REACTION TO PHYTOPHTHORA-INFESTANS OF POTATO GENOTYPES WITH RACE-SPECIFIC RESISTANCE

Phytophthora infestans isolates collected in Poland in 1987-1995 were evaluated on detached leaflets of a differential series consisting of potato genotypes possessing resistance genes R1-R11. On the same diffe rentials was evaluated over three years a highly virulent P, infestans isolate MP 245. Isolate MP 245 and those from natural populations of P. infestans were seldom or inconsistently pathogenic to R5, R8 and R9 . When leaflets of the differentials were infected with MP 245 the les ions were often small and sporulation weak. A statistically significan t interaction was found between years and pathogenicity of MP 245 to i ndividual differentials. Attempts to train the isolate MP 245 for path ogenicity to R9 were unsuccessful. The variation in specific resistanc e of potato genotypes may complicate the evaluation of their resistanc e to P. infestans, as well as the evaluation of the virulence spectrum of natural fungus populations. This variation may be due to changes i n specific pathogenicity of the fungus or in specific resistance of th e potato. Some differentials have a possibly useful resistance to P. i nfestans.