Fitness of virulent (avrLm1 and avrLm4) or avirulent (AvrLm1 and AvrLm4) isolates of Leptosphaeria maculans on Brassica napus without the corresponding resistance genes Rlm1 and Rlm4 was investigated in controlled environment (CE) and field experiments. Results indicate that there is a measurable fitness cost for virulent isolates compared to avirulent isolates in terms of number of lesions, si...