Anyone in the canola industry banking on clubroot-resistant varieties alone to stop the disease is in for a shock this spring, as Alberta researchers have discovered 16 more fields where resistance has broken.
In 2014 researchers identified a clubroot pathotype, christened 5x, that had worn away resistance. Not all the resistance-busting pathotypes in the 16 new fields have been identified as 5x, although Dan Orchard says those results are preliminary.
“So there could be many (virulent) strains out there,” says Orchard, an agronomist with the Canola Council of Canada.
The new fields are spread throughout Alberta’s clubroot zone in central Alberta, Orchard says. They aren’t clustered near the original field with the 5x pathotype, he adds.
“It seems like they’re all independent of each other,” Orchard says. “There’s no real relationship to think that they had the same equipment or shared anything.”
Some of the infested fields are also in areas where clubroot traditionally hasn’t had high spore loads, which Orchard says comes as a bit of a surprise. Resistance wasn’t expected to break in areas where spore loads weren’t high, he explains. The next step is to find out whether the 16 fields individually have sky-high spore loads that weren’t identified for some reason.
Orchard isn’t familiar with all the fields, but he says some were under tight rotations.
Dr. Stephen Strelkov, a University of Alberta researcher, is heading up the next round of research. He’ll look at whether any canola varieties can fight off these new clubroot pathotypes, Orchard says. He’ll also confirm which strains the new, unidentified pathotypes belong to.
But there are several different varieties in the 16 fields, Orchard says, and they all seem to have the same clubroot resistance genetics. “So it would be unlikely that it’s just (specific to) that variety in that field.”
Strelkov’s earlier work showed that there appeared to be some different sources of clubroot resistance being used by different canola seed brands, Orchard says.
“But they don’t appear to be enough to say that rotating between Company A and Company B is going to slow down the shift because none of them are resistant to this 5x.”
Canterra Seeds is releasing a new variety — CS2000 — that has an intermediate reaction level to the 5x pathogen. Orchard explains that rating means plants will have between 30 to 69 per cent infection levels compared to the clubroot-susceptible check.
A canola variety rated resistant to clubroot has to rate 70 per cent compared to a susceptible check. That means in a field of clubroot-resistant canola, up to 30 per cent of the plants could potentially be infected. But in-house standards for seed companies would be in the 90 percentile for resistance, says Clint Jurke, agronomy director for the Council.
At any rate, this range of allowable infection made it more difficult to catch the resistance breakdown early on, Orchard says.
Old rules have come full circle
Orchard says the old rules that farmers were reluctant to follow “have come full circle.”
Chief among those is cleaning equipment. The Canola Council hasn’t asked farmers to clean equipment between each field for years now, says Jurke. Instead, they’re asking farmers to look at their own risk level before deciding how much cleaning they should do.
If a farmer is scouting diligently and he knows his farm is clubroot-free, Orchard still recommends knocking large dirt lumps from equipment.
Farmers whose fields are free of infestations, but who are on the fringe of the clubroot zone, will want to clean a little more thoroughly, Orchard says. They may also want to keep equipment from other farms out of their fields, he adds.
Farmers with infested fields will want to start field operations in the cleanest fields and work their way towards the most heavily-infested. They should clean equipment thoroughly once they’ve finished the worst field.
That approach is more realistic, Orchard says, “because farmers just do not have the time to do the sanitation that’s been asked by the industry.”
Equipment is the main way clubroot-contaminated soil infests new fields, but it can also blow in from eroded fields, Orchard says. That means farmers can’t skip scouting even if their equipment is pristine.
Jurke also notes that the disease may have arrived in some fields years ago, but escaped observation.
Both Jurke and Orchard emphasize that the greater the spore load, the more quickly clubroot will overcome resistance.
“Farmers need to identify it really early to keep that spore load as low as possible,” says Orchard. “And that way, the odds of it breaking resistance are much, much slimmer.”
Both Orchard and Jurke recommend using resistant varieties before farms have clubroot to prevent spore loads from building to uncontrollable levels. Clubroot seems to spread about 30 km a year, so farmers within 30 km of an infested field should use clubroot-resistant varieties, Orchard says.
Orchard also recommends farmers create a management plan even if their fields are still clubroot-free. For example, Orchard finds energy companies tend to comply with field entry protocols if farmers have them in place already.
In areas with few cropping choices, it’s even more important farmers keep clubroot out, Orchard says, to make sure they can potentially survive on a shorter rotation.
Rotate, rotate, rotate, rotate
The more often farmers grow clubroot-resistant canola, the more selection pressure they’ll apply to the clubroot pathogen, Orchard says.
Early greenhouse research showed that clubroot resistance started to erode within two canola crops, regardless of the time between those crops, Orchard says. The resistance break-down would be evident by the third canola crop. And that’s almost exactly what happened in the field, he says.
What this means on a practical level is that farmers with a two-year rotation have grown three resistant crops since clubroot resistance was introduced in 2009. These farmers can already see dead patches of canola from the swather, Orchard explains.
But farmers using a three-year rotation with clubroot-resistant varieties have less severe infestations right now because they’ve only used resistant varieties twice so far.
“We can scout their fields and find very small patches and evidence of this happening. But it’s not visible from a swather or a pick-up truck,” says Orchard.
Longer rotations not only slow the resistance erosion, but also give researchers more time to get ahead of the disease, Orchard says.
Orchard understands it’s a challenge to find other crops to lengthen rotations, but adds that it’s an important management practice for clubroot and other crop diseases.
But ultimately rotation alone isn’t enough to manage clubroot resistance.
Jurke explains that even with a four-year rotation, farmers using susceptible canola varieties will boost the number of virulent spores. In fact, a four-year rotation with susceptible varieties will build up more virulent spores than a two-year rotation with resistant varieties, assuming both farms start with low spore loads, he says.
The mantra must be to rotate crops and not use susceptible varieties anymore, says Jurke.