Manitoba canola growers are being urged to check their fields for blackleg infections after two severely infected fields were discovered recently in the province even though the varieties were rated as blackleg resistant.
"It's the worst blackleg I have ever seen and I've been looking at blackleg in canola since the late '90s," Manitoba Agriculture, Food and Rural Initiatives (MAFRI) oilseeds specialist Anastasia Kubinec said in an interview July 30.
"I've never seen blackleg so bad in a field that the actual plants have fallen over at the base."
Samples from the severely infected field are being tested to see if the infection was caused by an existing race of the fungal disease or a new one.
MAFRI isn't saying where the two fields were or naming the varieties.
Nothing can be done to help those fields, or others that are less severely infected, however, knowing a field is infected allows the farmer to take steps to reduce future infections, Kubinec said.
Growing canola less frequently in the same field is one of the key ways to reduce blackleg infections, she said.
"It has been a canola-wheat, canola-wheat rotation for about eight years," Kubinec said of one of the severely infected fields.
"I don't think we'd be seeing the blackleg quite as bad if he has thrown a few more crops in there and not just canola every other year."
Rotation has little impact on preventing sclerotinia in canola because that fungal disease is easier spread and it affects many more crops than canola, said MAFRI plant pathologist Holly Derksen. Blackleg has fewer hosts and isn't as easily spread.
In addition to longer rotations farmers should also should varieties to try and prevent blackleg infections, she said.
"I wouldn't be surprised if it wasn't a new race (of blackleg)," Derksen said. "We've been setting up for it by pushing canola rotations."
Even if it turns out to be an existing race it probably was one the variety was not exposed to during testing before commercialization. There are 16 races and they vary geographically, even within the province.
"I think we're going to see a lot more blackleg this year," Derksen said. "All the hail and strong winds have caused wounds on canola plants providing an entry point for the disease."
Just because a canola variety is rated "resistant" to blackleg doesn't mean it's immune to the disease, said Angela Brackenreed, the Canola Council of Canada's agronomy specialist for Manitoba.
"But things would definitely be much worse without those varieties," she said.
Many canola fields are infected with blackleg this year but many farmers don't know it because the symptoms aren't visible from the road, she said.
"It's on the base of the stem and you look and think it's fine," Brackenreed said. "Until you get out there and start digging around and looking under that canopy you don't realize it's there."
Farmers should look for round, whitish lesions with black spotting on canola leaves, as well as black lesions at the base of the stem. The disease kills the plant by preventing water and nutrients from moving through it.
Fungicides are no guarantee of preventing blackleg either, she said. They need to be applied between the two- and six-leaf stage and provide only a couple of weeks of protection.
"It's really important now to scout because if it's there now, chances are it's going to overwinter and be there the next time you grow a host crop," Brackenreed said. "Although it seems futile to scout now it's really important for planning for upcoming years."
Besides using longer rotations, farmers should plant treated, certified seed rated as blackleg resistant.