Germination of Oospores of Sclerospora sorghi in the Presence of Growing Roots of Host and Nonhost Plants

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Abstract
Oospores of S. sorghi maintained between porous membranes in soil germinated at low frequencies adjacent to growing roots of sorghum, corn, oats, wheat, soybeans and cotton. Germination occurred at similar frequencies in the presence of roots of sorghum seedlings of cultivars both resistant and susceptible to downy mildew, and also in nontreated field soil, autoclaved soil and sand. No germination was observed in the absence of plant roots. Germination always resulted in formation of single, broad, coenocytic germ tubes which were seldom branched and elongated up to 1.25 mm prior to autolysis. Germination occurred consistently with oospores stored at 25.degree. C for 8 mo., but not with oospores stored at 4.degree. C; however, when oospores were transferred from 4.degree.-25.degree. C, the frequency of germination increased steadily after 2, 4 and 6 wk. Most germination occurred within 3 days after growth of roots adjacent to oospores. In all experiments, numerous instances of false germination were observed, due to growth of hyphae of mycoparasitic fungi from within oospore walls. Criteria for distinguishing true and false germination of oospores of S. sorghi are discussed.

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