Hope ahead for more durable clubroot resistance

A combination of two resistance genes shows promise against multiple pathotypes

The efficacy and durability of clubroot resistance depends a lot on inoculum levels in the soil.

Since it was first discovered in Alberta in the early 2000s, clubroot has proven a wily opponent for plant pathologists and breeders.

Back then, researchers identified a handful of clubroot pathotypes, mostly minor, and one major one that was responsible for most infections and yield loss in Western Canada. Plant breeders focused on that and introduced the first clubroot-resistant (CR) canola varieties in 2009.

They worked for a while, but then something changed. “In 2014, we started seeing new pathotypes coming out of Alberta,” says Gary Peng, a research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon. “Just in a few fields, but we had a strong sense of resistance breaking down.

“We do have an arsenal of R (resistance) genes, but they are still limited in diversity, especially in terms of resistance mechanisms,” Peng says. “And now we have a total of 17 pathotypes identified in Canada. So the question is, do we have all the R genes against those patho­types, and what’s the best strategy to deploy our R genes for optimum durability and efficacy?”

With funding from WGRF and SaskCanola, Peng and his team set out to answer those questions. And what they found offers new hope for canola producers across the Prairies.

A happy surprise

Used on their own, none of the CR genes currently in the arsenal can be effective against all 17 pathotypes, so Peng was interested in learning how a multi-gene approach would work. Would having more than one CR gene in a cultivar provide more robust disease resistance? Would that resistance last longer?

To find out, canola lines with various combinations of multiple CR genes were tested under simulated, intensive growing conditions against pathotype 5X — one of the newly reported pathotypes that is virulent on all old resistant canola cultivars. The possibilities presented by one particular double-gene combo looked promising.

Nutrien Ag Solutions, a collaborator on this project, bred hybrid canola lines carrying one R gene from chromosome A3 and one from A8. “We wanted to use a commercial breeding process so that anyone could use the approach in their program,” Peng says.

He found that canola lines with these two particular CR genes showed highly effective resistance to the handful of old clubroot pathotypes and partial resistance to pathotype 5X. Peng then wanted to find out if this resistance would also be durable.

“We repeated the plant’s exposure to the patho­type,” he says, explaining that this is done by collecting the clubroot galls from canola roots, drying them, grinding them up, then reintroducing that inoculum to the growth medium before growing the canola in it.

“If you do that with single-gene resistance, it breaks down very fast,” Peng says.

“Our hypothesis was that, even with the two CR genes, the resistance wouldn’t last. But to our surprise, it lasted pretty well at over five generations of exposure,” he says. “And we also saw a reduced level of inoculum in the soil over time.”

Investigating that aspect a bit further, Peng found that these stacked-gene canola lines developed slightly fewer galls over each generation of exposure, and that the galls themselves got smaller over time as well.

Ultimately, Peng says, that meant a smaller amount of pathogen inoculum was returned to the growth medium in each canola generation. That, in turn, resulted in consistent resistance performance from the plant.

Lessons and cautions

“From what I know, this is the first time anyone has looked at the multi-genetic route for clubroot resistance,” Peng says, adding that it is still a relatively new disease of canola and that there is still a lot to learn about the clubroot pathogen itself, as well as resistance mechanisms.

“We may have only two or three R genes to choose from at this time, and pathotypes can have different modes of action. The new ones avoid mechanisms of resistance by the R genes.”

Peng is also quick to point out that, even with two CR genes present in a canola variety, the efficacy and durability of resistance depends a lot on inoculum levels in the soil. “If levels are high, resistance is more vulnerable; if low, it’s more durable, especially for varieties with stacked CR genes.”

Peng says that with a two-year break between canola crops, 90 per cent of clubroot inoculum can die off in soils. It sounds like a lot, but it’s all relative.

“Ten million spores per gram of soil reduced by 90 per cent is still one million spores,” Peng laughs. “But there is still benefit to bringing inoculum levels down in all cases — even in severely infested fields. By bringing that inoculum level down, you allow even the resistant variety to perform better.”

Peng has two conclusions from this study to offer growers:

“One, the double gene will work effectively on older pathotypes and give you moderate, durable resistance against the new pathotype 5X. Two, reducing the amount of inoculum in the soil is good for resistance performance and durability.”

So what now? “I hope to move on to the next stage and look at the new pathotype 3A, 3B and 3D to ensure the resistance performance of a double-gene hybrid.”

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