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New strain of clubroot beats resistance genes

This article is from way back in our 2014 Fall edition of the Farming Smarter Magazine!


by Helen McMenamin

Plant pathologists found a new pathotype of clubroot thriving even in resistant varieties of canola. So far, they have only found the new pathotype in the Edmonton area, where clubroot first appeared in canola some years ago, but scientists rush to find out more about this new strain of the disease and help develop management strategies for it.

“We don’t know yet whether this pathogen has always been present or if there’s been a genetic change,” says Alberta Agriculture plant pathologist, Mike Harding. “Maybe, it was present at low levels before and it’s increasing because we’re selecting for it by growing resistant varieties or there may be a recent genetic change in the disease organism.”

The biology of the disease is important to develop a strategy against it.

 If resistant varieties have selected for the new pathotype, researchers need to find different resistance genes than the ones currently in commercial canola varieties to effectively control the disease. However, this is a less useful strategy if the clubroot organism is genetically unstable and mutates frequently.

Harding and other plant pathologists are building a research program to better understand this new clubroot pathotype and find the best ways to manage it.  Clubroot is already a challenging disease for scientists because it’s an obligate biotrophe – it only grows in a living host. Harding works in a biosecure disease nursery within the research greenhouse facility at Brooks.

Picture by Ken Coles

So far, the new pathotype only shows up in small patches of fields already diagnosed with clubroot. Even though the growers used clubroot-resistant varieties, some plants were infected and showed typical symptoms. Although the new pathotype is only in the Edmonton area, so far, pathologists diagnose new fields every year.

The drier, higher pH soils of southern Alberta are less favourable to the pathogen. The pest surveillance branch of Alberta Agriculture runs an ongoing survey for clubroot that samples about 15 fields in each county every year.

“Infections aren’t leveling off,” he says. “We’re still finding new infections and the south is not immune, the cool, wet springs we’ve had these last three years were ideal for the swimming spores that move in soil moisture to host plants.”

Harding advises scouting for clubroot no matter where you farm.

“To me, scouting is the first, best practice you can use to protect your canola,” he says. “You don’t know whether you have clubroot unless you look for it. Sometimes you see wilting, stunted plants and yellowing, but not always.  Mild or late infections may not cause any visible symptoms above ground.”

The most likely spot for clubroot is around the field entrance – the bottleneck for soil entering fields on vehicles or machinery and it’s where scouting finds 90% of infections. But, it can also arrive in wind- or water-eroded soil.

Harding advises producers to pull 30 to 50 plants near the field entrance, ideally at or before swathing, and look at the roots. Include brassica weeds in your sample because clubroot can infect them too. Look for clubbing, galls or tumors on the roots. The disease causes unregulated growth of root tissue. Not all growths on roots are due to clubroot, the Canola Council website has some excellent pictures, but even pathologists have trouble identifying clubroot symptoms in the field.

“Sometimes it’s easy to confuse clubroot galls with herbicide damage or hybridization nodules,” says Harding. He advises sending roots with suspicious nodules to a lab for molecular testing.

Clubroot is long-lived, but spores have a half-life of about four years, so a 4-year rotation cuts down the amount of inoculum and the severity of crop damage. During non-canola phases of the rotation, controlling brassica weeds– volunteer canola, mustards, shepherds purse, flixweed and stinkweed – is important to reduce inoculum levels in fields.

Harding’s other advice is to keep soil in its own field. “Minimize tillage operations,” he says. “That limits the spread of disease organism. Whether or not you sanitize equipment between fields or farms is an individual call when you’re busy, but even knocking soil off helps. I definitely recommend sanitizing before moving in equipment or vehicles from an area where clubroot exists.”