Notes on Wisconsin parasitic fungi. XX.

Abstract

This series of notes is, with som:e exceptions, based on collections made during the seasons of 1949 and 1950. The phytoplankton of Wisconsin lakes, particularly the Bacillariaceae, Desmidiaceae and Volvocaceae, as well as some of the filamentous algae, including Spirogyra, Zygnema anid Mougeotia, are rather frequently parasitized by chytridiaceous fungi, but determination of these forms, owing to their small size and transitory nature, is all too often highly uncertain. A. F. Bartsch, in a doctoral thesis (Univ. Wis. 1939) enititled "A Taxonomic Study of the Aquatic Chytridiales," lists for the state about twenty possible parasites, some very doubtfully so, mostly on filamnentous algae, and a few on water molds and pollen grains. Powdery mildews which did not form perithecia and are hence indetertmninable were noted oll 1) Brassica arvensis. Dane Co., Madison, September 10, 1950, and 2) Verbena hybrida (cult.). Dane Co., Madison, September 25, 1949. MICROSPHAERA ALNI (Wallr.) Wint. occasionally infects leaves of Ceanothus americanus without evidence of any particular damage to the host plants. However, in August 1949 the developing fruits of many large specimens of Ccanothus in the University of Wisconsin Arboretum at Madison were devastatingly blighted by a powdery mildew which did not form perithecia, but whicn I assume was M. alni. So heavy was the infection that, at a distance, the plants appeared to be still in full bloom, with characteristic white flower clusters. MYCOSPHAERELLA, so far undetermined, occurred in profuse development on overwintered leaves of Botrychium virginianum collected in May 1950 in the New Glarus Woods in Green Co. In my Notes XIV the immature phase of the fungus was described as being actively parasitic on the still green leaves in August 1949. The fungus appears to have had a preservative influence, since in the spring only heavily infected leaves were found, whereas in a section of the woods in which no infection was noted the previous summer, no overwintered leaves could be found. When the overwintered material was first examined it was found that, while asci had been delimited, mature ascospores were not yet formed. Accordingly, the leaves were placed in a moist chamber for 72 hours, when it was noted that well-developed ascospores had been produced. The perithecia are subglobos,e to globose, black, mostly about 60-80t diam.; the clavate asci are 7-8 x 30-40[t; the hyaline, uniseptate, slightly curved ascospores are 10-12 x 3-3.5j. There was no evidence of any welldefined imperfect stage preecding the Mycosphaerella, although in the August 1949 collection certain of the p,erithecial bodies contained bacillary microconidia about 4 x 1 u.