Shaw (1978) provided a useful description of the genus, as follows: “Mycelium parasitic in higher plants, hyaline, coenocytic; imperfect state like Sclerospora except that conidia are always produced rather than sporangia. Conidiophores produced at night, erect, dichotomously branched two to five times; sterigmata conoid to subulate, usually two, but three or four in some species. Conidia ellipsoid, ovoid or cylindrical, wall of uniform structure, neither operculate or poroid, always germinating by a single germ tube. Oogonia subglobose to spherical. Oospores globose or subglobose, 25–55 μm in diam; oospore wall partially or ompletely fused to the wall of the oogonium, oospore wall of three layers: exosporium chestnut to reddish brown at maturity, irregularly ridged, 1.0–3.0 μm thick; mesosporium very thin, hyaline; endosporium hyaline, uniformly thick, 1.5–3.5 μm thick.
Notes: The distinction between what we now recognize as Peronosclerospora and the genus Sclerospora was first pointed out by Ito (1913), who split Sclerospora into two subgenera based on differences in asexual spore germination, which occurs directly by germ tubes in Peronosclerospora and indirectly by zoospores in Sclerospora. Ito recognized that two taxa would fall into the new subgenus Sclerospora subgen. Peronosclerospora; namely Sclerospora sacchari and Sclerospora graminicola var. andropogonis-sorghi (Ito 1913).Sclerospora subgen. Peronosclerospora was described as the genus Peronosclerospora in 1927 (Shirai & Hara 1927), with just one species (Peronosclerospora sacchari) transferred as the generic type (Shirai & Hara 1927). The original description of Peronosclerospora went unnoticed among some members of the scientific community, resulting in the description of several non-zoosporic species in the genus Sclerospora rather than in Peronosclerospora (Sclerospora dichanthiicola, Sclerospora philippinensis, Sclerospora sorghii, Sclerospora westonii), and a second, superfluous description of the genus in 1978 (Shaw 1978, Shaw & Waterhouse 1980).
From a practical standpoint, discriminating between Peronosclerospora and Sclerospora based on differences in asexual structures is not a trivial matter. Development of asexual spores by members of both genera is nocturnal under natural conditions. In Peronosclerospora, structures persist for just a few hours in the early morning until they germinate under conducive environmental conditions (e.g., Sriinivasan et al. 1961). After germination, the asexual spores and related structures rapidly collapse, leaving no trace behind. As a result, asexual structures are not well preserved on herbarium materials and other collections on non-living host material, hindering identification and taxonomic study. Structures of Sclerospora last longer, but within a few days can also vanish. The challenging application of asexual spore morphology for Peronosclerospora identification is further complicated by the impact of environmental effects, such as host species, variety, and climate, on spore size and shape (Delanie 1972, Leu 1973, Kimigafukuro 1979, Bock et al. 2000, Dudka et al. 2007, Runge & Thines 2011).