Not the usual: New hot, dry foe for canola

Verticillium stripe looks like blackleg or sclerotinia, but it’s neither and its been growing in recent droughty conditions

Canola growers in Manitoba have increasingly reported verticillium stripe in their crops this summer, but it’s still unknown how much yield damage the disease can inflict.

Verticillium stripe is a stem disease caused by the fungal pathogen verticillium longisporum that causes stem cross-section discolouration that can look somewhat like blackleg and bleaching of the stem that can look somewhat like sclerotinia stem rot, said Justine Cornelsen of the Canola Council of Canada.

She said verticillium stripe thrives in the hot and dry weather, which large swaths of the canola-growing areas in the Prairies experienced this summer.

“I think that’s why we’re seeing more symptoms of it this year, and we know what we’re looking for. The last few years have been dry so we kind of built up that inoculum in the soil and that’s why we are starting to notice it a lot more,” Cornelsen said.

It’s common for canola growers in Europe and Australia to have to contend with verticillium stripe in their canola crops.

Canadian potato growers also deal with a verticillium species, but it’s a separate species that affects brassica crops.

Cornelsen said the Canadian Food Inspection Agency did an extensive survey in 2015 and it was able to isolate verticillium longisporum across six provinces, including the Prairie Provinces.

Even though the disease can be found across the Prairie canola growing regions so far it has only been a problem reported in Manitoba.

“I can find it from the Red River Valley all the way up to the Swan River Valley,” Cornelsen said.

The Canola Council of Canada is involved in research projects funded through the Canadian Agricultural Partnership program that examine the disease in Canada, including how big of an economic bite it can take out of a canola crop.

“Originally we thought that it came in a little too late to cause a real yield loss. Usually it’s taken up by the plants around flowering or right prior to flowering and by the time it gets its way up into the stem we are already harvesting,” Cornelsen said.

“Over in Europe I believe they report something like 30 percent yield loss from this disease, but they are also growing winter varieties that are in the ground much longer.”

A reason it’s difficult to know how much yield loss the disease causes is because other canola diseases like blackleg and sclerotinia are often present where verticillium stripe is found, so it’s difficult to know where the yield loss is coming from.

“But in some of the fields we are seeing that premature ripening and the plants are shelling out. It can cause damage but we’re not sure what the perfect conditions need to be,” Cornelsen said.

Verticillium stripe is a soil borne disease that is taken up by the root and moves up to the stem, which is where the visible symptoms of it are typically found.

Because it’s a soil borne disease management is similar to that of clubroot.

“It has micro sclerotia, which are the equivalent of clubroom resting spores,” Cornelsen said.

“They will move with the dirt, they’ll move on the combine during harvesting. That’s the one real concern.”

Symptoms of verticillium stripe are easier to identify around harvest. Growers should look for bleaching that can be on one or all sides of mature stems.

After harvest, infected stems will have a peeling of the outer skin of the stem, the epidermis, with signs of micro sclerotia just under the outer layer.

“Half of the stem will be shriveled in or shrunken in and the other half will be green and look good. That’s a characteristic feature of verticillium, and I think that in the past it’s been misdiagnosed as fusarium wilt,” Cornelsen said.

She said verticillium micro sclerotia are small grey spots similar to the specks inside a blackleg lesion. However, verticillium specks are smaller, more numerous and under the epidermis while blackleg specks are always on the surface.

Sclerotinia-infected stems are brittle and when shredded open will have larger mouse-dropping sized sclerotia inside the stem.

“That’s also a real key piece when you’re trying to diagnose it between blackleg, or grey stem, both of those pathogens are going to be on the outside of the stem, and grey stem you can scrape off with your nail. Whereas the verticillium micro sclerotia are underneath that outer stem layer.”

Symptoms of verticillium stripe can also be found in the root tissue. At first glance the roots will look healthy, but when they are cut into, as is done when scouting for blackleg, there will be grey or brown discolouration across the cross section and there will be micro sclerotia develop there also, Cornelsen said.

“With blackleg there is usually a big predominant black dot, verticillium will cover that entire root cross section,” she said.

It’s important growers know how to distinguish verticillium from other diseases, because they have options if it’s another disease such as blackleg damaging their crop.

“We obviously don’t know a lot about verticillium yet, but because it is soil borne and can live in the soil for a considerable amount of time. Extended crop rotations, for any plant disease is a big part that’s allowing you to reduce your inoculum over time. Sanitation, so the cleaning of equipment to minimize soil movement is also important”.

Reducing tillage, controlling host weeds like wild mustard and increasing soil fertility can also improve canola hardiness.

So far there are no seed treatments, fungicides, or varietal recommendations that can help to control verticillium.

Manitoba Canola Growers Association is offering free verticillium stripe testing of canola samples submitted to the PSI Lab in Winnipeg, and Cornelsen said if growers see the disease in the other prairie provinces they should send it to the provincial diagnostic labs to help get it on their radar.

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