato wart "cluster".

Considerations

• Potato wart detections in PEI have been grouped into "clusters" based on the linkages between detections (e.g., common equipment, soil movement, seed potato movement). The CFIA invests significant efforts into confirming the potato wart pathotype associated with each cluster. To date, in PEI there have been 5 clusters associated with the 37 index fields. 26 of these fields have confirmed symptomatic tubers associated with them. Of the remaining 11 index fields, one is associated with seed potato movement and the remaining 10 are linked through soil / equipment movement.
• Index fields will be identified by these proposed classifications in the potato wart land management tool.
• A full trace-out will occur following a confirmed "detection" and both index field types and their associated field categories will be regulated the same, irrespective of the detection being via spores or tubers.

Proposed approach

In a situation where there are no symptomatic tubers in the same detection cluster and only spores are confirmed as Synchytrium endobioticum, the following process will be used:

• Soil collected from the "Index – viability not confirmed" field will undergo laboratory bioassay in an effort to generate symptomatic tubers to confirm viability and for use in determining the potato wart pathotype present in the field.
• Up to five consecutive rounds of bioassay will occur, or until tuber symptoms or other evidence of a reproducing population of S. endobioticum is confirmed (i.e. an observed increase in spore detections in the soil that has undergone the bioassay via microscopic analysis) or there is a detection of symptomatic tubers in an associated field.
• Should no confirmation of spore viability be demonstrated (in the field or in fields in the associated "cluster") following five rounds of bioassay, the field will be reclassified as a "primary contact" field and will follow the process for release of phytosanitary measures for primary contact fields described in the National Potato Wart Response Plan. All associated fields that were categorized during the trace-out will be released from all restrictions (unless also associated with another potato wart cluster).

CFIA proposal

Index fields where symptomatic tubers have been confirmed in the same detection cluster will be classified as "Index field – viability confirmed". Index fields where symptomatic tubers have not been confirmed will be classified as "Index field – viability not confirmed". Both classifications of index fields will have identical risk mitigation measures applied at the time of detection. Naming conventions for all other field classes (adjacent, primary contact, other / secondary contact) will remain the same regardless of the classification of the associated index field.

An "Index field – viability not confirmed" will undergo up to five rounds of bioassay. If no evidence of a reproducing population of S. endobioticum is confirmed following the 5^th round, all associated fields will be released, and the index field will be reclassified as a primary contact field.

Fields of concern

CFIA proposal

• A field where only one spore is detected and confirmed, via morphology and molecular analysis, will be designated a "field of concern".
• A field of concern will be placed under notice and if the field is not already classified as a primary contact field, equivalent risk mitigation measures will be applied, including the requirement for freedom from soil and restrictions on potato crop usage. The field will be considered "restricted" as per the requirements of the Potato Wart Order and the US Federal Order of April 1, 2022, and potatoes from a "field of concern" will not be certified for export. Both owner and land user will be notified.
• Intensified soil sampling of the "field of concern" will be prioritized.
  • If one or more additional spores is detected, the field will be considered "detected" for potato wart.
  • If the "detection threshold" (symptomatic tubers and/or two spores confirmed via two methods) is not met following laboratory analysis of the soil from the intensified sampling, the field will remain under restrictions equivalent to a primary contact field and will follow the process for release from phytosanitary measures described for primary contact fields in the National Potato Wart Response Plan.

Considerations

• If CFIA were to do a full trace-out on a field of concern and categorize and restrict all associated fields, it would effectively be lowering the detection threshold to "one spore".
• The need to find only a single additional spore in subsequent sampling aligns with the new detection threshold of two spores in a field.
• Fields with one spore identified morphologically, but not confirmed via PCR, are outside the scope of this proposed field classification and will be deemed as "not detected" for potato wart. Owners and the land user will be notified when a single unconfirmed spore is detected as this information may inform best practices on their farms.
• Fields with empty, plasmolyzed (appears to have water loss or shrinking) remnants of spores but no intact spores are outside the scope of this proposed field classification as they do not meet the requirements to be morphologically identified as potato wart.