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The study of the genetics of resistance in pepper to the oomycete pathogen Phytophthora capsici has been complicated due to a lack of use of a common set of pathogen isolates and host genotypes. We have developed a differential series for this system using eleven pepper genotypes and thirty-four isolates of the pathogen from California, New Mexico, North Carolina, and Turkey. Through differential patterns of virulence of the isolates on the hosts, we identified fourteen different physiological races of P. capsici. There appears to be no restriction of races to particular geographical locations. Isolate mating types were also determined, and both mating types were found in one field in California. The significance of the characterization of physiological races and existence of both mating types in the field to pepper growers and breeders is discussed. [ABSTRACT FROM AUTHOR]