Assessment of mating types and resistance to metalaxyl of Canadian populations of Phytophthora infestans in 1997

The distribution of A1 and A2 mating types, and of metalaxyl-sensitive (MS) , metalaxyl-intermediate (MI), and metalaxyl-resistant (MR) strains of Phyt ophthora infestans, the causal pathogen of late blight, were determined in Canada in 1997. Potato plants or tubers and tomato plants or fruits, suspec ted to have late blight infection, were received from seven Canadian provin ces in 1997, and were assessed for the presence of P. infestans. Among 220 samples suspected to have late blight, P. infestans was recovered from 132 samples (60%) received from British Columbia (BC), Manitoba, New Brunswick, Prince Edward Island, and Quebec. From the 132 samples, 364 isolates were obtained and tested for their mating type and response to metalaxyl. Popula tions of P. infestans within Canadian provinces were homogeneous for mating types, except within BC, where populations comprised both Al and A2 mating types both within fields and sometimes within samples. The A2 mating type was found in all provinces where late blight was present. The Al mating typ e was found only in Manitoba (1 isolate) and in BC (38 isolates). Unlike th e old Al isolates, those found in BC in 1997 mere highly resistant to metal axyl. These Al isolates are even more resistant than A2 isolates. Based on assays with 10 mu g.ml(-1) of metalaxyl, 14% of all tested isolates were MS , 69.8% were MI and 16.2% were MR. In addition, all provinces contained MS, MI, and MR isolates. No specific mating type or metalaxyl sensitivity leve l were specific to tomato or potato.