OBSERVATIONS ON CHYTRIDIACEOUS PARASITES OF PHANEROGAMS VII. A PHYSODERMA ON LYCOPUS AMERICANUS

IN THE COURSE of routine collections of fungi, made during the summer of 1956 in northern Michigan, a species of Physoderma was found on Lycopus arnericanus. Since no species of this fungus genus have been described as attacking this host, a brief account of the main features of structure, reproduction and life history of the parasite seemed of interest. The fungus remained subepidermal and induced on the host stem somewhat ellipsoidal, slightly raised pinkish or pale-brown pustules which were distinct or more or less confluent. They were up to 1.5 mm. long by 0.5 mm. wide and were primarily on the fifth and sixth internodes of the stem. These internodes were probably in the bud at the time of infection. It is possible, however, that the apical meristem was not infected; at least internodes above the sixth rarely harbored the fungus. ~ery heavy infections produced a marked swelling and dark purplish discoloration of the internode for a distance of 1.0 em. or more. Less often, the leaves were infected, again, primarily those at the fifth and sixth nodes. Here, the pustules were much smaller than those on the stem and were blackishbrown. They were more commonly found on the petiole than on the lamina. In the latter region they were either scattered over the lamina or aggregated near the leaf apex. The precise reasons for the striking localization of the infection must await further study. Quite obviously, a relationship exists between water level, time of germination of the overwintered resting spores, time of infection and stage of growth of the host. It will be recalled that species of Physoderrna havc two very distinct phases in their life history. The well-known endobiotic one consisting of an extensive much-branched strongly polycentric rhizoidal system bears septate turbinate cells and, at maturity, numerous disproportionately large, thick-walled, amber or brown· resting spores. In certain species (as P. menyanthis) , the latter assume somewhat the shape of their confining host cell, or in others (as P. maydis) lie loosely in the host cells and are somewhat hemispherical in outline with a more or less flattened face. This phase induces the formation of dark-colored pustules, streaks, galls etc. on the host. The so-called "ephemeral sporangial phase" on the other hand, is invisible macroscopically. It consists of an epibiotic, typically chytridiaceous zoosporangium-