44 Our Advertisers Appreciate Your Business Better Farming | September 2024 Ag Insights USING BLACKLEG RACE TESTING Blackleg remains a factor when it comes to growing canola. By Allie Noble, Saskatchewan Ministry of Agriculture Each year, canola producers struggle with many pests attempting to rob yield. For over five decades, blackleg has caused yield losses in canola fields across the Prairies. There have been many advances in fighting the disease since it was first found. Over the years, fungicides, resistant cultivars, and crop rotation have all been major tools in managing yield losses. More recently, blackleg race testing has been used to combat the disease by supporting producers in making canola cultivar decisions. Blackleg – the disease on your farm Blackleg is caused by a complex of two species, Leptosphaeria biglobosa and Leptosphaeria maculans. L. biglobosa is less virulent and often present in fields, with the onset typically so late in the season that it does not have major impacts on yield. L. maculans can begin to affect the crop at the seedling stage and is associated with more severe infections that will girdle the stem and can cause major yield loss. The main symptoms to scout for are lesions with pycnidia, stem cankers, and discoloured tissue in cross sections of the stem. In the 1990s, resistant cultivars were introduced that reduced yield losses significantly, and since then, they have performed the heavy lifting for blackleg management. Over the last decade in Saskatchewan, we have begun to see incidence of blackleg in canola crops increase based on data from annual general canola disease surveys conducted in the province. Producers who are experiencing blackleg increases causing major yield losses should consider doing blackleg race testing. The results from these tests can be used to help identify the corresponding resistance package needed in a canola cultivar. Always keep in mind that although blackleg resistant cultivars are valuable, we should not rely on them as our only management method. Understanding blackleg resistance There are two types of resistance that exist for blackleg: Qualitative and quantitative. Quantitative resistance reduces the speed at which blackleg causes yield loss, with many genes involved, each having a small effect. Qualitative resistance has race-specific genes involved, so when the blackleg pathogen is detected, the plant will initiate a defence response. Qualitative resistance is also known as “major gene resistance,” meaning there is a specific gene for gene relationship between an avirulence (Avr) gene (from L. maculans) and a resistance (Rlm) gene (within the canola cultivar). A blackleg pathogen race has an accumulation of multiple Avr genes. In each canola cultivar with qualitative resistance, we have a resistance gene (major gene) that will recognize an Avr gene from a specific blackleg race. The resistance gene doesn’t necessarily fight off the infection; it is what sounds the alarm that an infection is beginning. When a resistance gene does detect the corresponding Avr gene, it sets off a hypersensitive reaction where the plant will kill off cells to stop the disease in its tracks. If the blackleg race doesn’t have the Avr gene that matches the resistance gene, the resistance gene won’t recognize the pathogen and a virulent response will occur (see Figure 1). If the blackleg race in a field is known, we can determine the corresponding major gene and choose a canola cultivar with the correct resistance group to combat blackleg. Blackleg can cause major yield losses. Canola Council of Canada photos
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