Multiplex PCR Specific for Genus Phytophthora and P. Nicotianae with an Internal Plant DNA Control for Effective Quarantine of Phytophthora Species in Japan

To prevent threats from pathogens such as Phytophthora species from international plant trade, molecular identification techniques are needed for rapid, accurate quarantine inspection. Here, for quarantine control in Japan, we developed a simple DNA extraction for plants and a practical detection method that combines multiplexed PCR using primers specific for Phytophthora species, for P. nicotianae, which is the only non-quarantine Phytophthora species, and as internal controls, for plants. For the new genus-level primer set, we modified previously reported genus-specific primers to improve detectability. The new primers were able to detect mycelial DNA of 155 taxa among Phytophthora clades 1–10, with a sensitivity of 100 fg/µL for three representative species, P. ramorum, P. kernoviae and P. nicotianae. In the PCRs using DNA from non-target species, amplification was observed for only three taxa, and for some strains, four taxa in a closely related genus. Duplex and triplex PCR of the genus-specific primers combined with previously reported plant primers verified the success of DNA extraction and PCR detection from diseased plant samples, and in the triplex PCR, whether the pathogen was diagnosed as P. nicotianae or not by the species-specific primer. The new method detected the pathogen in naturally infected and inoculated plants. The amplicons using the genus-specific primer have enough variation to be sequenced to identify the species. This new method can be used immediately for detecting Phytophthora species and for quarantine control in Japan.