Submitted to: Phytoprotection
Publication Type: Review Article
Publication Acceptance Date: 12/15/1999
Publication Date: N/A
Technical Abstract: Late blight of potato and tomato, caused by the oomycete Phytophthora infestans, was effectively controlled for decades in most developed countries by careful crop sanitation and judicious use of fungicides. Prior to 1976, populations of the pathogen worldwide were dominated by a single clonal lineage of the pathogen, designated US-1. Increased disease occurred in Europe during the mid-1980s and in the United States and Canad during the early 1990s. Analyses with molecular markers revealed that the increased disease problems were caused by new populations of P. infestans that had migrated from Mexico, the center of origin for the pathogen. These new migrating populations were resistant to fungicide, were more virulent, and were much more aggressive than the populations that had occurred previously. They also had much higher fitness than the old US-1 clonal lineage, and rapidly replaced US-1 in all locations sampled, usually ywithin 2-4 years. The mechanism of migration of new genotypes of P. infestans into the United States and Canada probably was by importation of infected tomato fruits from coastal production zones in northwestern Mexico. There are now two likely outcomes for P. infestans in the United States and Canada: the development of sexually reproducing populations due to the presence of both mating types; or a gradual turnover of clonal genotypes. Analyses of populations in northwestern Mexico have revealed a gradual turnover of clones from year to year, and this may be occurring already on tomatoes in the United States. These results highlight the importance of preventing pathogen migrations and of the use of molecular markers for detection and tracking of new genotypes.