Publication no. D-2000-0421-01R

Implications of Sexual Reproduction for Phytophthora infestans in the United States: Generation of an Aggressive Lineage. P. D. Gavino, DuPont Fellow, Department of Plant Pathology, Cornell University, Ithaca, NY; C. D. Smart and R. W. Sandrock, Department of Plant Pathology, Cornell University, Ithaca, NY; J. S. Miller, Department of Plant Pathology, Washington State University, Pullman; P. B. Hamm, Department of Plant Pathology, P.O. Box 105, Oregon State University, Hermiston 97838; T. Yun Lee, Department of Plant Pathology, Cornell University, Ithaca, NY; R. M. Davis, Department of Plant Pathology, University of California, Davis 95616; and W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY. Plant Dis. 84:731-735. Accepted for publication 5 March 2000. Copyright 2000 The American Phytopathological Society.

Phytophthora infestans isolates (n = 26) collected in the Columbia Basin of Oregon and Washington in 1993, which had been characterized (previously for mating type, metalaxyl sensitivity, and alleles at the glucose-6-phosphate isomerase locus), were analyzed for nuclear restriction fragment length polymorphism (RFLP) bands detected by probe RG57 and mitochondrial haplotype. Analyses involving the larger set of markers indicated that this group of isolates satisfied expectations of a sexual progeny: they contained much greater genetic diversity than has been reported for most other epidemic populations of P. infestans in the United States and Canada (16 unique multilocus genotypes); both mating types were present in proximity; all possible combinations of alleles occurred at many pairs of polymorphic loci; and two distinct mitochondrial haplotypes were distributed among the isolates. An in vitro laboratory cross involving the putative parents (US-6 and US-7) as parental strains produced progeny with the same general characteristics as the field isolates. Among the field progeny were two genotypes, US-11 and US-16, that had been described previously but from subsequent and largely clonal collections. Isolates obtained from epidemics on tomatoes (n = 40) and potatoes (n = 7) in 24 counties in California in 1998 were analyzed as described above, and all except one US-8 isolate from potatoes were of the US-11 clonal lineage, consistent with the hypothesis that the US-11 lineage is an especially fit clonal lineage that has survived over time and can dominate pathogen populations over a large area. We conclude that the 1993 Columbia Basin collection represents a sexual progeny that generated the US-11 lineage, and that this lineage is particularly fit when tomatoes are part of the agroecosystem.