OBSERVATIONS ON CHYTRIDIACEOUS PARASITES OF PHANEROGAMS. XXVII. A FURTHER STUDY OF PHYSODERMA HYDROCOTYLIDIS VIÉGAS AND TEIXEIRA

A further study of Physoderma hydrocotylidis from California on its host Hydrocotyle ranunculoides gives further details on its structure and development. EM micrographs reveal that the notably thin wall of the resting spore consists of an outer, an intermediate, and an inner layer. The complete wall thickness from measurements of these EM preparations is only 0.5 um, the thinnest of any known Physoderma resting spore. Such resting spores germinate readily in 2-5 h by the dehiscence of a broad cap and formation of a protruding endosporganium. Zoospores from the latter infect epidermal host cells and produce either the endobiotic, polycentric rhizoidal system with tenuous filaments, turbinate cells and eventually resting spores, or a hitherto unknown epibiotic, monocentric, rhizidiaceous sporangial stage. Zoospores from the latter may all bear colorless lipid globules or all faintly orange-colored ones. IN an earlier publication (Sparrow, 1968) a brief account was given of the obligate parasitic fungus Physoderma hydrocotylidis, first described from Brazil by Viegas and Teixeira (1943). Notes on its distribution in the Eastern United States and California, its appearance on the host leaves and germination of its resting spores were included in this account. No clear picture of its method of infection, polycentric endobiotic system on which the resting spores were formed, and no information on the monocentric epibiotic stage were given. Primarily, these considerable gaps in information were the result of the failure to obtain seedlings of the host which could be used to follow the development of the fungus from zoospore to zoospore in both stages; mature host tissue was too opaque to be of value in this respect. Recent studies, however, on the nearly transparent underwater bud scales of the Hydrocotyle, have obviated these difficulties and these essential features of the parasite were followed with great clarity. MATERIALS AND METHODS-Infected host material collected in California in 1967 and maintained in tanks at the University of Michigan Matthei Botanical Gardens was used in this study. Unopened leaf and floral buds together with a portion of the shoot just below the node bearing them were excised and placed in shallow dishes of charcoal water containing swimming R.S. (resting spore) zoospores of the fungus and left in daylight in damp chambers (petri dishes) at 22-26 C. These R.S. zoospores had been ob:'Received for publication 14 December 1976; revision accepted 15 May 1977. tained from resting spores of Physoderma teased out from infected, rotted blades and petioles of the host which had been lying in water. Such spores germinated readily in the dishes of charcoal water and provided an abundance of the zoospores aforementioned. At intervals, starting with 12 h of exposure tc R S. zoospores, bud scales were excised and ex amined for stages of the fungus, particularly for the epibiotic stage earlier suspected of being absent. This species of Physoderma is unique in my experience in the ease and speed with which its resting spores germinate. It will be recalled that it is the aggregation of these bodies, formed endophytically in the unhypertrophied but eventually killed local areas of host parenchyma, that produces the cinnamon brown to black pustules on petioles and leaves and occasionally on rhizomes of the host (Sparrow, 1968). These somewhat ellipsoidal to irregularly shaped resting spores, 15-25 /m x 12-16 /m, slightly flattened on one face, have two remarkable features. The first of these is an extremely thin, faintly brown wall, which, with a Filar micrometer eyepiece measured 0.8-0.9 /%m and from EM preparations was calculated as being even thinner (0.5 gm). This is the thinnest known wall of a Physoderma rest-