Characterization of changes in populations of Phytophthora infestans in Canada using mating type and metalaxyl sensitivity markers

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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 response between the high sensitivity of the original Al population and the highly insensitive Al populations in B.C. The exception was a few A2, MS isolates obtained from B.C. in 1996. The correlation between recovery of MI isolates and metalaxyl use was poor. There was a tendency for the proportion of MI isolates from fields managed with or without metalaxyl to increase as the growing season progressed. Both mating types of P. infestans were found in the same tissue sample a maximum of three times in a given year; self-fertile isolates were not recovered. Of 627 tissue samples examined over 3 years, one was found to have oospores of P. infestans present, indicating that sexual reproduction had occurred. These results demonstrate that populations of P. infestans in Canada have changed dramatically from 1994 to 1996 and that the original Al, MS population was rapidly displaced.