In theory, the last couple of relatively hot and dry growing seasons should have slowed the progress of clubroot to canola fields across the Prairies. In practice, they didn’t. Why?
Researchers and plant breeders say the answer is that just as other weeds, pests and diseases evolve to thwart control methods, clubroot is doing the same. While they’re coming up with solutions to stay ahead, applying them will mean that farmers have to learn a bit of clubroot biology so they can make the right decisions when selecting canola varieties.
CLUBROOT PATHOTYPES 101
Until recently, researchers have primarily used the reaction to the Williams differential host-set to classify clubroot into five pathotype groups. These were identified as pathotypes 2,3,5,6 and 8 when the disease was first found in the St. Albert area of Alberta in 2003.
Overuse of the first resistant varieties resulted in the development of new pathotypes needing new sources of resistance.
Researchers realized they needed a better way of classifying the pathotypes. Through work at the University of Alberta and Agriculture Canada, the Canadian Clubroot Differential (CCD) set was developed. The CCD builds on the Williams differential set and has more host lines, which are now further classified as 3A, 2B, 5C, etc. for a total of 17 clubroot pathotypes.
The Williams differential host set was a tool that allowed pathologists and researchers to identify between five pathotypes.
“The CCD provides the ability to hone in and better understand the pathotypes affecting the fields… where we once thought a product was resistant to pathotype 3, now we know that there are many different pathotypes within group 3,” says Eric Gregory, Director of Marketing for BrettYoung Seeds.
Under the CCD, 3H has been identified as the primary pathotype in new areas, most recently in Saskatchewan and Manitoba.
“Clubroot moves into an area, the resting spore population develops. What was once considered 3 and is now known to be 3H, is typically the pathotype most predominant in early infection stages,” says Gregory. “It was likely 3/3H which started the clubroot problems in the Edmonton area.”
Researchers agree that more work must be done to fully understand the pathotypes and how they are evolving.
MULTIGENIC RESISTANCE
Rene Mabon, Agronomy and Regulatory Manager with BrettYoung, sees two major problems facing growers. “First, the disease continues to spread to new areas. Second, in the areas where disease resistance was first identified, now some of the new pathogen types are becoming more significant — 3A is the most prevalent pathotype where breakdown of genetic resistance occurs.”
Gregory says the last few seasons have not been considered prime for clubroot development. “It’s been pretty hot and dry — not really conducive to disease development — and I am personally surprised by the number of new fields that we are finding with clubroot symptomology. It has been a bit scary how quickly this has developed.”
While growers can rely on older resistant varieties in areas where there is no clubroot yet, those in areas with a history of the disease need to be mindful of its continual evolution and manage accordingly.
“Once you are into areas where there might be newer pathotypes, you need to look at varieties that have multigenic, or multiple sources of resistance. This gives growers a broader base of resistance,” says Mabon.
The multi-genic varieties bring together more genes and sources of resistance and improve the performance by having two types of resistant genes. This will help with resistance to various strains within a group.
“If you are in an area at risk of clubroot, you should be growing resistant varieties. In the newer areas, growers are likely dealing with 3H and we have heard from growers, for instance, in Manitoba, that the single gene resistance is working,” says Mabon.
MORE RESISTANT VARIETIES COMING
Traits related to higher yield or shatter resistance have been a big focus for growers but Gregory says in the future the focus will need to shift to clubroot resistant varieties.
In 2018, only 21 percent of overall Prairie canola acres were seeded to resistant varieties, but regional numbers illustrate the disease’s spread: 44 percent of Alberta’s acres were clubroot resistant, compared to 12 percent in Saskatchewan and 10 percent in Manitoba. Overall, 36 percent of RoundUp Ready acres were CRresistant varieties.
Gregory emphasizes that clubroot is everyone’s problem but adds that there are limitations to what can be done in the short term.
“There isn’t enough clubroot product on the market today for growers to purchase, even if they wanted to. These products take time to develop.”
Many researchers and seed growers may consider developing only resistant varieties in the next few years, as that trait becomes a minimum requirement.
“It wouldn’t surprise me if BrettYoung Seeds and other companies move to marketing only clubroot-resistant varieties,” says Gregory.
TIPS FOR GROWERS
- Know the pathotype: Some companies and growers are still using only the Williams differential set to identify whether the variety is resistant. Gregory says this is a missed opportunity to manage the problem effectively. “I’m not sure if growers know if they are selecting for pathotype 3H, which is equivalent to the old 3. I hope everyone is moving towards using the new CCD, but the industry isn’t there yet.”
- In order to manage this disease effectively, growers need to know what’s in their field and what tools are available to manage it. Understand which pathotypes they are at risk with and ask questions of their seed rep and educate themselves so they can select the appropriate variety.
- Know the soil: Send in a soil sample this fall or next spring to get a better picture of what is happening in their fields so they can manage accordingly. The Canola Council of Canada’s website lists labs which offer this service.
- Research and rotation: Both Mabon and Gregory promote longer canola rotations, rotating varieties to manage the issue in each field and using the Canola Council of Canada’s website (clubroot.ca), which has extensive clubroot information. Mabon adds: “Growers need to scout their fields and adapt their management to include different varieties. If you are growing a crop intensively, you need to manage intensively.”
