Genes for race-specific resistance against blackleg disease in Brassica napus L.

Specificity of interaction at the cotyledon stage was recently demonstrated between the blackleg pathogen, Leptosphaeria maculans. and Brassica napus. Three pathogenicity groups were distinguished, PG2 avirulent towards 'Quinta' and 'Glacier', PG3 avirulent towards 'Quinta', and PG4 virulent on the two cultivars. The genetic control of the interactions was investigated on both the pathogen and the plant. Tetrad analysis was performed following PG3 x PG4 and PG2 x PG4 crosses. 'Quinta' and 'Glacier' were crossed with the susceptible winter oilseed rape cultivar 'Score'. The analysis of F-1, F-2 and testcross populations suggested that the incompatible interaction between 'Quinta' and PG3 isolates is conditioned by the presence of the dominant single: resistance allele Rlml in 'Quinta' and the matching avirulence gene AvrLml in L, maculans. Race-specific resistance of 'Glacier' to PG2 isolates was conditioned by the matching gene pair Rlm2/AvrLm2. Finally, the data suggest that two avirulence genes matching two dominant loci control the 'Quinta'-PG2 interaction. The consequences of the occurrence of race-specific resistance in B. napus are discussed with respect to future breeding for blackleg resistance.