Changes in race structure of Canadian populations of Phytophthora infestans based on specific virulence to selected potato clones

A collection of 80 isolates of Phytophthora infestans, representing multilo cus genotypes of the pathogen found in Canada from 1994 to 1996, was screen ed on a series of nine differential potato host cultivars to determine the physiological race of the isolates. A total of 28 pathotypes were found amo ng the 11 genotypes present in the collection. There was a significant incr ease in the complexity of pathotypes from 1994 to 1996 which reflected the displacement of the US-1 genotype (mean of 2.2 host differentials infected) by the US-8 genotype (mean of 8.1 host differentials infected). Eighteen o f the US-8 isolates could overcome all nine of the resistance genes tested. Virulence genes which overcame host resistance conferred by R8 were the ra rest in the populations studied. The predominance of complex populations of P, infestans in Canada implies that the introduction of cultivars with spe cific R genes for resistance would be an ineffective control measure.