Are leaf symptoms a way to check an increase of virulent populations on Rlm7 hybrids?

Abstract: New genotypes of oilseed rape were recently introduced commercially in France. Their excellent resistance to blackleg (Leptosphaeria maculans) is mainly due to a new major resistance gene, Rlm7. Such varieties are potentially exposed to a resistance breakdown. The main objective of extension bodies is to promote the durable management of resistance and try to anticipate any possible breakdown. This objective necessitates being able to detect as early as possible the first step of the pathogen population switch towards virulence. Two ideas were developed for such an aim. Firstly, a comparison between leaf symptom density on cultivars having or not having the targeted specific resistance may be used as an indirect indicator of the increase of virulent isolates in the population. The second way was to directly check virulence profiles of the pathogen population. These two methods have been tested since autumn 2004 in the central region of France at fields located between Issoudun and St Florent sur Cher. The first approach appeared to work during the first two experimental years. Nevertheless, epidemics were too low during the last two years to allow us to make a clear conclusion. Virulence profiles were surveyed in this region and also in experimental fields in Versailles, Grignon and Rennes, from isolates sampled from leaf symptoms. The survey showed that AvrLm7 (avirulent) spores were able to produce leaf symptoms on Rlm7 varieties. However the leaf lesions were often smaller than those due to virulent isolates, with a huge variability of leaf symptom morphology, even on the same genotype, in the same field, under the same climate. Only a low number of isolates from Rlm7-leaf symptoms were identified asavrLm7 (virulent). These results underline the risk of overestimation of virulence allele frequencies based on foliar symptoms, the need for pedagogic documents and teaching sessions to help experimenter’s and local advisors to detect the beginning of resistance breakdown in a bio-vigilant approach, and the difficulties of doing such a survey with limited costs and limited sampling.