Late blight of tomato and potato, caused by the fungal pathogen Phytophthera infestans, is currently the most destructive disease of tomato (Lycopersicon esculentum) in the Pacific Northwest. Geographically this includes regions stretching from the San Francisco Bay in California to the coastal islands of British Columbia. The importance of this disease, which has had far-reaching effects worldwide since causing the Irish Potato Famine in the 1840s, has increased substantially in the early 1990s when new clonal lineages of the A2 mating type migrated from the Toluca Valley of Mexico (Fry and Goodwin, 1997). Previous to this only genotypes of the A1 mating type of P. infestans had been detected outside of the Toluca Valley.
