Blackleg disease (phoma stem canker) ofBrassica napus(canola, oilseed rape) is caused by the fungusLeptosphaeriamaculans. In some regions of Australia, resistance in oilseed rape cultivars derived fromB. rapasubs. sylvestris(e.g. cv.Surpass 400) became ineffective within three years of commercial release. The genetic control of avirulence inL. maculanstowards cv. Surpass 400 is described. When Australian field isolates were screened on this cultivar, three phenotypicclasses were observed; virulent, intermediate and avirulent. Analysis of crosses between fungal isolates varying in theirability to infect cv. Surpass 400 demonstrated the presence of two unlinked avirulence genes,AvrLm1andAvrLmS.Complementation of isolates (genotypeavrLm1) with a functional copy ofAvrLm1, and genotyping of field isolatesusing a molecular marker forAvrLm1showed that virulence towardsRlm1is necessary, but not sufficient, for expressionof a virulent phenotype on cv. Surpass 400. Taken together, these data strongly suggest that cv. Surpass 400, with‘sylvestris-derived’ resistance, contains at least two resistance genes, one of which isRlm1.