Novel genotypes of Phytophthora infestans in Canada during 1994 and 1995

Populations of Phytophthora infestans, causing late blight of potato, chang ed significantly in Canada in the mid-1990s. The US-1 genotype (A1, metalax yl-sensitive [MS]) was displaced by novel genotypes (mainly US-8) in much o f the country in 1994 and 1995. Many of the new genotypes were insensitive to metalaxyl and were of the A2 mating type, although metalaxyl-insensitive (MI) A1 isolates of the g11 (or US-11) genotype were found in British Colu mbia. A total of 14 unique mnltilocus genotypes were described over the two years of the study based on mating type (Al or A2), metalaxyl sensitivity (MS or MI), allozyme banding patterns (Gpi and Pep), and random amplified p olymorphic DNA (RAPD) markers. Six of these genotypes could be distinguishe d by allozyme banding patterns at the Gpi locus alone; RAPD analysis reveal ed additional variation within allozyme groupings and defined additional ge notypes. Among 82 isolates collected from various Canadian provinces in 199 4, US-1 and US-8 constituted 30% and 49% of isolates, respectively; other g enotypes occurred in minor frequency. Among 50 isolates collected in 1995, US-1 and US-8 constituted 10% and 74%, respectively. Isolates of g11 (US-11 ) were found in British Columbia in 1995. Diversity of genotypes was greate r in 1994 (11 genotypes) than in 1995 (six genotypes). We hypothesize that sexual reproduction may have been responsible for generating some of the ge netic variability in P. infestans populations, during a time period in whic h A2 isolates first migrated into many provinces in Canada.