U of A researchers find new clubroot strains

The nine new pathotypes detected in a field survey in 2017 and 2018 brings the total number in Western Canada to 36

New strains of clubroot, a costly canola disease, are popping up in farm fields across the Canadian Prairies.

According to researchers at the University of Alberta, nine new clubroot pathotypes were detected on western Canadian farms during a field survey conducted in 2017 and 2018.

That brings the total number of unique clubroot pathotypes detected on prairie farms to 36.

At least 19 of the documented pathotypes are no longer affected by “first generation” genetic resistance that has been built into recently developed clubroot-resistant canola varieties.

Details of the newly detected strains were the subject of an academic paper published late February in the Canadian Journal of Plant Pathology, which can be found at bit.ly/3r3gMP6.

“In Canada, clubroot management relies heavily on the planting of resistant cultivars, but since 2013, resistance has been broken in an increasing number of fields,” the paper’s abstract states.

Before the introduction of resistance, clubroot (P. brassicae) pathotype 3H was predominant in Alberta, the abstract adds.

However, in testing of pathogen collections from 2014 to 2016, pathotype 3A was most common, indicating rapid shifts in the pathogen population.

The research results suggest “significant diversity in the virulence of (clubroot) populations and an increasing prevalence of resistance-breaking … strains.”

Keisha Hollman, a U of A plant pathology expert and lead author of the research paper, said her research is focused on analyzing the virulence of new pathotypes.

Understanding more about new pathotype virulence is important in guiding canola breeding efforts and expanding the efficacy of commercially available clubroot-resistant cultivars.

“Every year, we’re finding more and more pathotypes so we’re up to 36 unique pathotypes now,” Hollman said in a March 18 interview.

“We’re also starting to find more and more resistance-breaking pathotypes, which is obviously a big concern for producers.”

“We found the first cases of resistance breaking pathotypes in Manitoba this past year.”

So far, only two pathotypes have been detected in Saskatchewan farm fields.

Hollman described the Saskatchewan strains as older pathotypes that can still be controlled by “traditional resistance” found in clubroot resistant canola genetics.

But she cautioned that over-reliance on genetic resistance alone may be contributing to the emergence and proliferation of new, resistance-breaking pathotypes.

The discovery of new strains highlights the importance of using a multi-pronged strategy for clubroot control, she added.

To limit the spread of both well-established and new pathotypes, growers should stick to longer rotations, adopt enhanced bio-sanitation measures such as cleaning of equipment between field moves and grassing entrances to fields.

Soil amendments can also help in cases where the pathogen is evident in a small area of a field.

Many of the newer resistance breaking pathotypes that were documented in Hollman’s research were detected in isolated areas and are not believed to have widely spread.

This gives researchers, farmers and the canola industry a greater chance of containing potentially costly pathotypes in localized areas and ensuring that they don’t become widespread threats to production.

At the same time, other resistance breaking pathotypes such as pathotype 3A and 3D are already established over wide production areas, particularly in hotspots of central Alberta, Hollman said.

Pathotype 3A was the most dominant pathotype found in Alberta during the field survey.

“From what I understand, breeding is now focusing a lot on pathotypes 3A and 3D because they’re becoming so prevalent,” Hollman said.

“That’s one of the biggest concerns obviously is the breeding programs being able to keep up with how many new pathotypes we’re finding.”

“Right now, they’re kind of focused on the more dominant (strains). For these new ones … genetic resistance might not be an option for a very long time so we’re (trying to promote) more integrated strategies and integrated approaches to managing clubroot, rather than relying solely on genetics.”

“Genetic resistance is of course a great tool,” she continued. “However, I think relying solely on genetic resistance is why we’re seeing this influx in pathotype shifting.”

Pathotype shifting occurs when the population of dominant pathotype within a field is reduced or minimized, giving less prevalent pathotypes an opportunity to expand their populations.

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