Verticillium stripe – Identification and management

Verticillium stripe, a relatively new disease of canola in Western Canada, is causing yield loss in some fields. Researchers don’t yet have a way to measure that loss, but they’re working on it.

Caused by the pathogen Verticillium longisporum, verticillium stripe was first detected in Manitoba in 2014. In 2015, Canadian Food Inspection Agency (CFIA) surveyors used soil DNA testing to identify the pathogen in six provinces, including all three Prairie provinces. By 2020 and 2021, the incidence rate in some fields was enough to cause yield loss. The Manitoba 2021 disease survey found it in 30 per cent of fields surveyed, and in fields with the disease, 15 per cent of plants, on average, had symptoms.

Identification

The soil-borne fungus infects roots and travels up the water-transporting xylem in the stem. It will eventually plug the xylem, cutting off the flow of nutrients. The Canola Encyclopedia has more details, including a life cycle graphic.

With early infection, half of the stem can be healthy and green while the other half is discoloured and diseased. This is where the “stripe” name comes from. Leaves can show similar symptoms – healthy on one side, diseased on the other. The only other disease to cause similar symptoms is fusarium wilt, but current canola cultivars all have resistance to that pathogen.

The disease is easiest to scout at or after harvest when symptoms are most obvious. Look for brittle stems with a peeling outer layer. As verticillium infection advances, microsclerotia – that look like tiny grey specks – will show up on the underside of peeling stem skin and throughout the inside of the stem. These specks may seem similar to blackleg pycnidia, but they’re much smaller. Both blackleg and verticillium stripe will cause a darkening of the stem cross section, but blackleg tends to be darker and cause distinct wedge shapes of black. Verticillium stripe tends to be grey and somewhat diffuse throughout the stem cross section, and gets continually darker as microsclerotia build up. (See the table below for comparisons.) Severely diseased stems can break, especially with high winds, and can be confused with lodging.

Wang works with canola pathologists Stephen Strelkov and Sheau-Fang Hwang to research a verticillium stripe rating scale for canola. Wang is doing field work in Alberta to measure the level of disease. “I did see yield loss related to verticillium stripe infection in 2021,” she says.

Yixiao Wang, a PhD student in applied plant pathology at the University of Alberta, discovered another way to distinguish blackleg from verticillium. Blackleg stem infection is concentrated in the crown – the point at ground level where root and stem meet. Verticillium darkening can extend well up the stem. By splitting infected stems lengthwise, Wang discovered she could distinguish the two diseases based on how far the darkening stretches longitudinally.

For a final confirmation, some labs (PSI Labs, Discovery and 20/20) will test plant tissue for the verticillium pathogens.

Cross section of canola stems, clipped through the root just below ground level.
Verticillium darkening (above) can extend well up the stem while blackleg stem infection is concentrated in the crown. Photo credit: Yixiao Wang
Blackleg stem infection (above) is concentrated in the crown – the point at ground level where root and stem meet, while verticillium darkening can extend well up the stem. By splitting infected stems lengthwise, University of Alberta researcher Yixiao Wang discovered she could distinguish the two diseases based on how far the darkening stretches. Photo credit: Yixiao Wang
Late season verticillium stripe symptoms: Peeling outer layer of skin with microsclerotia – the tiny specks.

Management

Distinguishing verticillium stripe from sclerotinia stem rot and blackleg is important for farm management planning.

Verticillium microsclerotia are soil-borne, so anything that keeps soil in place – like equipment sanitation and reduced tillage – will reduce spread of the pathogen.

No fungicide or soil amendment is known to be effective on verticillium stripe.

Crop rotation and genetics are likely to provide some benefit, but there is not enough research to make specific recommendations at this time. In general, two- or three-year breaks between canola crops is good for disease management. Dilantha Fernando, professor in the department of plant science at the University of Manitoba, leads a study “Verticillium disease – etiology and nursery“. Results so far suggest the pathogen survives in the soil for a number of years.

Fernando’s lab has found genetic resistance in Canadian and Chinese germplasm, but differences among commercial canola cultivars have not been identified. While waiting for seed companies to identify resistance levels, genetic diversity from using different cultivars could buffer the risk. Hossein Borhan is drilling down into the genes involved in resistance. The Agriculture and Agri-Food Canada researcher from Saskatoon leads a study called “Genetics and genomics of Brassica-Verticillium interaction”. The work includes screening 50 Brassica napus (canola) lines within the AAFC nested association mapping (NAM) program to compare their verticillium stripe resistance. The NAM system is a tool to show genetic differences among canola lines and isolate genes responsible for those differences. Some lines are resistant. Borhan’s lab is mapping the location of resistance genes to generate markers that canola breeders could use to develop resistant canola cultivars.

More…