Vol. 74, Part 3 Trans. Br, mycol. Soc. 74 (3) 449-457 (1980) Printed in Great Britain June 1980 TRANSFER OF THE PHYSODERMATACEAE FROM THE CHYTRIDIALES TO THE BLASTOCLADIALES By LENE LANGE Danish Government Institute of Seed Pathology for Developing Countries, Ryvangs AIle 78, DK-2900 Hellerup, Denmark AND L. W. OLSON Institute of Genetics, University of Copenhagen, 0ster Farimagsgade 2A, DK-1353 Copenhagen K, Denmark The relationship of Physoderma to the Chytridiales and the Blastocladiales based on light and electron microscopical studies is discussed. Electron microscopical observations of Physoderma resting sporangia and their spores are presented, providing new elements for a discussion of the taxonomic validity and position of the family Physodermataceae; the light microscopical appearance of Physoderma spores is re-evaluated; growth and morphology of the sporangium of Physoderma are described, relating it to the Catenariaceae (Blastoclad- iales) as well as to the Cladochytriaceae (Chytridiales). Based on similarities in spore structure, a conservative taxonomic character, transfer of the genus Physoderma (coexten- sive with the family Physodermataceae) from the Chytridiales to the Blastocladiales is proposed, maintaining it as a separate family. Species of the obligate plant parasitic genus Physoderma Wallroth (1833) were for several decades confused with, or considered closely related to, the rust and smut fungi (DeBary, 1864) and to Protomyees (Fuckel, 1866). This confusion was caused by the similarity in the host symptoms produced by Physoderma, and in many cases because of the similarity in form and colour of the resting sporangia of these fungi. The plant parasitic nature of Physoderma species (especially P. maydis Miyabe (1909), a serious pathogen of maize) attracted the interest of many plant pathologists and mycologists. How- ever, the difficulties in inducing the thick-walled resting sporangium of Physoderma species to germinate, and in obtaining controlled infection of its host, have prevented a full understanding of the life cycle of these fungi and the taxonomic position of the genus. This confusion is exempli- fied by the fact that the 50 species included in the monograph of Physoderma by Kading (1950) have previously been ascribed to 12 different genera (Caeomurus, Cladochytrium, Doassansia, Entyloma, Melanotaenium, Oedomyees, Olpidium, Protomyees, Pyenoehytrium, Synehytrium, Uromyees, Uro- phlyetis). Demonstration of ephemeral, epibiotic, thin- walled sporangia in Physoderma pulposum Wallr. (Schroeter, 1882) led to the placement of the genus in the Chytridiales, closely related to the genus Cladochytrium: Later, Fischer (1892) placed Physoderma along with Urophlyetis as subgenera of Cladochytrium. Sparrow (1942) proposed the transfer of Physoderma from the Cladochytriaceae to its own family (Physodermataceae) in the Chytridiales. This was based on the presence of an epibiotic, ephemeral, monocentric zoosporangial stage in addition to the polycentric endobiotic phase as opposed to Cladoehytrium in which only a polycentric thallus is formed bearing both resting sporangia and zoosporangia. The family Physo- dermataceae was later formally described by Sparrow (1952). Magnus (1902) attempted to distinguish Urophlyctis from Physoderma by differences in sexuality, resting sporangium mor- phology, and in the symptoms produced in the host. Although only the last character was appar- ently consistent, the concept of separating Uro- phl yetis from Physoderma was recognized by most mycologists and plant pathologists in the first part of this century (Minden, 1915; Gaumann, 1926). In a revision of the genus Physoderma, Kading (1950) merged Urophlyetis with Physoderma, as previously suggested by Sparrow (1943). Corres- pondingly, the genus Physoderma is co-extensive with the family Physodermataceae. The concept that the Physodermataceae constitutes a well- defined group of plant parasitic phycomycetes was brought about by extensive experimental and light microscopical studies, e.g. Sparrow (1964a, 1974, 1975 a, b) which demonstrated, for several species, most of the stages of the life-cycle (germination of 0007-1536/80/2828-6030 $01.00 © 1980 The British Mycological Society Vol. 74, Part 2, was issued 21 April 1980 15 MYC