Submitted to: International Agronomy Congress Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: June 9, 1999
Publication Date: N/A
Technical Abstract: Late blight of potato and tomato, caused by Phytophthora infestans, was effectively controlled for decades in most developed countries by careful crop sanitation and judicious use of fungicides. This changed during the mid 1980s in Europe and the early 1990s in the United States and Canada due to changes in the pathogen populations. New populations of the pathogen were resistant to fungicide, were more virulent, and were much more aggressive than the previous populations. Analyses with molecular markers revealed that the new populations rapidly replaced the populations that occurred previously, usually within 2-4 years. Migrations into the United States and Canada probably occurred by movement of infected tomato fruits from coastal production zones in northwestern Mexico. The US-8 genotype was first detected in New York during 1992 and in the seed producing regions of Maine by 1993. During 1994-1996 this genotype spread throughout tthe potato-producing regions of the United States and Canada, where it is still the predominating genotype. There are now two likely outcomes in the United States and Canada: the development of sexually reproducing populations; or a gradual turnover of clonal genotypes. Analyses of populations in northwestern Mexico have revealed a gradual turnover of clones from year to year. These migrations have serious ramifications for plant health and quarantine operations. The initial migrations probably occurred on tomato fruits, which were not screened for late blight. The new genotypes moved rapidly because they were resistant to fungicide and were more aggressive than the previous genotypes. This highlights the importance of preventing pathogen migrations and of the use of molecular markers for detection and tracking of new genotypes.