Look closely for verticillium stripe in 2022
By Clint Jurke
Verticillium stripe caused yield loss in some canola fields in 2020 and 2021, and probably will again in 2022. The disease can be confused with blackleg, sclerotinia stem rot and lodging, and – unlike these other common problems – verticillium stripe tends to be worse in dry conditions.
Verticillium stripe, caused by the pathogen Verticillium longisporum, was first detected in canola Manitoba in 2014, and surveys in 2015 found the pathogen’s DNA all across the Prairies. It seems to be getting worse – likely because it is still establishing itself. The bottom line: It’s here and we want people to know about it. That is why SaskCanola held a verticillium workshop in February. A video recording of the workshop is posted at www.youtube.com/canolacouncil.
Identification
Symptoms are easiest to see at harvest. Look for brittle stems with a peeling outer layer. Some stems may be lodged or broken. Tiny specks called microsclerotia form under the peeling skin.
Verticillium stripe, like blackleg, will cause discolouration inside the stem. Blackleg stem infection tends to be darker and moderate levels of infection cause distinct wedge shapes of black. (Severe blackleg will leave the stem completely black and dead.) Verticillium stripe tends to cause grey discolouration throughout the stem cross section, getting continually darker as microsclerotia build up.
At the workshop, Yixiao Wang, a University of Alberta PhD student researching verticillium stripe, shared 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. Using a knife to split stems longitudinally, Wang discovered she could distinguish the two diseases based on how far the darkening extended up the stem.
Management
We don’t have a fungicide option, so management comes down to genetics and rotation. Unfortunately we don’t have confirmed recommendations for either of those options.
All four presenters at the workshop are researching genetics, and they have found clear differences. Some canola breeding lines are quite resistant. Some are very susceptible. But we don’t know the level of resistance in commercial cultivars in Canada, and we don’t have an official method to rate this resistance.
For rotation, anecdotal evidence suggests that longer breaks between canola crops may help to reduce disease severity. It certainly works for blackleg and clubroot. We need more research to tell us how long a break will provide an effective reduction in soil inoculum levels of the verticillium pathogen.
The good news is that research continues in Alberta, Saskatchewan and Manitoba. These studies will help us learn more about the pathogen and the disease. The Canola Research Hub at canolaresearch.ca has updates for ongoing and completed projects. Enter “verticillium” in the keyword search.
In the meantime, keep an eye out for verticillium stripe in your canola in 2022. For more images, identification tips and a lifecycle graphic, please see the verticillium stripe chapter in the Diseases section at canolaencyclopedia.ca.
Clint Jurke is agronomy director for the Canola Council of Canada. Email jurkec@canolacouncil.org.
Verticillium stripe symptoms are easiest to see at harvest. Look for brittle stems with a peeling outer layer. Some stems may be lodged or broken. Tiny dark specks called microsclerotia form under the peeling skin. Credit: CCC
Verticillium stripe, like blackleg, will cause discolouration inside the stem. Blackleg infection (centre) tends to be darker with distinct wedge shapes. Verticillium stripe tends to cause grey discolouration throughout the stem cross section (left). Credit: CCC
