Use of allozyme markers to determine genotypes of Phytophthora infestans in Canada

A comparison of allozyme banding patterns at two loci, for glucose-6-phosphate isomerase (Gpi) and peptidase (Pep), with markers for mating type, metalaxyl sensitivity, and cultural morphology revealed eight distinct genotypes among 726 isolates of Phytophthora infestans collected in Canada between 1994 and 1996. Banding patterns for allozymes of the Gpi locus alone proved to be adequate for distinguishing seven of the genotypes. Five of these genotypes (US-1, US-6, US-7, US-8, and g11 (or US-II)) were comparable to genotypes described in the United States. The US-I genotype (Al mating type, metalaxyl-sensitive) was recovered commonly (41% frequency) throughout Canada (outside the province of British Columbia) during 1994. By 1996, the US-I genotype was no longer recovered from any samples of potato or tomato tissue taken from across Canada. The US-8 genotype (A2 mating type, metalaxyl-insensitive) dominated populations of the pathogen in Ontario, Quebec, and New Brunswick during 1994 and by 1996, the US-8 genotype was the dominant genotype in Canada outside British Columbia. In British Columbia, the g11 (US-11) genotype (Al mating type, metalaxyl-insensitive) was the dominant genotype recovered from tissue samples received during 1995 and 1996. Allozyme banding patterns were correlated to mating type, metalaxyl sensitivity, and cultural characteristics, implying that populations were mainly clonal. Only minor variation in metalaxyl sensitivity was found among provincial populations of the US-8 genotype.