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The occurrence of race-specific resistance genes to the stem canker fungus, Leptosphaeria maculans, was analysed in 453 accessions of B. napus, mainly originating from the Institut für Pflanzengenetik und Kulturpflanzenforschung (IPK) GeneBank. Major resistance genes Rlm1, Rlm2, Rlm4 and the putative RlmBBA gene were investigated using genetically improved strains of the fungus harbouring as few corresponding avirulence genes as possible. In addition, a screening with fully virulent isolates was used to uncover novel resistance sources. Major resistance genes were rarer in frequency and diversity in spring-type cultivars compared to winter types. In the former, 65.7% of the accessions were fully susceptible to all isolates, whereas only 12.2% of the winter types were devoid of at least one R gene. In spring cultivars, the most common R gene, Rlm4 was found in 26.6% of accessions, whereas the other R genes were rare. In winter cultivars, the most common R genes were Rlm2 (more than 45.9–54.0% of the accessions) and Rlm4 (26.4–27.7% of the genotypes). In winter types however, the improvement of the quality of oils, through the generation of single- and double-low genotypes improved the homogeneity of the cvs, whereas it impoverished R gene diversity, including the loss of complete resistance that was harboured by 18.4% of the less advanced accessions, and a reduction in the ratio of accessions harbouring Rlm1. Correlation between the R gene(s) present in the accessions and their field resistance is discussed. [ABSTRACT FROM AUTHOR]