Migration from northern Mexico as the probable cause of recent genetic changes in populations of Phytophthora infestans in the United States and Canada

Isolates of Phytophthora infestans from five U.S. states (California, Florida, New York, Washington, and Wisconsin) and one Canadian province (British Columbia) collected between October 1987 and October 1991 were analyzed for mating type and for genetic variation at two allozyme loci. A subset of these 155 isolates, which included all of the allozyme genotypes plus 26 isolates from our culture collection that had been collected in the United States and Canada during 1979-1987, were analyzed for DNA fingerprint variation. These analyses revealed 18 genotypes among the 120 isolates that were analyzed for all markers. One genotype (US-1), which occurred in New York and Wisconsin in 1987 and 1991, respectively, and also in older isolates dating back to 1979, has been found throughout the world and may have persisted asexually in the United States and Canada since the first reports of late blight disease in the 1840s. Four other genotypes that appeared to be members of the same clonal lineage were variants of US-1. The most common genotype in the collections from 1987 to 1991, US-6, was identical to previously characterized isolates from northwestern Mexico, so it was probably a recent migrant. Five additional genotypes were variants of US-6 and differed from it by a single allozyme or DNA fingerprint allele; thus, they were probably members of a single clonal lineage. Overall, 13 of the 18 genotypes identified in isolates collected during 1979-1991 could have resulted from two probable migration events. Only two isolates (1.3% of the total sample) from a single field in British Columbia in 1991 were A2, and these also were probably introduced recently from Mexico. Each field sampled appeared to have been colonized by a single clonal lineage. Therefore, future surveys in the United States and Canada can be accomplished most efficiently by sampling only a low number of isolates per field but increasing the number of fields sampled. These data provide a baseline for monitoring future genetic changes in populations of P. infestans in the United States and Canada.