Yearly surveys of populations of Leptosphaeria maculans reveal an unexpected distribution of virulence/avirulence phenotypes for AvrLm genes, between regions in France
Résumé
Leptosphaeria maculans is a phytopathogenic fungus causing stem canker on Brassica napus. This disease is mainly controlled by using varieties carrying major resistance genes (Rlm). The intensive use of the few Rlm genes leads to a strong selection pressure on populations of L. maculans and the selection of virulent isolates. As virulent isolates increase in populations, Rlm genes can rapidly become ineffective, as observed for Rlm1 (15% of virulent isolates in 1997 to 93% in 19991). Durable management of Rlm genes requires frequent monitoring of the frequency of virulent isolates in field populations, and the characterization of molecular events leading to virulence2. From 2021 to 2023, 2411 isolates were collected from 8 to 11 sites in France. The phenotyping for their virulence was conducted against five to six Rlm genes. For Rlm4, a bypassed resistance, the results were as expected, with most isolates showing virulence (87% in 2022 up to 98% in 2021). On the contrary, RlmS deployed since 2018, is still efficient, with 0% of virulent isolates in 2021, 0.8% in 2022 and 0.1% in 2023. Samples from the Bourgogne region, whose populations had not previously been characterized, highlighted the specificity of these populations, with drastically different frequencies of virulent isolates. To go further, the MPseqM tool3, combining multiplex PCR and sequencing, should allow us to identify allelic diversity and may explain the specificity of the population from the Bourgogne region.
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