Department of Plant Pathology, Michigan State University, East Lansing 48824.
Phytophthora capsici Leonian is a destructive soilborne pathogen that infects economically important solanaceous, cucurbitaceous, fabaceous, and other crops in the United States and worldwide. The objective of this study was to investigate the genetic structure of 255 P. capsici isolates assigned to predefined host, geographical, mefenoxam-sensitivity, and mating-type categories. Isolates from six continents, 21 countries, 19 U.S. states, and 26 host species were genotyped for four mitochondrial and six nuclear loci. Bayesian clustering revealed some population structure by host, geographic origin, and mefenoxam sensitivity, with some clusters occurring more or less frequently in particular categories. Bayesian clustering, split networks, and statistical parsimony genealogies also detected the presence of non-P. capsici individuals in our sample corresponding to P. tropicalis (n = 9) and isolates of a distinct cluster closely related to P. capsici and P. tropicalis (n = 10). Our findings of genetic structuring in P. capsici populations highlight the importance of including isolates from all detected clusters that represent the genetic variation in P. capsici for development of diagnostic tools, fungicides, and host resistance. The population structure detected will also impact the design and interpretation of association studies in P. capsici. This study provides an initial map of global population structure of P. capsici but continued genotyping of isolates will be necessary to expand our knowledge of genetic variation in this important plant pathogen.