Breeding varieties for increased disease resistance is a major means to control epidemics. However, the deployment of resistance genes through space and time drives the genetic composition of the pathogen population, directionally altering pathotype frequencies. In France, Leptosphaeria maculans causes stem canker on Brassica napus oilseed rape crops but not on B. juncea condiment mustard. Prior to the deployment of winter B. napus varieties with Rlm6 resistance gene introduced fromB. juncea, the aim of our study was to investigate if this deployment could impact disease control in condiment mustard. We assessed the presence of resistance genes against the disease in a set of current FrenchB. juncea varieties and breeding lines with a set of five differential L. maculans isolates carrying know Avr/avr genes.Rlm6was detected in all 12 condiment mustard varieties.Rlm5was also detected in eight varieties. No additional resistance genes were detected with the set of isolates used. Because the frequency of isolates virulent onRlm6is very low in France, these results indicate thatRlm6gene is a major component of disease control in the French B. juncea mustards tested. Using Rlm6 in oilseed rape varieties will very likely induce an increase in frequency ofRlm6virulent isolates. This raises the acute concern of a wise deployment of oilseed rape around the condiment mustard growing area. Scientific knowledge on adaptation dynamics, spatial segregation of crops and cooperation between actors is currently available in order to mitigate the risk and advert negative consequences of the introduction ofRlm6resistance gene in oilseed rape varieties.