Genotypes of the late blight pathogen (Phytophthora infestans) in British Columbia and other regions of Canada during 1993-1997

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Abstract
Isolates of Phytophthora infestans were collected from British Columbia at various times during 1993-1997 and from eight other provinces in Canada during 1993-1995 and characterized for mating type (either Al or A2), growth response to metalaxyl (either sensitive-MS or insensitive-MI), and random amplified polymorphic DNA (RAPD) patterns. Among the isolates tested, 312 were from diseased potato cultivars, while 14 originated from tomato or hairy nightshade plants with blight symptoms. A composite of RAPD patterns generated by nine primers following PCR amplification was used to identify novel genotypes based on presence of polymorphic bands. The occurrence in Canada of genotypes that corresponded to US I, US 6, US 7, and US 8 was confirmed during this study. The RAPD primers also identified 15 novel genotypes from British Columbia (BC) and 4 from New Brunswick, regions in which both Al and A2 mating types of P. infestans have previously been found and oospore production reported. In 1993, genotypes US 6 and US 7 were found in British Columbia, while US 1 was found in five other provinces (Alberta, Manitoba, New Brunswick, Prince Edward Island, and Quebec). In 1994, genotype US 8 was found in New Brunswick and US 1 in Alberta, Prince Edward Island, Quebec, and Saskatchewan. In 1995, US 8 was identified from all provinces except Alberta and British Columbia, while genotype US 1 was found in Alberta, Manitoba, and Prince Edward Island. Among the novel genotypes from British Columbia and New Brunswick, many were localized to specific geographic areas and most were ephemeral and not recovered in subsequent years. Only two genotypes from British Columbia - BC 1 (A2, MI) and BC 11 (Al, MI) - were recovered over several years and from different locations, indicating they had overwintered and likely spread through movement of infected plant materials or inoculum. Genotypes US 7 and BC 11 were recovered from tomato, and BC 11, BC 13, BC 14, and BC 15 from hairy nightshade, illustrating the importance of other solanaceous hosts as potential sources of inoculum. The results from this study illustrate the complex and changing genotypic structure of P. infestans, which may, in part, be responsible for the increased prevalence of late blight on potato and tomato in Canada. A complex genotype structure, such as that observed in British Columbia, may be characteristic of other regions in which both mating types are in close proximity. The diversity observed is likely the outcome of sexual recombination after oospore formation and germination under natural conditions.