The A2 mating type of Phytophthora infestans was first reported in the United States in 1990. Concurrently, P. infestans strains resistant to metalaxyl were found in the Pacific Northwest. Collaborative surveys were undertaken during 1991-1993 to investigate the frequency of occurrence of A2 mating types and metalaxyl resistant strains in populations of P. infestans isolated from outbreaks of late blight in potato and tomato crops in North America. In vitro testing indicated that isolates from the northeastern U.S. and Atlantic Canada were primarily (52/55) metalaxyl sensitive and all were A1 mating types. Among 85 isolates from late blight epidemics in Florida and Texas, greater than 61% were both metalaxyl resistant and A2 mating type. Metalaxyl resistance and A2 mating types were identified also in a few tomato isolates from North Carolina. Although the majority of 134 isolates from the Pacific Northwest (British Columbia and Washington) were metalaxyl resistant, only 2 isolates from Washington were A2 mating types. Among 111 isolates from 2 sites in central Mexico, 63% and 77% were both metalaxyl resistant and A2 mating types. The data indicate also a higher frequency of metalaxyl resistance in A2 isolates, than in A1 isolates, among isolates from Florida and Texas. Highest metalaxyl resistance levels were found, however, in A1 isolates from California, where no A2 isolates were recovered.
