Mysterious blackleg affecting Manitoba fields

Two Manitoba canola fields have been destroyed by what is likely a resistant race of blackleg.

Earlier this month, Manitoba Agriculture staff detected blackleg on the stems of canola plants on two fields. However, oilseed and plant disease specialists with the government aren’t disclosing the location of those fields or the canola variety that is infected with blackleg.

Nonetheless, the damage to the fields is extensive, said Holly Derksen of Manitoba Agriculture.

“To the point where plants are falling over and there’s most likely going to be a big yield loss,” she said, noting 40 percent of the canola plants in one field had fallen over because of the stem rot.

At this point, Derksen thinks a unique race of blackleg is causing the damage but laboratory tests will be required to confirm her suspicion.

A distinct race of the pathogen is likely responsible because the infected fields were seeded with a canola variety with resistance to blackleg.

Furthermore, at another field close to the infected fields, a grower seeded the same variety of canola and there are no symptoms of blackleg, Derksen said.

The infected canola fields were grown under a tight oilseed rotation.

Taken together, this case suggests that races of blackleg not only vary from region to region, but from field to field.

“This is showing that the regions (with the same race of blackleg) can be even smaller than we originally thought,” Derksen said.

To be precise, the resistant race of blackleg could be a new and unique version of the disease. It could be a race that isn’t present or prevalent in variety trials, Derksen said.

“It’s definitely unique from wherever they tested (against) in their variety trials.”

Curtis Rempel, vice-president of production for the Canola Council of Canada, isn’t convinced this is a new race of blackleg.

“Dr. Randy Kutcher at AAFC Melfort showed that there are 16 races present in Canada (published in 2010) and it is very likely that the race we are seeing in Manitoba is one of these,” Rempel said, in an e-mail to The Western Producer.

“Research is ongoing to determine how rapidly new races of blackleg can develop. (But) normally blackleg is not a problem when R-rated varieties are used.”

It’s premature to speculate that this is a new race of the disease until laboratory tests are completed, Rempel said.

In a few weeks, Derksen and others will have a better sense of the extent of blackleg in the province because Manitoba’s annual disease survey is now underway.

Over the past five years in Manitoba, the number of canola fields infected with blackleg has been steady.

But when the pathogen does invade a particular field, the damage is now more severe, Derksen noted.

Dilantha Fernando, a plant pathologist and blackleg expert at the University of Manitoba, said every summer new races of the pathogen infect canola that is supposedly resistant.

Consequently, growers shouldn’t put too much stock in blackleg resistance ratings.

“They may be useful as a general guide (as they have been tested in co-op sites against some strains/races),” he noted. “The more races they are tested against, under field conditions, the better.”

Rempel said the existing rating system for blackleg resistance does serve growers, along with germplasm and seed developers, very well.

But, he added, there is a chance that the blackleg resistance of certain canola varieties may not stand up to “intense” disease pressure.

“Disease pressure varies from year to year and so some varieties that may have been developed during periods when blackleg severity was fairly low may show some more susceptibility to the disease when disease pressure is very intense, which appears to be the case this year in parts of Manitoba.”

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