Clubroot in resistant canola?

Erosion of resistance? | Researchers don’t know if it is a new virulent phenotype of clubroot

The discovery of a possible new strain of clubroot in two fields in the Edmonton area is raising concerns that resistance is already breaking down in new clubroot resistant canola varieties.


Stephen Strelkov, a plant pathologist with the University of Alberta, said clubroot galls collected from fields in the Edmonton area and regrown and tested in the greenhouse are raising concern about a new virulent phenotype of clubroot. 


“There were a couple cases where there actually was increased infectivity by those strains in the greenhouse conditions. It did suggest it was a new virulent phenotype,” said Strelkov.


“What we don’t know is if this was a naturally occurring strain that may have existed or whether it was in fact an erosion of the resistance.”


Strelkov said researchers have just finished their greenhouse experiments and are conducting a second phase of experiments to learn more conclusively if what they’ve found is a new strain of clubroot or resistance erosion.


“Right now, in this case, I can’t conclude one way or the other. I don’t think we have enough data to make such a conclusion. Right now I will call it a couple fields of concern.”


Clubroot was first detected in a canola field in 2003 and has slowly spread to other parts of the Prairies. The soil-borne disease creates galls, or clubs, on the roots of canola plants, which dramatically decrease yields.


Researchers were quick to come up with new clubroot resistant varieties to allow farmers to grow canola in the clubroot hot spots. 


Many municipalities and counties had implemented bylaws allowing canola to be grown on infected land only every five to seven years in an effort to reduce the clubroot spores in the soil. 


However, the new resistant varieties allowed farmers to once again grow the valuable crop on infected fields.


Strelkov said it wouldn’t be a surprise if resistance has started to break down. Clubroot resistance was relatively short lived in parts of the United States and Europe after growers grew resistant varieties year after year, he added.


Resistance breaks down when the same varieties are grown continually on the same field and new pathogens start to attack the resistant genotypes within the canola plant.


“Resistance is such an important management tool. It is our main management tool,” Strelkov said.


“It would be a shame if we started to lose resistance options because people are not rotating properly, particularly if they know they have heavy clubroot. It would be a shame.”


Curtis Rempel of the Canola Council of Canada said the discovery of the new pathotype has become a red flag, and the organization wants to conduct field surveys this summer to determine the spread of the new pathogen. 


Samples were taken from several fields, and the infected galls were found in two fields.


Rempel said it’s not known if the potential new pathotype is confined to a small area or if the findings are indicative of more fields potentially having the new pathotype.


Rempel wants government, growers and industry to put together a surveillance program this summer for a more detailed search for the pathogen in the Edmonton and Westlock areas.


“Let’s say you have a new patho-type. The best way to manage it is to look at vigilant scouting and sampling and see how broadly spread it is,” he said.


“You also have to treat it like a new disease and keep the soil from moving. Then you have contained it.”


The best way to reduce resistance is to not grow canola back to back, not grow canola on fields that already have a high clubroot infestation level, scout for clubroot, don’t plant the same resistant canola variety in a rotation and try to limit soil movement between fields.


“With farm consolidation, some growers are managing a lot of acres. What your ideals are and what reality happens when you have a late spring, you have to start making compromises,” said Rempel.


“I think among a segment of growers there is an understanding that resistance is a tool and one of an integrated pest management approach. Those producers understand that to preserve that tool we need to use other practices if we want to preserve resistance.”


Rempel said spending the summer documenting any potential new pathotypes would help researchers know if it is a new pathotype, the environment or just a variety of factors that have come together to create a more aggressive strain for a single year.


“If we’re starting to find it in more fields that are spread out, then we can start saying it’s around and now we need a different approach,” he said.


“Now it’s contained in two to three fields and can we really work with these producers and really rally focus on keeping the soil in that field.”


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