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Abstract
Sphaerotheca fuliginea infecting cucurbits was demonstrated to be heterothallic. Isolates were obtained from pumpkin (Cucurbita pepo) in New York and summer squash (C. pepo) in Florida. Heterothallism was demonstrated by growing isolates singly or in pairs on detached leaves of summer squash in culture. These leaves were in two joined petri dishes with the leaf blade in the upper dish and the petiole in soil in the lower dish. Cleistothecia formed only when two compatible isolates were grown together. Six isolates were assigned to mating type MAT1-2 and 59 were assigned to MAT1-1. Special stimulatory conditions, such as low temperature or senescence of host tissue, were not required for cleistothecial formation on detached leaves or on greenhouse-grown plants. Hyaline cleistothecia were observed as soon as 8 days after inoculation and became brown in 1–5 days. Unequal distribution of mating types may at least partially account for the rare occurrence of cleistothecia worldwide.