Race Structure of Leptosphaeria maculans and Its Influence on Canola Cultivar Resistance in the Western Cape Province of South Africa

Blackleg, caused by the fungal pathogen Leptosphaeria maculans , is a serious constraint to canola production in the Western Cape province of South Africa. Host resistance forms an essential part of integrated blackleg management and includes the deployment of major resistance genes in canola cultivars. However, the intensive use of resistance genes can result in resistance breakdown. To manage the efficacy of resistance genes, changes in avirulence gene frequencies present in pathogen populations should be monitored. This study analysed whole genome data of 264 L . maculans isolates collected between 2020 and 2022 in the Western Cape province, South Africa, to determine genotypes of alleles at 12 avirulence loci. The resistance of commercially available cultivars was also evaluated in field trials conducted at four locations from 2021 to 2023. Results revealed low frequencies of AvrLm1 (6.44%), AvrLm4 (0.38%) and AvrLm6 (36.82%), indicating pathogen adaptation to the corresponding resistance genes, Rlm1 /LepR3, Rlm4 and Rlm6 , extensively deployed over the last two decades. In contrast, AvrLm7 , corresponding to the recently deployed Rlm7 , was present in 98.86% of isolates. In support of these findings, results from the field trials showed that the presence of Rlm7 in cultivars is associated with lower blackleg disease severity. Lower severity in some cultivars lacking Rlm7 suggests the influence of minor gene resistance. As expected, high frequencies (> 90%) were observed for avirulence genes corresponding to resistance genes not deployed locally. To prolong Rlm7 efficacy and potentially restore other resistance genes, rotation of resistance genes and regular avirulence allele monitoring are recommended.