By Canola Council of Canada
May 8, 2015 - Erosion of clubroot resistance is showing up across Alberta. Recent research by Alberta Agriculture and Rural Development (AARD) and the University of Alberta (U of A) has confirmed the continued spread, with multiple virulent pathotypes suspected. Protecting against further erosion of current clubroot resistance requires a strategy focused on minimizing clubroot resting spore spread and keeping spore loads as low as possible.
In early 2014, U of A researcher Dr. Stephen Strelkov identified the presence of a different clubroot pathotype virulent on current forms of clubroot resistance. Further field surveillance in 2014 by AARD, the U of A and the Canola Council of Canada (CCC) confirmed that clubroot pathotypes capable of causing high levels of disease in resistant cultivars were present in 16 of 27 fields investigated across Alberta. The clubroot pathogen isolated from many of these fields was virulent on all clubroot resistant cultivars. It is suspected that there may be multiple pathotypes causing this loss of function of clubroot resistance.
"These new fields aren't clustered around the original location of resistance breakdown. They are hundreds of kilometres apart and throughout the clubroot infested areas of the province," says Dan Orchard, agronomy specialist and clubroot lead with the CCC. "We need to continue to focus on both minimizing the spread of these new pathotypes and the buildup of resting spores in all fields."
Since equipment contaminated with clubroot-infested soil is the key mechanism for clubroot spread, good sanitation on equipment during all field operations — including seeding and spraying — is recommended. "The level of sanitation should be based on the level of clubroot risk, but at least 90 per cent of clubroot spores that move from field to field can be stopped by just scraping off 90 per cent of the soil," says Orchard.
"Preventing the buildup of resistant spores is best managed through longer rotations and the use of resistant varieties," he adds. "If growers wait for clubroot to show up before choosing resistant varieties, the selection pressure for new virulent pathotypes is literally millions of times higher than if growers begin using these varieties before the disease shows up."
AARD and the U of A continue to monitor the situation closely. Actions undertaken include ongoing surveillance for clubroot pathotypes capable of overcoming resistance, and development of a research program to evaluate and characterize the new clubroot pathotypes.
The CCC also advises canola growers and agronomists to scout their canola fields this summer with extra effort on clubroot identification. The best time to scout for clubroot symptoms on roots is late in the season, approximately two weeks before swathing, when root galls should be easy to identify. Soil samples can be collected as well at any time of year and from non-canola fields to determine if the clubroot pathogen is present.
If resistance breakdown is suspected, contact your CCC agronomy specialist. "Identifying breakdown will aid the entire industry in agronomic research and varietal development efforts to manage this disease," Orchard says.
To learn more, check out this Canola Watch article on management strategies and visit www.clubroot.ca for more details on clubroot identification and best management practices including sanitation, long rotations, weed management and minimizing tillage.