OCCURRENCE OF THE A2 MATING TYPE OF PHYTOPHTHORA-INFESTANS IN POTATO FIELDS IN THE UNITED-STATES AND CANADA

Ten isolates of P. infestans collected from blighted potato fields in the U.S. and Canada during the years 1983-1989 were examined for mating type on lima bean, oatmeal, and rye grain media. Two of the ten isolates produced oospores in less than 15 days when cultured in the presence of known A1 mating types from the USA, Mexico, and Europe, indicating they are A2 mating types. When the two A2 isolates (one from Pennsylvania and one from Vancouver, B.C.) were cultured singly or with known A2 isolates, no oospores formed after 6 weeks of culture. Pathogenicity tests showed that both isolates produced typical late blight symptoms on potato foliage and stems, and there appeared to be no difference in virulence between these two A2 and the eight A1 isolates. Inoculations with a mixture of A1 + A2 sporangia also produced oospores in host tissues. This is the first report of the presence of the A2 mating type of P. infestans in the U.S. and Canada.