Physoderma

Physoderma is a genus of chytrid fungi. Described by German botanist Karl Friedrich Wilhelm Wallroth in 1833, the genus contains some species that are parasitic on vascular plants, including P. alfalfae and P. maydis, causative agents of crown wart of alfalfa and brown spot of corn, respectively. Of the chytrid genera, Physoderma is the oldest. However, species were confused with the rust fungi, the genus Synchytrium, and the genus Protomyces of Ascomycota. Members of Physoderma are obligate parasites of pteridophytes and angiosperms. There are approximately 80 species within this genus (depending on whether one includes those traditionally belonging to Urophlyctis).

==Taxonomic history== The genus was erected in 1833 on the basis of resting spore development. and included 6 species. Unfortunately, his original diagnosis was very similar to that of Protomyces, which led others to place species in the wrong genus. In 1877, Nowakowski erected the genus Cladochytrium in the Chytridiales, which led to the transfer of Physoderma to the Chytridiales as well by Schroeter in 1883. Just prior to that (1882), Schroeter added an additional 4 species to the genus and noted, for the first time, epibiotic, ephemeral zoosporangia. He also claimed that sexual reproduction was through the fusion of two cells and resulted in the resting spores. In 1889, Schroeter created the genus Urophlyctis for those species with epibiotic, ephemeral zoosporangia and sexually derived resting spores. He placed both in the same subfamily as Cladochytrium. In 1891, Fischer refuted Schroeter's observations on sexual reproduction and merged Physoderma and Urophlyctis with Cladochytrium. In 1897, Schroeter separated them once more. Magnus, in 1901, used characteristics of the resting spore and host plant reaction to distinguish between Physoderma and Urophlyctis. He claimed that resting spores from Physoderma were globose and ellipsoidal, and those from Urophlyctis were flattened on one side. Physoderma species cause discoloration and slight malformation, while Urophlycits cause significant malformation and hypertrophy. Sparrow, in numerous publications, expressed concerns over the characters used to distinguish the two genera. In 1943, Sparrow suggested that the genus be merged with Urophlyctis, which was done by Karling in 1950. He also moved Physoderma to its own family, the Physodermataceae. Sparrow, in 1962, decided the genera were distinct based on morphology and host reaction. However, at this time, the two genera are considered synonymous. Typically, it was thought that Physoderma was related to the polycentric genera Cladochytrium and Nowakowskiella Based on the ultrastructure of the zoospore, it was realized that Physoderma belongs to the Blastocladiales, which later became the Blastocladiomycota. Recent phylogenetic analyses indicate that Physoderma and Urophlycits might be separate genera.