There have been few confirmed cases of clubroot in southern Alberta but recent confirmation of the canola disease in Rocky View County, southeast of Calgary, shows the region is not immune.
Clubroot leaped onto the crop disease scene in 2003, in areas around Edmonton. Since then it has steadily spread outward and has also been identified in Saskatchewan and Manitoba canola fields.
Southern Alberta appeared to be a bit of an oasis, perhaps leading farmers there toward complacency.
“There’s been a feeling down here, when we talk about clubroot, that a lot of growers think that it just won’t come here, whether it’s our higher pH or our lower moisture,” said southern Alberta Canola Council of Canada agronomy specialist Autumn Barnes.
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Michael Harding, Alberta Agriculture crop pathologist, said it’s true that the soil-borne disease hasn’t been as prevalent in the south but any thoughts of immunity are illusory.
Until the Rocky View confirmation, only three fields in southern Alberta, in the County of Newell, had been confirmed with clubroot infection. The first was found in 2007.
“We pretty much went 10 years without a confirmed new positive in southern Alberta until last year. And then last year there were three new fields in the County of Newell that were positive for clubroot … but it was not at the (field) entrance,” Harding said.
“It had flown under the radar in our surveys, and the County of Newell does exhaustive canola field surveillance and nobody saw it because it wasn’t at the entrance.”
His reference to field entrances relates to original survey work on clubroot by Stephen Strelkov, a University of Alberta professor and researcher who has led clubroot surveillance. He found that in fields infected with the disease, it was present at the field entrance 90 percent of the time.
That led future surveyors to test field entrances and move on to other fields if no clubroot spores were found. However, the disease has since been found inside fields that had no evidence of clubroot at the entrance.
Harding said that wasn’t necessarily a revelation. “Soil moves in all kinds of ways. It doesn’t surprise me that much at all that we could find it anywhere in the field.”
Its assumed clubroot spreads from soil that drops off farm equipment moved from field to field. Folding and unfolding that equipment at field entrances is common.
However, a farmer might use a different entrance or start work in a field at some other spot, thus explaining clubroot’s presence elsewhere on the site.
It can also spread by wind and water erosion, ATV or myriad other ways.
Even so, clubroot hasn’t made the same inroads in the south as it has elsewhere.
“The fact that we’ve gone 10 years, from 2007 to 2017, without a confirmed positive indicates to me … that the conditions just aren’t quite as conducive for that rapid spread,” said Harding.
“It could be our soil conditions or soil type or our dryer climate or whatever, that’s sort of giving us a bit of an advantage and so it could be that the pathogen is introduced at the field entrance but it doesn’t cause any field symptoms and then it gets spread around a little bit until it finds a pocket where the environment is favourable and then that’s where you start to see field symptoms.”
The other southern Alberta factor is irrigation. Farmers with access to it tend to have more crop options leading to longer rotations. Many higher-value crops are based on contracts with processors, another potential factor.
“A lot of those crops are grown under contracts that specifically stipulate that it should be a one in four rotation, or you can’t grow (the same crop) two years in a row,” said Harding.
That supports the concept that longer rotations help manage clubroot.
“People should certainly not be growing canola in tight rotation and they should be considering using a resistant variety, especially in counties where there are confirmed cases,” he said.
Given clubroot’s relative ease of spread, it’s difficult if not impossible in many cases to determine how it gets into a particular field.
“We are rarely going to know for sure how it was introduced,” said Harding.
In its clubroot information, the canola council recommends two- to three-year rotations as one tool against the disease.
“No, crop rotation will not stop the spread of clubroot since this disease travels by soil movement,” it says in a Q and A brochure.
“However, crop rotation does slow the buildup of clubroot resting spores, which slows the development of pathotypes virulent on canola and extends the effective durability of clubroot resistant varieties. Protect current clubroot resistance genes and reduce spore numbers by allowing two to three year breaks between canola crops on infested soils.”
