A path of least resistance opens door to clubroot ‘disaster’

Canola growers should keep one simple message in mind when choosing their varieties for next year’s growing season: Every acre needs to be clubroot resistant.

“What we’re trying to do is really focus on using clubroot resistance as part of a proactive, integrated prevention or management plan for every canola acre,” said Autumn Barnes, agronomy specialist with the Canola Council of Canada.

There’s been some misunderstanding among growers in recent years about whether they should use clubroot-resistant cultivars before the disease turns up in their fields, said Barnes.

But scientists and seed companies agree every acre should be seeded to resistant varieties to slow clubroot’s spread across the Prairies.

“That’s the direction our industry is going,” said Barnes. “All the new cultivars the seed companies are putting out are going to have clubroot resistance in the next few years.”

The reason is that this pathogen is sneaky, and so can be difficult to spot in the field before it gets established. Soil testing and regular scouting can help with that, but unless the right spot is scouted and tested when clubroot arrives, it can become established without a farmer knowing.

But growing resistant varieties can help prevent a “catastrophic multiplication of spores,” said Barnes.

“If some clubroot spores ended up in a field of mine and the first canola they encountered was susceptible, those spores would basically turn the roots into little multiplication factories,” she said.

“They can reproduce quite effectively, so you automatically have that multiplication of clubroot spores. But if the first canola those spores encounter is resistant, you’re automatically muting that ability to reproduce.”

That doesn’t mean growers will never see clubroot galls on their canola.

“It’s a lot more complicated than that, but it’s so much better than having susceptible cultivars in there.”

Producers still need to take a multi-pronged approach so spore loads are kept low and local.

“When your field encounters clubroot spores, you might not ever really know that you have clubroot if you have a one-in-three crop rotation, you’re scouting for it, you’re growing clubroot-resistant cultivars, and you’re controlling all your brassica weeds,” she said.

“The spores may be there, but they may never be to the level that you have problems.”

But fail to do those things and the reverse can happen.

“If you’re doing things like growing a tighter rotation, not looking for clubroot in your canola every year, and growing a susceptible hybrid, that’s a recipe for disaster.”

Ideally, using an integrated approach will catch an infestation “when it’s just a small patch and prevent that patch from spreading.”

“It’s really about being proactive and trying to prevent a problem from getting bigger. The risk of waiting is that you’re going to have a gigantic problem instead of something small that’s manageable.”

Integrated strategies

The first step is a one-in-three-year crop rotation.

“We know that about 90 per cent of clubroot spores break down during a two-year break between canola crops.”

But that break also has to include brassica weed control, and that’s something a lot of growers don’t think about, she added.

“A one-in-three rotation is really important, but if you have a one-in-three rotation and in your non-canola years you have a lot of volunteer canola or other brassica weeds, you’re still multiplying spores,” she said.

“A break in a crop rotation is about having a period of time where there’s no host for those spores, so it’s important to control those brassica weeds.”

Regular scouting is the next step.

“I’ve been in a number of fields over the course of my career in areas where people don’t think clubroot will be a problem so they’re not scouting for it, and all of a sudden, they’ve got a whole field polluted with clubroot,” said Barnes.

“So always look for it in any canola year by pulling up plants, especially in higher-traffic or higher-moisture areas.”

Reducing the movement of soil is the last — and perhaps most critical — step.

“Clubroot spores move anywhere soil moves, so it’s really only a matter of time before each field encounters spores in some way,” she said.

“But there are ways we can keep spores local and prevent them from moving around.”

That includes knocking big chunks of dirt off equipment between fields, avoiding field work when it’s wet, reducing tillage, and sanitizing footwear.

“Not a lot of growers out there want to be pressure washing equipment every time they leave a field, but there’s a lot of different things we can do for bio­security that isn’t the three-hour pressure washing of a seed drill every time it leaves a field.”

But it’s not enough just to do one or two of these things.

“You can’t just pick one management strategy. You can’t just do crop rotation. Growing a clubroot-resistant cultivar on its own is not enough. You need to take an integrated approach.”

Otherwise, these resistant traits could break down, leaving growers without their most effective tool to manage and prevent the spread of clubroot.

“There will be a point where, if we don’t respect these traits and steward them, they’re not going to be around long term and they’re not going to be effective,” said Barnes.

“Clubroot-resistant traits are extremely important, and they are not unlimited. We need to focus on stewarding those hybrids.”

– Jennifer Blair is a reporter for the Alberta Farmer Express. Her article appeared in the Nov. 29, 2021 issue.