Last year’s flooding may have been bad news for canola growers, but there may be a bright side.
Studies have found that flooding results in reduced sclerotia inoculum present in the soil because the fungal spores rot under wet conditions.
“Although sclerotia are built to withstand harsh conditions, if you have enough flooding in an area, possibly up to 65 per cent of the total sclerotia in the soil can rot,” said Holly Derksen, a field crop pathologist with MAFRI, at a recent Excess Moisture Meeting hosted by the Canola Council of Canada.
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But don’t break out the champagne. The spores can last up to seven years in the soil.
“However, 65 per cent is not 100 per cent. If 35 per cent of the sclerotia remains behind, unfortunately, that’s more than enough to cause a disease issue in your field.”
Sclerotinia problems may have fallen off last year, but blackleg, which thrives under a variety of conditions, continued on as before.
Blackleg in canola over the past five years has remained steady, with 50-60 per cent of all Manitoba fields showing some degree of infection.
The number of plants within those fields is growing, however.
Five years ago, about five per cent of plants showed signs of blackleg, and now that figure has risen to 10-15 per cent.
“The more we push the canola rotations, the more these numbers are going to rise,” she said.
New blackleg-resistant varieties offer only temporary relief. Resistance doesn’t mean zero infection. In fact, it means that infection will continue at a low level and the pathogen population will still be out there.
“It’s going to be able to adapt to the varieties that we’re growing,” said Derksen.
Blackleg spores overwinter as little black bumps on crop residue, and present a management problem throughout the growing season.
Scouting is important, because knowing if fields have it is the first step in controlling the disease. Cutting through infected stems will show discolouration inside, even if lesions are not visible outside.
“That’s why we urge random sampling,” she said. “Go out, pull some plants, then cut them open at the base of the stem.”
Longer rotations
Canola rotations should be greater than three years. If that’s too onerous, rotating resistant varieties can help. It may not make immediate economic sense to plant other crops, but taking a long-term view of cropping choices is best.
Fields that went unseeded in 2011 can only be considered a non-canola year if volunteer canola didn’t appear as the dominant species, she added.
Using fungicides at the two- to six-leaf stage to control blackleg only offers a 10- to 14-day window, basically a “head start” for the growing plants. After that, thepathogen can return and infect the stems and lower leaves.
Clubroot hasn’t arrived yet in Manitoba from its base in Alberta. The spores can survive up to 20 years in a dormant state, so practising good sanitation both froma whole-farm and a field-to-field perspective is wise.
Wet soil is stickier than dry soil, so taking the extra effort to clean up equipment in wet years is also a good idea. Just removing the dirt gets rid of 99 per cent of the pathogens, she added.
Neighbours who push rotations can send sclerotinia spores drifting into nearby crops, but with blackleg, the pathogen isn’t as mobile. In such cases, problems may be limited to adjacent field edges.
For cereal growers, wheat streak mosaic virus presents a “scary” threat, with the Pilot Mound and Manitou areas lately becoming “hot spots” for the disease.
Widespread early seeding of winter wheat last fall may have created a “green bridge” window for the virus to jump over from the ripening spring wheat crop.
“This spring, if you do have a lot of wheat streak mosaic symptoms in your winter wheat, consider destroying that crop and growing a non-cereal,” she said.
