Hyphae rather irregular in width, up to 6 µm, with spherical to ellipsoid, monilioid or catenulate hyphal swellings, rare in some isolates, common in others. Sporangiophores 2 µm wide, branching in a close sympodium on agar media, or in a morelax sympodium when this is transferred to liquid culture. Sporangia broadly ovoid or obpyriform, av. 57 x 36 (max. 75 x 42) µm, with a short pedicel but non-deciduous; apex scarcely papillate, often slightly flattened with shallow thickening (up to 3,5 µm). Oogonia usually abundant in culture and in the host tissues, spherical, av. 33 µm (max. 46 µm) diam., wall at first thin and colourless but becoming thicker and yellower after the oospore is mature. Antheridium 10 x 7 µm, usually paragynous. Oospore almost or quite filling the oogonium, wall 2 µm thick. A low temperature fungus with an opt. c. 20°, and max. near 23°C. Mycelium in agar culture forms ose shaped growth zones.