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National survey to document the extent of apple scab resistance to systemic fungicides, including sterol inhibitors and strobilurins, in apple orchards

Project Code: PRR11-060

Project Lead

Kelly Ciceran - Vineland Station


To conduct a national apple scab survey to determine the level of pathogen resistance to sterol inhibitors and strobilurins in the various apple growing provinces of Canada

Summary of Results


Apple scab, caused by the fungal pathogen Venturia inaequalis, is one of the most serious diseases affecting apples in Canada. Surveys by Ontario Apple Growers estimate that in 2009 a total of 4,000 tons of Ontario apples were affected by apple scab, resulting in financial loss of over $1.8 million.

In the past, diseased leaf samples submitted to Cornell University and Michigan State University have shown that resistance to strobilurin and sterol inhibitor (SI) fungicides was present in apple scab populations from some Ontario orchards. The lack of efficacy in applied fungicides resulted in an increased number of fungicide applications and significant economic damage at harvest.

The extent of the pathogen’s resistance to fungicides throughout Canada was not known. Although protocols for testing for apple scab resistance had been developed, there were no facilities in Canada that provided this service to growers. Reliance on services for resistance testing provided by United States research institutions was not sustainable either, due to potential trade barriers limiting the transport of diseased plant samples across the border.


A 2-year national apple scab fungicide resistance testing project was launched in 2011. All apple-producing provinces of Canada participated: Nova Scotia, New Brunswick, Quebec, Ontario and British Columbia. In total, 98 sites across Canada were tested for apple scab fungicide resistance.

In each of the project years, 49 growers cooperating in the project across Canada left a small number (6-8) of apple trees unsprayed for disease until apple scab lesions appeared. Leaves with fresh primary scab lesions were collected and sent to the University of Guelph’s Pest Diagnostic Laboratory (PDL) for resistance testing to both SI, or Group 3 (Nova 40W, Nustar & Inspire) and strobilurin, or Group 11 (Sovran, Flint 50 WG & Pristine WG) fungicides.

Two methods for testing resistance were used:

With guidance from Dr. Kerik Cox, Cornell University, and Dr. George Sundin, Michigan State University, results were interpreted from baseline relative growth thresholds set to determine the sensitivity levels of scab to the SI (Nova or Inspire) or strobilurin (Flint) fungicides tested.


The results for Nova indicated that pathogen populations tested were shifting or have shifted from susceptible to resistant. The only susceptible populations were found in an abandoned, unsprayed orchard, near Brighton, Ontario. In orchards with a resistant pathogen population, Nova cannot be relied upon to manage apple scab, but may be useful for powdery mildew.

For Inspire, the results indicate that the majority of the pathogen populations tested were still susceptible. However, a few samples were in the shifting phase. Caution should be recommended to growers that this product be used with sound resistance management strategies.

For Flint, the distribution of resistance among populations ranged from susceptible to resistant. All orchards that were classified as shifted or resistant to Flint also tested positive for the G143A mutation, which would confer resistance to all strobilurin fungicides. Caution should be recommended to growers that this product also be used with sound resistance management strategies.

The cooperating growers were notified of the results and recommendations for use of SI and strobilurin fungicides in their orchards. Presentations, posters and newsletter articles have been delivered to the apple growing industry.

As a result of this project, the University of Guelph’s PDL has developed an apple scab fungicide resistance test established as a standard diagnostic protocol and, since 2013, offers diagnostic service to growers for a fee.

The service enables growers to send samples and obtain an indication about the status of fungicide resistance in their orchards. By having resistance testing capabilities in place, the apple industry has the opportunity to test the status of scab resistance to a range of fungicides, including SIs, strobilurins and other products, such as dodine. This new tool allows for better decisions regarding the use of fungicides, including tank mixes, timing and number of applications, and helps to keep the use of these chemicals to a minimum.

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