GENES FOR RACE-SPECIFIC RESISTANCE AGAINST BLACKLEG DISEASE IN BRASSICA-NAPUS L

Specificity of interaction at the cotyledon stage was recently demonst rated between the blackleg pathogen, Leptosphaeria maculans. and Brass ica napus. Three pathogenicity groups were distinguished, PG2 avirulen t towards 'Quinta' and 'Glacier', PG3 avirulent towards 'Quinta', and PG4 virulent on the two cultivars. The genetic control of the interact ions was investigated on both the pathogen and the plant. Tetrad analy sis was performed following PG3 x PG4 and PG2 x PG4 crosses. 'Quinta' and 'Glacier' were crossed with the susceptible winter oilseed rape cu ltivar 'Score'. The analysis of F-1, F-2 and testcross populations sug gested that the incompatible interaction between 'Quinta' and PG3 isol ates 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 c onditioned by the matching gene pair Rlm2/AvrLm2. Finally, the data su ggest 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 b reeding for blackleg resistance.