A canola disease that still lingers

More than 90 per cent 
of surveyed canola fields had blackleg

Blackleg: One of the oldest diseases in canola is still one of the most common.

While many canola growers are worrying about new diseases like verticillium wilt or the growing threat of clubroot, it may be an old threat that causes them the most problems in the coming seasons — blackleg.

“We’ve seen a continued trend of increased prevalence,” said Angela Brackenreed, speaking at Brandon’s Keystone Centre during Ag Days. “We’re getting to over 90 per cent prevalence, meaning over 90 per cent of the fields we surveyed (in 2014) had some level of blackleg.”

Brackenreed, an agronomist with the Canola Council of Canada, said that, on average close to 25 per cent of plants in infected fields showed signs of the disease last summer, although the level of infection on each plant was at the lower end of the spectrum.

“Plant infection wasn’t extremely bad, but it was high enough that we can expect that there was a yield drag,” she said. “It’s something that we should be managing or else it will get to the point where there is significant yield loss potential.”

Complicating the situation this year was the appearance of root rot, which in some cases made identifying blackleg more difficult.

“Producers wanted to know what came first, was it blackleg or was it root rot, and the reality is that by observation that was difficult to tell,” said the agronomist.

“But from what I saw, they were likely occurring in tandem, so it’s most important for producers to know that blackleg was there and it was able to infect. And if you’re using an R-rated variety, you need to observe these things and understand that there is infection occurring, regardless of if the root rot came in first.”

As always, scouting for the disease is key, she stressed. Farmers should be going into the field, snipping plants at their bases and looking for the characteristic blackening that gives the disease its name.

Producers may also want to re-evaluate their rotation schedules, in addition to the canola varieties they’re growing.

“Blackleg has seen a big increase in prevalence over the past number of years and a big reason for that is that the pathogen is shifting so that it can infect plants that currently have resistance,” Brackenreed explained. “The resistance mechanism that we are relying on is called major gene resistance… which means that plant is relying on one gene to confer resistance, in comparison to minor gene resistance this is a good type of resistance because it usually confers almost immunity.”

The downside of major gene resistance is that it can be broken down fairly easily. Blackleg has a sexual and asexual cycle, which allows the pathogen to reconfigure itself to overcome resistance and few if any varieties on the market today address this.

“Unfortunately… most of our commercial varieties are relying on the same sort of major gene resistance,” Brackenreed said.

What producers can do is lengthen their crop rotations to lessen the armour of spores or pathogens in their fields.

“The canola council’s traditional messaging came mostly from blackleg management, and the reason for that is that blackleg can overwinter on the stubble,” said the agronomist. “Over three or four years they suspected that the stubble had enough time to decompose and that spore load would be low enough, or acceptable enough that we can go back into canola.”

Waiting longer to go back to canola and changing varieties may help keep the disease manageable.

“Because this is something that we definitely need and want to manage,” she added.

“Blackleg has seen a big increase in prevalence over the past number of years and a big reason for that is that the pathogen is shifting so that it can infect plants that currently have resistance.”

About the author


Shannon VanRaes is a journalist and photojournalist at the Manitoba Co-operator. She also writes a weekly urban affairs column for Metro Winnipeg, and has previously reported for the Winnipeg Sun, Outwords Magazine and the Portage Daily Graphic.



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