Farmers’ poor stewardship has resulted in the loss of one weapon in the war against clubroot.
Since the disease was first discovered in Alberta, and later in Manitoba and Saskatchewan, agronomists have recommended farmers practice good crop rotation to keep clubroot damage at bay.
However, attractive canola prices prompted many to plant canola on canola or canola every other year in the same field.
Now varieties once resistant to clubroot have lost that resistance to some forms of the disease and it will be years before more can be developed.
Alberta Agriculture oilseed specialist Murray Hartman delivered this sobering message at a Brooks, Alta., field day and a Lethbridge canola disease crop walk in recent weeks.
“We were warning that this could happen,” said Hartman. “It didn’t seem to be taking a lot of traction with the growers because … they were growing canola more than once every four years.
“Some had four canola crops in five years. Is that stewardship?”
A new pathotype of clubroot recently identified in Alberta has overcome resistance bred into certain varieties of canola. Now there are no varieties resistant to that particular strain.
That doesn’t mean the nastier form of clubroot is present everywhere, but it is a signal that resistance will eventually break down.
Hartman said growers are always hopeful that new resistant lines will be developed in time to combat the problem before it affects their yield and profits, but that won’t happen for five to 10 years.
Polish rapeseed and some vegetables have clubroot resistance, but it is difficult to transfer that into Argentine canola.
Failing that, farmers have two options: planting canola less frequently and not planting it at all in areas of heavy clubroot infestation.
“The worst thing you can do, then, is canola every second year or back to back on a clubroot field that’s got really heavy infestation,” said Hartman. “Once you’ve got clubroot in the field, you really have to back off to a one in four rotation. Otherwise you’re going to really pressure for that resistance to break down.”
Some farmers also erred by planting non-resistant canola because clubroot hadn’t yet been found in their area or fields.
“If you would have planted a resistant variety and really slowed the establishment in the field, you would have really extended the durability of that variety a lot longer (rather) than waiting until you have a problem and then putting strong selection pressure on it.
“So some of the producers I think took a backward strategy,” said Hartman.
And once clubroot is in a field, it is there to stay. Crop rotation doesn’t rid fields of disease spores. It only prolongs resistance until other solutions can be found.
Hartman said researchers plan to survey fields and discover how many have the new clubroot pathotype that defies available resistance.
Besides canola, clubroot can affect all other members of the cabbage and mustard families. That has implications for weed control.
Hartman said galls with clubroot spores can develop on plants within two or three weeks, creating a reservoir for the spread of infection.
Volunteer canola, mustard and stinkweed could be that reservoir if not controlled within two or three weeks of growth.
“We might be considering things like more a fall application of herbicides program to control volunteer canola but also some of the winter annuals in the brassica family to prevent clubroot spores from being produced,” he said.
Alberta has found a high number of clubroot-infected fields, particularly in the Edmonton area. Relatively few have been found in the other two prairie provinces.
But once Saskatchewan and Manitoba have more cases, Hartman said there will likely be a greater push there to plant only resistant varieties of canola.