Aphanomyces mitsuba sp. nov. causing stem rot of “mitsuba”, Cryptotaenia japonica, in hydroponic culture

A new species of the oomycete Aphanomyces mitsuba sp. nov. was isolated from rotting stems and petioles of hydroponically grown mitsuba seedlings. Stem and petiole rot was observed in hydroponic cultivation of “mitsuba”, Cryptotaenia japonica , in Japan. An isolate of Aphanomyces , which has non-septate mycelia and forms bitactic zoospores from filamentous zoosporangia, was obtained from the rotted parts of the plant. In this study, we identified the isolate based on molecular phylogenetic analyses, morphological characteristics, and growth temperature response, and investigated its virulence on “mitsuba”. The rDNA ITS sequence-based molecular phylogenetic analysis showed that the mitsuba isolate was monophyletic and closely related to Aphanomyces iridis . Multigene phylogenetic analysis of the phytopathogenic Aphanomyces species, which was based on the combined sequences of the rDNA ITS region, β-tubulin, cox 1, and cox 2, also indicated that the mitsuba isolate was located in a monophyletic clade. Morphologically, the isolate is characterized by no-branching, no-tapering zoosporangia, occasionally connected oogonia, and rare coiling of antheridial stalks on oogonial stalks. It is a relatively high-temperature growing species, with an optimum growth temperature of 30 °C, with the capability of growth at 35 °C. The results indicated that the mitsuba isolate is a new species, Aphanomyces mitsuba sp. nov. The new species was pathogenic to “mitsuba”, causing stem and petiole rot and significantly suppressing growth, suggesting that this is a phytopathologically distinct species.