On this page
- About potato wart in Canada
- How it's detected: soil sampling and visual inspection
- What happens once it's detected in a field
- Why soil is testing necessary
- Why we don't test for spore viability
- National survey
- How potato wart is tested internationally
- Looking ahead
- More information
About potato wart in Canada
Potato wart (Synchytrium endobioticum) is a soil-borne fungus that causes cauliflower-like growth on the surface of the tuber. To date, this pest has only been detected in Newfoundland and Labrador and in Prince Edward Island (PEI).
Potato wart is not always evenly distributed in a field and, while potato wart naturally spreads slowly within a field, it can spread rapidly within and to other fields by the movement of soil in association with potatoes, footwear, farm equipment, cultivation practices, and planting of seed potato tubers from infected fields.
The spores can survive for many decades in soil. A low level of spores in soil is enough to spread the disease. There are no treatments available to control potato wart; therefore the most effective way to control the disease is to prevent its spread to other locations. The risks associated with the transmission of potato wart from fresh potatoes from PEI remain negligible when appropriate risk mitigation measures are in place and followed.
How potato wart is detected: soil sampling and visual inspection
In order to regulate a field, the Canadian Food Inspection Agency (CFIA) requires the identification of two potato wart spores. Before restrictions are implemented, the two spores must be confirmed to be potato wart by the CFIA Charlottetown laboratory.
The CFIA uses the following process for identification and detection of potato wart.
Potato wart is typically identified in two ways:
- Visual inspection: Symptoms of potato wart are visible on a potato tuber and the potato tuber is submitted to the CFIA Charlottetown laboratory for confirmation of the disease; or
- Soil sampling: Soil samples are collected by the CFIA and submitted for analysis to the CFIA Charlottetown laboratory.
In both situations, a detection is confirmed by the CFIA and triggers a regulatory response when:
- Two or more spores are identified. This method isolates potato wart spores and scientists use a microscope to evaluate the visual appearance of the spores; and
- Synchytrium endobioticum DNA is detected using a molecular testing method.
Tare and grid soil sampling
Tare soil is the soil found on potatoes after harvest. The soil falls off during handling and storage. This sampling method collects soil falling from potato tubers while they are being handled, where each truckload represents a portion of a field. Laboratory analysis for tare soil samples follows the same strict methodology used for grid field-collected samples.
Tare soil sampling was recommended by the International Advisory Panel as an effective means to collect soil. Grid sampling is another option and is used during potato wart investigations and in the national survey, but tare soil sampling can be more efficient as a single inspector can complete the activity as the potatoes move into storage.
What happens once potato wart is detected in a field
When potato wart is detected and the regulations are triggered, the CFIA takes immediate action to put restrictions on the field (referred to as an index field) to help prevent the potential spread of the fungus through movement of potatoes and soil. Fields associated with the index field, identified through tracing activities, are also placed under restrictions based on the nature of their linkage to the new index field.
Why soil testing necessary
Visual inspection is a tool that has been used for many years by different countries, including Canada and the United States (U.S.), to help identify detections of potato wart. However, visual inspection is limited in its ability to detect low levels of potato wart spores.
Based on its biology, this disease can be present and undetected for many years, even decades, before visible signs of the disease (such as warts) are observed on potato tubers. Because the fungus (Synchytrium endobioticum) produces symptoms mainly on plant parts that are below-ground, a visual survey of above-ground parts is not reliable for detecting potato wart. Therefore, soil sampling and testing remain the most effective way to detect the disease.
Given there is potato wart in PEI, surveillance based on soil sampling and testing is also an expectation of Canada's main trading partner, the U.S., to support the movement of potatoes for human consumption. In addition, the U.S. expects Canada to maintain its national surveillance program (which is based on soil sampling and testing) and expand it to include unrestricted fields in PEI such as the PEI survey to increase confidence in Canada's long-term management of potato wart. Given the last 2 years of sampling and testing in PEI was heavily focussed on the restricted fields as part of the investigations launched in October 2021, PEI only participated in the national survey in 2021.
