Mycoparasitism of oospores of Pythium and Aphanomyces species by Hyphochytrium catenoides

Oospores of Pythium myriotylum in nonsterile soil extracts became invaded by a fungus that was isolated and identified as Hyphochytrium catenoides. Sporangia of H. catenoides formed as spherical or ovoid lobes within the Pythium oospore and as radial catenulate swellings on agar media. In water, these sporangia formed short exit tubes through which anteriorly monoflagellated zoospores were released. Oospores of P. myriotylum and Aphanomyces euteiches placed in suspensions of motile zoospores of H. catenoides became extensively invaded within 48 h. Oospores of Pythium aphanidermatum and P. ultimum were also attacked, but these species were less susceptible to invasion than was P. myriotylum. Sporangia, hyphae, and zoospores of the Pythium spp., A. euteiches, and several other phycomycetes were not observed to be invaded. Zoospores of H. catenoides were formed and released in water adjusted to pH 5.0–7.0 and at temperatures of 10–35 °C, but were produced most abundantly and were most highly motile at pH 6.0–7.0 and at 20–30 °C. The roots of several crop plants did not appear to be parasitized by H. catenoides. Oospores of P. myriotylum, P. aphanidermatum, and P. ultimum in moist soil artificially infested with these species and H. catenoides were destructively invaded by the mycoparasite. Its presence in several soils and its ability to reproduce quickly in its parasitic stage suggest that H. catenoides may play a role in the natural decline of susceptible Oomycetes in soil.