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Population Structure and Resistance to Mefenoxam of Phytophthora capsici in New York State

In 2006, 2007, and 2008, we sampled 257 isolates of Phytophthora capsici from vegetables at 22 sites in four regions of New York, to determine variation in mefenoxam resistance and population genetic structure. Isolates were assayed for mefenoxam resistance and genotyped for mating type and five microsatellite loci. We found mefenoxam-resistant isolates at a high frequency in the Capital District and Long Island, but none were found in western New York or central New York. Both A1 and A2 mating types were found at 12 of the 22 sites, and we detected 126 distinct multilocus genotypes, only nine of which were found at more than one site. Significant differentiation (F_ST) was found in more than 98% of the pairwise comparisons between sites; approximately 24 and 16% of the variation in the population was attributed to differences among regions and sites, respectively. These results indicate that P. capsici in New York is highly diverse, but gene flow among regions and fields is restricted. Therefore, each field needs to be considered an independent population, and efforts to prevent movement of inoculum among fields need to be further emphasized to prevent the spread of this pathogen.