Why we don't test for spore viability
The term "spore viability" refers to whether a spore is dead or alive. Dormant potato wart spores may be alive, but in a dormant state for decades. The specific conditions that break potato wart spore dormancy or stimulate spore germination are currently not known.
Currently, there are no reliable scientific testing methods available to determine if a potato wart spore is dead, alive or alive but dormant.
Current scientific testing methods, including microscopic examination, pot bioassay, and field bioassay testing methods, have known limitations in determining the viability of a spore. At present, none of these tests can conclusively determine whether a potato wart spore is dead, alive, or alive but dormant.
Given the limitations of existing testing methods, the CFIA does not currently consider spore viability when determining whether or not a field should be regulated following a confirmation of the presence of potato wart spores through microscopic examination and molecular testing of full spores.
National survey
The CFIA’s national potato wart survey in seed fields is an important element of the Government of Canada's efforts to help contain, control, and prevent the spread of potato wart. Soil samples from seed potato fields across the country are collected and analyzed. This survey builds on previous survey data collected since 2021, as well as specific and general visual surveillance that has been ongoing in PEI since 2000. The sampling rate is approximately one soil sample per acre.
How potato wart is tested internationally
Testing in the United States (US)
Canada and the U.S. have closely integrated potato sectors that prohibit the importation of fresh potatoes, including seed potatoes, from all countries in the world (except each other) because of the risks posed by regulated pests that may be spread through the movement of potatoes and associated soil.
Potato wart is not currently present in the U.S. since it was declared eradicated by the United States Department of Agriculture (USDA) in 1994. However, in 2020, the USDA published the New Pest Response Guidelines for Synchytrium endobioticum (Schilb.) Percival Potato Wart Disease (PDF). The purpose of these guidelines is to outline the USDA's response plan in the event that potato wart is found in the U.S., including how they would test to confirm a detection.
The guidelines describe a similar approach to sampling and testing to the one used by the CFIA; that is, microscopic examination of soil samples for the presence of resting spores, coupled with a confirmatory molecular test. The USDA has not detected potato wart since the establishment of their response guidelines and protocols for testing spore viability are not included as part of the response plan should the disease be introduced to the US.
Testing in the European Union (EU)
For many EU countries, potato wart has been present for over a century. Given their long history with potato wart in major potato production areas and higher spore concentration, many EU countries take a different approach to testing and regulating fields than Canada in order to manage the risk of further spreading the disease. For instance, jurisdictions in the EU that have a high concentration of potato wart in localized areas may also have a different risk tolerance than Canada when determining how and for what reasons they choose to use testing methods like bioassays in their potato wart programs, recognizing the limitations of these testing methodologies when used for viability.
As a core part of their management programs, EU countries have relied heavily on growing resistant varieties of potatoes to help suppress potato wart populations. Many countries also prohibit potato production in index fields for extended periods of time and mandate that large buffer zones surround fields as a means of controlling the spread of the fungus. In Canada, we encourage the use of potato varieties that are resistant to potato wart and research is ongoing to find more resistant varieties that are adapted to our climate and meet market demands.
Given the absence of potato wart in most parts of Canada and the limited number of fields with a positive detection for potato wart in PEI, Canada is taking a precautionary approach to regulating potato wart with the long term objective of eradication of this disease in PEI. For instance, Canada continues to have a zero tolerance policy for the importation of soil from regions like the EU, where pests like potato wart are present, to protect the environment and Canada’s agricultural sectors.
Looking ahead
While the current science does not support a change in the CFIA's practices at this time, as a science-based regulator, the CFIA is closely monitoring and considering whether any emerging viability testing methods may be used for potato wart detection and management in the future.
The CFIA continues to work actively with Agriculture and Agri-Food Canada and other plant protection organizations around the world with a shared goal of advancing scientific research in this area. For example, the CFIA is working with partners to look at the potential for molecular assays that would allow the specific detection of viable resting spores and looking at how it could be used in the regulatory context. If an accurate and reliable method for testing spore viability is identified, the CFIA will look at how to incorporate it into its management program as quickly as possible.
The CFIA is also exploring the possibility of jointly developing sampling and testing guidelines for potato wart with the USDA, which could include spore viability testing. Efforts to advance this discussion are currently underway.