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What you thought was sclerotinia might be verticillium stripe, and the Canola Council is making it a research priority

Verticillium microsclerotia are small grey spots, almost like the pycnidia inside a blackleg lesion. Verticillium specks are tinier, more numerous and under the epidermis, while blackleg fruiting bodies are always on the surface.

It first appeared in Manitoba fields in 2014, and now it’s showing up in Manitoba’s crop reports. “Canola yields tend to be disappointing for farmers, with heat blast and verticillium stripe suspected as contributing factors,” said a provincial summary in mid-September.

The Canola Council of Canada (CCC) says verticillium stripe (VS) is caused by V. longisporum, a soil-borne fungus that infects roots and travels up the water-transporting xylem, plugging it late in the growing season. It can kill part of the plant or the entire plant. Surveys by the CCC and the Canadian Food Inspection Agency have confirmed the presence of the pathogen in canola stubble from British Columbia to Quebec and it’s now a CCC research priority.

Justine Cornelsen, CCC’s western Manitoba and southeastern Saskatchewan agronomy specialist, is among those keeping a close eye on the pathogen. She confirms that more VS was seen in Manitoba this year.

“We had a delayed spring with a lot of moisture in July and hot and dry conditions in August. We don’t know about yield implications. On a normal year, we think the pathogen comes in too late to impact yield. Last year, growers were harvesting into October and that’s when it was noticed.”

Preliminary results of the Manitoba disease surveys were to be released at the Western Forum on Pest Management by the end of October. Manitoba is the only province that does a comprehensive canola disease survey.

Discovering resistant lines

The CCC and others are sponsoring several VS research projects, including investigations of pathogen biology and canola resistance genetics by University of Manitoba professors Dilantha Fernando and Mario Tenuta. Some of the genetics research is being conducted in tandem with Hossein Borhan at Agriculture and Agri-food Canada in Saskatoon. There are four main projects.

“The first one involves screening our own line germplasm and that of seed companies that wanted to take part,” Fernando says. “It’s the first year for that and we’re still gathering data. The results for private company germplasm will be given confidentially to each company involved and results for the entire screening project will be published in a year or two, but the companies will not be named.

“Once the team has resistant germplasm identified, analysis of the genes present in our lines will be undertaken, and perhaps that of company lines as well, if they wish to take part. This was what we did with blackleg and it worked very well.”

A second project involves looking at the distance the pathogen can travel through the soil from the inoculum source in the stubble. Susceptible varieties have been planted at various distances from sources. The third project is a look at how long the microsclerotia can survive in the soil, and the fourth is an investigation of which V. longisporum genes might be relevant to its pathogenicity.

After harvest, infected stems will have a peeling epidermis (the outer skin of the stem) with signs of microsclerotia just under that outer layer. photo: Justine Cornelsen, CCC

Meanwhile, Sheau-Fang Hwang and Stephen Strelkov, professors of plant pathology at the University of Alberta, have a long-term project in which they are looking at how inoculum density affects yield severity, with greenhouse studies and eventually field studies with various cultivars. This will enable them to create a yield impact model. Hwang says evaluation of yield losses from VS will be important to help farmers and industry determine whether control measures are warranted.

These scientists are also investigating how infection levels are affected by plant growth stage and inoculation techniques (spore or microsclerotia). This will facilitate further research to evaluate control measures and also to more effectively screen genetic material for resistance.

Identification critical

The CCC is encouraging producers to learn to identify VS, since management options differ from other diseases.

Later in the season, VS is easy to identify. A recent Canola Watch newsletter says “look for bleaching of mature stems, noting that bleaching could be on one side or all around the stem. After harvest, infected stems will have a peeling epidermis (the outer skin of the stem) with signs of microsclerotia just under that outer layer. This is a distinct symptom of VS.”

While sclerotinia-infected stems will be brittle with a texture similar to tissue paper, with larger mouse dropping-sized sclerotia inside the stem, VS microsclerotia are small grey spots, “almost like the specks inside a blackleg lesion, but verticillium specks are tinier, more numerous and under the epidermis while blackleg specks are always on the surface.”

Before harvest, at around 60 per cent seed colour change, look for senescence, similar to fusarium wilt, of half the stalk. It will be shrunken on one side of the stem. photo: Justine Cornelsen, CCC

Cornelsen says that before harvest, at around 60 per cent seed colour change, “you are looking for senescence, similar to fusarium wilt, of half the stalk. It will be shrunken on one side of the stem. That’s what we’re seeing right now (the start of September). The stalks will shred when pulled at and that happens with sclerotinia, but it’s the half the stem that’s indicative of verticillium.”

Sanitation measures

The CCC says that if VS is confirmed on your farm, you must minimize spread by keeping soil in place, for example through equipment sanitation and reduced tillage. In addition, increase your rotation between canola crops, manage brassica host weeds (like mustards) and increase soil fertility to improve canola hardiness. There are no foliar or seed treatment fungicides registered for control of VS in canola.

For sclerotinia in canola, management is very different, involving fungicide application to flowering crops when moist conditions favour pathogen growth. With blackleg, the three most effective management tools are crop rotation, growing resistant cultivars and rotation of resistant cultivars.

Cornelsen hopes that in a few years, there will be more specific management options for VS.

“We’ll look at options being used in other jurisdictions, but it will likely be similar to that of clubroot, where biosecurity, minimizing of soil movement and extended crop rotation are very important. There are lots of unknowns but research is underway.”


Testing

Because verticillium is more common in Manitoba, the Manitoba Canola Growers Association currently offers free VS testing of canola samples at PSI Lab in Winnipeg. Visit mbpestlab.ca/field-testing/ for more information.

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