The clubroot story in western Canada continues to be written as researchers are finding additional strains of the pathogen that are capable of overcoming genetic resistance.
So far, 13 new variants have been identified since the breakdown of resistance was first confirmed in 2013, explains Stephen Strelkov, plant pathology researcher at the University of Alberta.
“We think resistance has broken independently in different fields as resistant varieties were grown in short rotations,” he says in this episode of Canola School.
Fortunately, these new versions of the pathogen have only been found on a small fraction of fields where clubroot has been confirmed.
The total number of clubroot cases found since surveying began 12 years ago is expected to reach the 2,500 mark after samples from the 2016 season are processed, he notes.
Out of those approximately 2,500 fields, there have been 42 confirmed cases where new strains of clubroot have been found. Strelkov says another 30 or so fields of concern have been flagged in 2016.
“It’s still a relatively small percentage of the overall clubroot that’s out there, but it serves as a warning sign,” he notes. “Resistance is still effective in the vast majority of fields, but we have this worrying trend.”
In addition to high selection pressure, Strelkov explains part of the reason they’re finding new strains of the disease is the methods for differentiating between pathotypes “weren’t truly capturing the full pathogenic diversity until now.”
“Let’s say two strains might be pathotype 5, but they were a very different pathotype 5. One couldn’t attack resistant canola and another one could. So we’ve been working on what we call a Canadian clubroot differential series,” he says.