CHARACTERIZATION OF CHANGES IN POPULATIONS OF PHYTOPHTHORA INFESTANS IN CANADA USING MATING TYPE AND METALAXYL SENSITIVITY MARKERS

Isolates of Phytophthora infestans were obtained from individual fields of potato and tomato and from potato storages across Canada in 1994 (142 samples yielding 555 isolates), 1995 (185 samples yielding 914 isolates), and 1996 (300 samples yielding 1013 isolates). Characterization of these isolates according to mating type and sensitivity to metalaxyl revealed the changing nature of P. infestans populations in Canada. In 1994, isolates of the traditional Al, metalaxyl-sensitive (MS) phenotype were common, but by 1996, they were no longer recovered from any tissue samples. Isolates of the A2 mating type, which were predominantly insensitive to metalaxyl (MI), were prevalent in 1996, except in British Columbia (B.C.). In B.C., Al isolates highly insensitive to metalaxyl predominated. Isolates of the Al, MI phenotype were also found in a sample of tomatoes from Ontario in 1996. New populations composed of the A2 mating type were predominantly insensitive to metalaxyl, but they showed an intermediate respons...