A Leptosphaeria maculans set of isolates characterised on all available differentials and used as control to identify virulence frequencies in a current French population

Abstract

The characterization of virulence frequencies has to be regularly updated to identify which genes are currently efficient and use this information to advise gene deployment by choosing varieties depending on the current composition of local pathogen population. In L. maculans on Brassica napus, because different genes were characterized by different teams, because new interactions are continuously identified and seed of differentials are difficult to obtain, we today still lack isolates characterized on all current resistance genes. On the one hand, we assembled a set of 12 isolates characterized on 13 of the 17 described resistance genes, having clearly compatible and clearly incompatible isolates for each interaction. This set can be used to characterize the L. maculans - B. napus interaction at cotyledon stage. Expanding the set of isolates with clearly virulent ones allowed us to detect inconsistent behaviour or intermediate (avirulent) phenotypes. On the other hand, we used this set of isolates as controls to identify virulence frequencies in a current French L. maculans population sampled in 2018 at Le Rheu. We provide the current status for 13 avirulence frequencies, including LepR1, LepR2 and LepR3 available in near isogenic lines of spring canola but not yet documented in France. Avirulence frequencies on the genes Rlm1, Rlm2, Rlm3, Rlm4, Rlm7, Rlm9 and LepR3 were low, indicating the lack of efficacy of these genes against the current population. In the opposite, all or most of isolates were avirulent for the genes Rlm5, Rlm6, Rlm10, Rlm11, LepR1 and LepR2. An optimistic point of view could conclude that there are ample resources for oilseed rape breeding. However, as compared to previous studies, so far all the resistance genes used on significant acreage without additional management practices have lost efficacy and only avirulences corresponding to resistance genes not deployed in France retain efficacy. While the call to wisely manage the available host resistance genes is not recent, it is still relevant. Adding, management practices to the deployment of resistance genes in order to reduce inoculum carry-over from one growing season to the next and to lower population sizes is key to maintain their efficacy over time.