Abstract
Blackleg, caused by the fungal pathogen Leptosphaeria maculans, is a serious threat to canola (Brassica napus L.) production in western Canada. Crop scouting and extended crop rotation, along with the use of effective genetic resistance, have been key management practices available to mitigate the impact of the disease. In recent years, new pathogen races have reduced the effectiveness of some of the resistant cultivars deployed. Strategic deployment and rotation of major resistance (R) genes in cultivars have been used in France and Australia to help increase the longevity of blackleg resistance. Canada also introduced a grouping system in 2017 to identify blackleg R genes in canola cultivars. The main objective of this study was to examine and validate the concept of R gene deployment through monitoring the avirulence (Avr) profile of L. maculans population and disease levels in commercial canola fields within the Canadian prairies. Blackleg disease incidence and severity was collected from 146 cultivars from 53 sites across Manitoba, Saskatchewan, and Alberta in 2018 and 2019, and the results varied significantly between gene groups, which is likely influenced by the pathogen population. Isolates collected from spring and fall stubble residues were examined for the presence of Avr alleles AvrLm1, AvrLm2, AvrLm3, AvrLm4, AvrLm5, AvrLm6, AvrLm7, AvrLm9, AvrLm10, AvrLm11, AvrLepR1, AvrLepR2, AvrLep3, and AvrLmS using a set of differential host genotypes carrying known resistance genes or PCR-based markers. The Simpson’s evenness index was very low, due to two dominant L. maculans races (AvrLm2-4-5-6-7-10-11 and AvrLm2-5-6-7-10-11) representing 49% of the population, but diversity of the population was high from the 35 L. maculans races isolated in Manitoba. AvrLm6 and AvrLm11 were found in all 254 L. maculans isolates collected in Manitoba. Knowledge of the blackleg disease levels in relation to the R genes deployed, along with the L. maculans Avr profile, helps to measure the effectiveness of genetic resistance.
Highlights
Blackleg, caused by the fungal pathogen Leptosphaeria maculans (Desm.) Ces. & de Not, is an economically important disease of canola (Brassica napus L.) in many parts of the world, including western Canada, due to yield loss and trade conflicts (Fitt et al, 2006; Van de Wouw et al, 2016; Zhang and Fernando, 2018)
The new resistance labeling scheme identifies the specific R genes deployed within a cultivar, allowing producers to rotate cultivars based on major resistance gene groups (Zhang and Fernando, 2018; Van de Wouw and Howlett, 2019)
The use of commercial fields in this study provides insight into how farmers have been influencing the L. maculans population on their fields through the deployment of cultivars carrying different R genes
Summary
Blackleg, caused by the fungal pathogen Leptosphaeria maculans (Desm.) Ces. & de Not, is an economically important disease of canola (Brassica napus L.) in many parts of the world, including western Canada, due to yield loss and trade conflicts (Fitt et al, 2006; Van de Wouw et al, 2016; Zhang and Fernando, 2018). Regional monitoring over time has revealed changes within the population due to the use of resistance genes in many canola cultivars (Kutcher et al, 2007; Liban et al, 2016; Fernando et al, 2018; Soomro et al, 2020); the avirulence gene AvrLm3 had become scarce in the L. maculans population due to the overuse of Rlm resistance gene in Canadian B. napus germplasm. The new resistance labeling scheme identifies the specific R genes deployed within a cultivar, allowing producers to rotate cultivars based on major resistance gene groups (Zhang and Fernando, 2018; Van de Wouw and Howlett, 2019). The main objective of this study was to assess the concept of blackleg major R gene-labeled cultivar deployment through monitoring the avirulence profile of L. maculans population and disease levels in commercial canola fields within the Canadian prairies. The use of commercial fields in this study provides insight into how farmers have been influencing the L. maculans population on their fields through the deployment of cultivars carrying different R genes
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