Clubroot resistant lines of canola are under siege in Alberta.
In 2014, researchers identified a new strain of the disease that appeared in six fields in central Alberta in 2013. The strain, known as 5X, was able to overcome resistance built into new varieties.
Last week, researchers announced that a data analysis based on a 2014 survey of clubroot resistant canola fields had uncovered nine additional strains of the disease capable of overcoming existing resistance.
“It certainly complicates our clubroot situation quite a bit,” said Stephen Strelkov, a plant pathologist with the University of Alberta.
“This does definitely suggest that the problem is more widespread than we hoped.”
The results stem from a 2014 survey of fields growing clubroot resistant varieties. Researchers discovered 27 fields with unusually high levels of infection.
Further analysis determined that virulent strains of the disease were found in 16 of those fields, some of which were located 600 kilometres away from the six fields where the 5X virulent pathogen was discovered.
Strelkov hoped it was the same 5X pathogen that cased all the problems last year, but further lab work unveiled nine distinct new virulent strains of the disease.
The strains haven’t been fully characterized or named, but he is certain they are different than 5X.
He believes the outbreak is the direct result of growers pushing rotations and planting the same clubroot resistant variety every couple of years on a field or even in back-to-back years.
“(The farmer) is selecting for a new strain in that particular field, and that’s why they tend to be different from each other,” said Strelkov.
He said it is a risky game to play.
Resistance is holding up in most fields in central Alberta, where the disease is most prevalent, but it won’t last long if farmers continue to tighten rotations.
Growers need to view this development as a major warning sign, he added.
“We certainly don’t want to create more and more of the strains that become harder and harder to handle,” he said.
“If we were to lose clubroot resistance on a widespread scale, we’d be in trouble because genetic resistance is the most important tool we have to manage this disease.”
The new virulent strains will make it more challenging for breeders to create resistant varieties.
Jed Christianson, plant pathologist with Monsanto’s canola breeding group, was anticipating that there was likely more than just the 5X pathogen at work in the 16 Alberta fields.
“Nine was a little bit of a surprise. Nine is a little bit more than we were expecting,” he said.
He does not yet know how big a setback this represents to clubroot resistant canola breeding programs.
“It will depend a little bit on how similar some of these pathotypes are, whether it would be possible to find another single resistance gene that would be resistant to everything,” said Christianson.
He suspects that is unlikely.
Christianson said it is not a good sign that resistance has broken down to this degree five years after the first clubroot resistant variety hit the market.
“The risk is that any new resistance gene you put out will only last for a few cropping cycles,” he said.
It will take a minimum of four years for new lines resistant to the 10 new virulent strains of the clubroot pathogen to be commercially available.
One of the challenges is that many of the resistance traits originate from related species such as brassica rapa, brassica oleracea and vegetables.
“All of these present a little bit more challenge for a breeding organization because you’re working with material that is not canola quality, it’s not adapted to a spring canola growth habit,” said Christianson.
However, he said clubroot resistant lines are still holding up for most growers who are not pushing rotations.
One of the problems confronting Strelkov is that breeders at seed technology companies are unwilling to divulge the source of resistance in their varieties for proprietary reasons.
“We’re kind of operating in a black box,” he said.
“We know a lot about the pathogen but very little about the resistance.”
Another problem is that farmers have become overly reliant on genetic resistance as the only way to combat the spread of the disease.
Two other effective management strategies for those dealing with clubroot are to lengthen rotations and thoroughly clean equipment coming off infected fields.
“Neither of these strategies were popular with farmers and neither was widely adopted,” said Strelkov.
Growing canola in tight rotations is too tempting because it is the major cash crop for most farmers.
Even the Canola Council of Canada recently changed its long-running stance of advocating a one-in-four-year rotation for the crop.
“Growers have taught us that more intensive rotations can be managed sustainably and profitably in many soil zones and regions of the Prairies,” the council said in its 2025 vision document.
Strelkov said that may be the case in fields with no known issues with clubroot and blackleg, but tighter rotations are not an option in areas where disease has been prevalent, especially for growers planting clubroot resistant lines of canola.
Other control strategies have been explored such as liming the soil, applying fungicides and using bait crops and biocontrol agents, but they either gave inconsistent results or were too expensive to be practical.
One that has shown promise is targeted application of the soil fumigant metam sodium as spot treatment in fields where the disease is not yet well established.
“There’s probably a handful of situations where something like this would be useful,” said Strelkov.
Clubroot was first discovered in 12 fields near Edmonton in 2004 and infected more than 1,900 prairie fields last year.
Monitoring activities will continue in 2015. In the meantime, researchers will attempt to learn more about the new virulent strains, such as whether they are derived from the old pathotypes and if they are related from a genetic or evolutionary perspective.
Christianson said progress has been made in sequencing the clubroot genome, which could lead to marker tests that will help growers determine what strains of the disease are in their field and what variety of clubroot resistant canola they should grow.