LAUSR

BACKGROUND Leptosphaeria maculans, the cause of stem canker of oilseed rape, develops gene-for-gene interactions with its host and shows a high evolutionary potential to "break down" novel resistance genes (R, Rlm) deployed in cultivars over large areas. For optimal management of R genes, updated knowledge of the population structure of the pathogen is needed. In France, large-scale surveys have been done at 10 years intervals since 2000. Here we report the characterization of a large L. maculans population collected in France in 2019-2020. RESULTS A total of 844 isolates were collected from 11 sites in 10 French departments and were phenotyped for their virulence against nine Brassica napus R genes. All isolates were virulent toward Rlm2 and Rlm9. Very few isolates were avirulent on Rlm1 (1.8%) and Rlm4 (0.6%). Avirulent isolates toward Rlm7 ("AvrLm7") varied from 67% to 11.3%, depending on the site sampled, illustrating the ongoing breakdown of Rlm7. The decrease of AvrLm7 isolates (29.2% at the national level) compared to the 2010 survey (96.5%) was accompanied by an increase of avirulent isolates on Rlm3 (0% in 2010; 54% in 2019-2020). However, virulent isolates on both Rlm3 and Rlm7, previously rarely detected, were now found in all sites with a frequency of 17.3%. Finally, most or all isolates were avirulent on Rlm11 (96.1%), LepR2 (RlmS, 99.8%), and Rlm6 (100%), suggesting these three genes still effectively control the disease. CONCLUSION These data will help guide strategies for breeding and deploying resistant oilseed rape varieties against L. maculans in France. This article is protected by copyright. All rights reserved.