Canola cultivars are available with improved tolerance to sclerotinia stem rot. These cultivars have demonstrated reduced levels of sclerotinia stem rot infection in field and laboratory trials. The mechanisms for this increased level of tolerance or resistance are not well described, but they do appear to reduce lesion development in stem and other tissues. This reduced severity of sclerotinia stem rot is effective but may not match the same level of control as a fungicide application. Therefore under high risk of disease development applying a fungicide to these cultivars may still be necessary.
Because sclerotinia stem rot can be severe in heavily lodged fields or areas within a field, choosing cultivars that resist lodging may reduce sclerotinia severity.
Research and plant breeding efforts continue to improve genetic resistance, and work is underway to incorporate new sources of resistance into canola lines.
Dwayne Hegedus, a molecular biologist with Agriculture and Agri-Food Canada (AAFC), is part of an international collaboration of researchers from Australia, China, France and South Korea working on sclerotinia resistance. In an odd twist, genetic resistance for sclerotinia stem rot may actually come from turning off those genes responsible for the natural defence mechanism that promotes localized cell suicide. If the plant stops scorching itself, the disease has less fuel and infects more slowly.
The key for Hegedus and his AAFC colleagues and international collaborators will be to “turn off” the genes for highly specific response to sclerotinia stem rot, but “leave on” the genes that respond to blackleg and other diseases. That is part of the challenge, but to finally find “genuine and durable” genetic resistance to probably the most important disease — based on loss of yield and profits — of canola in Canada will be a major event.
Steven Whyard, professor at the University of Manitoba, is researching gene specific double-stranded RNA (dsRNA) as a means to control sclerotinia stem rot. Whyard’s lab has shown effective control of sclerotinia stem rot in canola with the use of RNAi tools through both foliar sprays and transgenic plants. This type of technology will provide diversity in crop protection options.
Meanwhile, a preventative fungicide remains the key defence against this plant-killing yield robber. Fungicides are effective, and their economic threshold — when around five per cent of plants in a field are infected — occurs more often that most farmers probably realize. Fungicides will typically remain active in the tissue they are applied to for two to three weeks depending on the product and its persistence, as well as the quality of application and its penetration into the canopy.