When Verticillium longisporum-infected canola was first discovered in Manitoba in 2014, it was quickly followed by countrywide soil surveys conducted by the Canadian Food Inspection Agency in 2015. Those surveys revealed V. longisporum populations in British Columbia and every province eastward as far as, and including, Quebec. And while it hasn’t reached the point where it is causing significant yield damage, as a soil-borne pathogen, once there, it doesn’t simply go away.
Why it matters: Mistaking verticillium stripe for sclerotinia stem rot or blackleg could cost you time and money.
Since verticillium stripe is new to Canada not much is known about how to manage it. In fact, much of what is known about the disease has been gathered from experts in Europe. Not all of it is applicable in Canada.
One of the confounding features of verticillium stripe is symptoms do not appear until later in the season, post-harvest, says Canola Council of Canada (CCC) agronomist Justine Cornelsen, who is the organization’s lead on the disease. In years where harvest is delayed, farmers found infected crop in a tangled mess around their combines.
“The plants become really fragile and so they start to strip away,” explains Cornelsen. “Very similar to if you have a bad sclerotinia case. We’re in the early stages of trying to figure out this disease and what it does, and what it potentially might do to our canola production here,” she adds.
What they do know is European farmers have reported losses ranging from 10 to 50 per cent. Cornelsen notes, however, that European farmers are growing winter varieties, which means their crops are in the ground much longer. This means the disease has a longer time to do damage to the crop, she points out.
Since verticillium stripe is a new disease in Canada, researchers and extension specialists have to start with the basics, including details such as identification, spread and life cycle.
It’s important to know what you’re dealing with, says Cornelsen. “Proper scouting and identification is going to change your management practices,” she says. “If you think you’re dealing with blackleg and it’s actually verticillium, you’re really going to change how you approach this disease.”
Three things you need to know
1. It looks like sclerotinia stem rot.
At the traditional scouting time, growers will notice senescing or lighter colouring on half of the stem. It looks a lot like sclerotinia, says Cornelsen, which will be particularly distressing to those who used a fungicide application to control the disease.
Cornelsen describes the senescing part as wilted, shrivelled and shrunken in. “At that point the plant is still going to be fairly hard because the other half is still green, so you’re not getting that shredding like you would with sclerotinia,” she says. “A sclerotinia plant can have white mould on it, and if the mould is dried up on it, it’s already shredding and it’s all the way around. It’s not just half of the plant.”
The key difference between verticillium and sclerotinia can be found in the stem. “When you crack open that stem, you are going to find a sclerotia body if it’s sclerotinia,” she says.
2. Microsclerotia will be present in infected stubble.
If you’re not sure what you’re dealing with at first sight, be sure to return to the field two to three weeks later, if possible. That’s when you’ll really start to see the plant shred, says Cornelsen. “Beneath it is where you see the real, true sign of the disease, which is the microsclerotia,” she says.
“The microsclerotia are extremely tiny grey flecks. The entire stem will be loaded with microsclerotia everywhere.”
Growers may confuse verticillium stripe with alternaria black spot and blackleg at this point. “As soon as you see anything black or grey, your mind automatically starts to go that way,” she says. “But that’s our true sign — the microsclerotia in the stem below the epidermis.”
3. Verticillium stripe is a soil-borne disease.
Because verticillium stripe is a soil-borne disease, the disease comes up through the root. Management tips will be similar to those used to manage clubroot, which includes rotation and equipment sanitation.
It should be noted, says Cornelsen, there is the potential for host resistance as well. Although they don’t know which varieties are more resistant than others to date, researchers are testing germplasm and they are finding differences.
“I’ve also noticed differences in variety trials in fields with verticillium. There are clear differences between varieties,” says Cornelsen.
“That’s very promising,” she adds. “We know then that we have something in the background that’s showing some levels of tolerance to this disease.”
In conclusion, Cornelsen notes, the disease can coexist with other species, so expect to see plants showing signs of blackleg and sclerotinia at the same time.