We aimed to simultaneously detect two pathogens causing strawberry diseases, Phytophthora nicotianae and P. cactorum, by multiplex polymerase chain reaction (PCR), and to survey their occurrence in the main strawberry production areas of Japan. Due to the need to combine different primer pairs for multiplex PCR and the low specificity of published specific primers for P. nicotianae and P. cactorum, new species-specific primers for P. nicotianae and P. cactorum were designed based on the internal transcribed spacer regions of ribosomal DNA and the ras-related protein gene Ypt1, respectively. Specificity of the designed primers was demonstrated using 68 isolates, including Phytophthora spp., Pythium spp., and other soilborne pathogens. Multiplex PCR discriminated between P. nicotianae and P. cactorum in DNA mixtures of mycelia of the two species. Moreover, both species were detected in artificially and naturally infested soils, indicating that these markers can be used in diagnosis of strawberry diseases. For investigation of the geographic distribution of the two pathogens in Japan, soil samples were collected in 89 strawberry fields from eight prefectures (Gifu, Saga, Nara, Tochigi, Chiba, Shizuoka, Yamanashi, and Hokkaido) of Japan. The method that was developed was successfully applied to survey P. nicotianae and P. cactorum, and distribution of the two pathogens in strawberry plantings in Japan was determined.