For the longest time, yield loss due to blackleg has been significantly reduced through the use of disease-resistant canola varieties. However, those genetics were first introduced nearly 20 years ago and are now starting to be overcome. For a disease that has mostly been ignored by farmers, this means there is a need to reintroduce blackleg mitigation strategies. Justine Cornelsen, Canola Council of Canada’s lead blackleg specialist, gives a rundown on what farmers need to know.
Currently, blackleg is present across the Prairies but at low levels. The disease rears its head a number of times throughout the season, but the best time to scout is right before swath timing when growers see 60 per cent seed colour change, says Cornelsen.
“That way, it’s still easy enough to identify the differences between the different plant diseases,” she says. “If you go later in the season, plants start to really decompose so you can’t really tell what’s causing the issue or what the symptoms are.”
To scout, go into the field, pull plants up and cut the plant through the root material right at the base of the stem and look for internal blackening. “Of course, not everything within that cross-section is necessarily blackleg, but it’s usually the predominant disease that we’re dealing with, and if you pull up enough plants you start to tell the difference between other diseases,” she says.
To mitigate yield loss, growers first need to know what they’re dealing with. “I think that’s where blackleg goes undiagnosed,” says Cornelsen. “Producers aren’t necessarily looking for it. Because of the introduction of resistant varieties, blackleg was kind of put on the back burner.”
With a good rotation — canola once every three years — farmers should be fine in terms of blackleg. Infected residue will break up and disintegrate under an extended rotation, says Cornelsen.
“The longer you’re out of canola and managing volunteers will help you to really lessen that inoculum in the field,” she added. “Scouting, crop rotation and the use of resistant varieties — those are our top three. If you’re using those three, then you should have no issues.”
Where growers are seeing issues is where rotations have tightened, building up the perfect storm for pathogens. In areas that restrict crop options, Cornelsen suggests that growers rotate genetics effectively.
At one time, many varieties had no resistance at all. “If you’ve never walked into a disease nursery or seen Westar — a susceptible variety — growing, it’s unbelievable seeing what this pathogen can do,” says Cornelsen.
Today, all canola varieties available in Canada are resistant to blackleg, but some genetics will better match a particular race of blackleg than others. For best results, test stubble to identify the race or strain you’re dealing with and choose the best variety accordingly.
When to apply fungicide
Growers who want to apply fungicides will want to scout for blackleg first thing in the spring. Look for old residue that is housing spores and leaf lesions on cotyledon canola. Assess if conditions for spore production have been ideal.
Fungicide use is effective when applied at the correct timing and on susceptible varieties. “When you know your resistant variety isn’t working as effectively as it should that’s where an early-season fungicide application would be warranted,” says Cornelsen. “Of course, the earlier the better. You’ve got to coat plants before the pathogen is producing spores.
“And that’s why it’s not widely used here in Canada,” she adds. “Because you’re pretty much spraying dirt when you’re spraying cotyledon canola.”
The recommendation for foliar fungicides is to spray at the two- to six-leaf stages. In the near future, expect fungicide seed treatments for blackleg, which will give canola seedlings protection during that stage where there’s the greatest risk for yield loss.
Clearing up confusion
There are a couple of confusing points around blackleg and canola disease in general, says Cornelsen. For example, blackleg is a stubble, residue-borne disease, whereas clubroot is soil-borne.
In Manitoba, in particular, growers are struggling to tell the difference between blackleg and verticillium stripe. The first case of verticillium stripe was found in Canada in 2014. Its impact on canola has yet to be determined.
“We’re thinking with some of our disease surveying here in Manitoba, we’ve probably been calling things blackleg when it’s actually verticillium stripe,” says Cornelsen.
Prior to swathing, blackleg will be prominent in stem cross-sections with black spots, whereas verticillium will be very subtle at that time, explains Cornelsen. Potentially, verticillium stripe will have a grey-brownish hue, starburst pattern or grey specks. Blackleg will produce pycnidia spores on the outside of the stem, she says. Verticillium stripe will have microsclerotia below the outer stem wall, but the outer layer must be peeled back to view them.
Symptoms for verticillium stripe are better seen later in the season, post-harvest.