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.
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.
Yixiao Wang, a PhD student in applied plant pathology at the University of Alberta, 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.
Wang 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.
|wdt_ID||Plant Disease||Sclerotinia||Blackleg||Blackleg||Verticillium Stripe||Grey Stem||Fusarium Wilt|
|1||Species||Sclerotinia sclerotiorum||Leptosphaeria maculans||Leptosphaeria biglobosa||Verticillium longisporum||Pseudocercosporella capsellae||Fusarium oxysporum|
|2||Stem Symptoms and distinguishing features||Bleached white appearance. Infected stems tend to shred and shatter very easily. White mouldy growth. Black sclerotia bodies will form inside the stem.||Stem lesions with pycnidia (black spots) forming inside the lesion. Base of stem (crown) becomes woody. Cross section cut reveals blackening.||Shallow stem lesions with pycnidia||Shredding of the stem tissue. Tiny black microsclerotia form beneath the peeling outer layer.||Large purple to grey-speckled stem. Pod lesions are also possible.||Discolouration of the stems; yellow or reddish-brown streaks on stems.|
|3||Pod||Withered pods if branch below is infected. Sometimes white mould, lesions.||No symptoms||No symptoms||No symptoms||Grey speckled pods.||No symptoms.|
|4||Crown (base of stem) exterior||Cankering|
|5||Crown cross-section||Clean, dried down||Solid black sections, often pie shaped. Fully black in extreme cases.||Typically does not reach stem in time||Greyish hue across entire cut. Gets darker as microsclerotia build up. Can extend many inches up the stem.|
|6||When to scout||Prior to swath timing||Prior to swath timing, 60% SCC.||Prior to swath timing, 60% SCC||Easier to ID post-harvest||Prior to swath timing, 60% SCC.||Prior to swath timing, 60% SCC.|
|7||Yield Loss||Potential yield loss in a field can be determined by: % Potential Yield Loss = % Infection x 0.5||For every unit of increase in disease severity, a 17% loss in plant seed yield can be expected.||Comes in too late in the season to cause a significant impact||Does occur but no system to measure at this time.||Not known.||Varieties are resistant, rare to find.|
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.
- Canola Research Hub has summaries of completed and ongoing verticillium studies, including the Verticillium stripe management one from University of Alberta
- The Hub also features the Verticillium stripe: Researching a new canola disease threat blog
- Canola Encyclopedia has a verticillium stripe lifecycle graphic and more
- Video summarizes verticillium stripe research projects
- Verticillium Stripe Workshop FULL RECORDING