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 sampl es yielding 555 isolates), 1995 (185 samples yielding 914 isolates), and 19 96 (300 samples yielding 1013 isolates). Characterization of these isolates according to mating type and sensitivity to metalaxyl revealed the changin g nature of P. infestans populations in Canada. In 1994, isolates of the tr aditional Al, metalaxyl-sensitive (MS) phenotype were common, but by 1996, they were no longer recovered from any tissue samples. Isolates of the A2 m ating type, which were predominantly insensitive to metalaxyl (MI), were pr evalent in 1996, except in British Columbia (B.C.). In B.C., Al isolates hi ghly insensitive to metalaxyl predominated. Isolates of the Al, MI phenotyp e were also found in a sample of tomatoes from Ontario in 1996. New populat ions composed of the A2 mating type were predominantly insensitive to metal axyl, 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. T he 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. Bot h mating types of P. infestans were found in the same tissue sample a maxim um of three times in a given :year; self-fertile isolates were not recovere d. Of 627 tissue samples examined over 3 years, one was found to have oospo res of P. infestans present, indicating that sexual reproduction had occurr ed. These results demonstrate that populations of P. infestans in Canada ha ve changed dramatically from 1994 to 1996 and that the original Al, MS popu lation was rapidly displaced.