Characteristics of populations of Phytophthora infestans from potato in British Columbia and other regions of Canada during 1993 to 1995

The incidence and distribution of A1 and A2 mating types of Phytophthora infestans in Canada and of metalaxyl-sensitive (MS) and -insensitive (MI) strains were monitored during 1993, 1994, and 1995. Diseased leaves from about 1,600 plants were collected from 36 cultivars in 168 potato fields from five province's at various times (June to September) during the growing season. The most extensive sampling (88% of total) was conducted in British Columbia (B.C.). About 500 isolates of P. infestans were characterized from over 1,000 collected. In 1993, the A1 mating type was found in all provinces sampled, while western Canada (B.C.) was the only region in which the A2 mating type was detected. In 1994, the A2 mating type was also found in eastern Canada (New Brunswick [N.B.]). In B.C., 23 fields sampled in 1993 had both A1 and A2 mating types, while in N.B. in 1994, both mating types originated from the same plant in seven different fields. When lesions from two of these fields were examined by microscope, sporangia of P. infestans and a single oospore were seen in leaf tissues, demonstrating the potential for oospore production in naturally infected leaves in western and eastern Canada if both mating types are present. Metalaxyl sensitivity tests measuring relative growth of isolates with metalaxyl at 0 and 50 micrograms/ml revealed that all isolates collected in 1993 from Alberta Manitoba, N.B., Nova Scotia, Ontario, Prince Edward Island, and Quebec were MS. In B.C., isolates showed a range of variation in growth with and without metalaxyl, and 76% of the isolates were MI. There was no correlation between recovery of these MI strains and use or nonuse of metalaxyl during the same growing season. Both MS and MI strains were present together in six fields. A low frequency of MS strains of both mating types was always recovered in B.C. When the frequency of occurrence of MS and MI strains throughout the growing season was examined, a high proportion of isolates collected early (June to July) from B.C. and N.B. in 1993 and 1994 was found to be MI. At the end of the season (September), MI isolates also occurred at a higher frequency in both locations and years. When the frequency of mating types was examined, the A2 type occurred at a higher proportion throughout the season in B.C. during 1993 and 1994. However, in 1995, the A2 mating type was rarely recovered and the A1 type predominated. In N.B. in 1994, the A1 mating type was recovered at a higher frequency than the A2 type in July, but most collections later were of mixed mating type. When the isolates were grouped into A1, A2, MS, and MI categories and growth rates were compared, isolates from populations in B.C. and N.B. in 1994 of A2 mating type grew significantly faster than A1; isolates that were MI from populations in B.C. in 1993 grew significantly faster than MS isolates. However, no differences could be detected in the extent of leaf colonization or sporulation between isolates representing A1, A2, MS, or MI. A preliminary survival study showed that P. infestans could overwinter under B.C. conditions on artificially inoculated tubers. These results illustrate the dynamic nature of populations of P. infestans within and between growing seasons in western and eastern Canada, and demonstrate higher variation in the population in B.C.