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Disease Note.

Cleistothecia of the Powdery Mildew Fungus Sphaerotheca fuliginea Observed on Pumpkin in New York. M. T. McGrath, Cornell University, Long Island Horticultural Research Laboratory, 39 Sound Avenue, Riverhead, NY 11901-1098. Plant Dis. 75:1075. Accepted for publication 2 July 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1075C.

Although the conidial state of Sphaerotheca fuliginea (Schlechtend.:Fr.) Pollacci occurs annually throughout North America, there have been only three published reports on the occurrence of cleistothecia: on Cucurbita pepo L. var. melopepo (L.) Alef. in the Imperial Valley of California in November 1978; on Cucumis sativus L. growing in a research glasshouse in Ontario, Canada, in November 1983; and on several cucurbit species in North Carolina during the fall of 1986 (1,2). Since these reports, cleistothecia have been observed rarely to recur in California and Ontario, have not been observed again in North Carolina, and have never been reported from Florida, Georgia, South Carolina, or Ohio (D. G. Kontaxis, W. R. Jarvis, L. F. Grand, C. W. Averre, K. L. Pohronezny, D. R. Sumner, C. E. Thomas, and R. M. Riedel, personal communications). On Long Island, New York, cleistothecia were observed on pumpkin (C. pepo) in research plots and in commercial fields during September 1989, approximately I mo after conidia were first seen. Each cleistothecium contained one ascus. Cleistothecia were grouped, primarily on the lower surface of heavily infected, senescing leaves but also on upper leaf surfaces, petioles, and green leaves. Cleistothecia were again seen in late August 1990. Environmental conditions, including frost and more hours of sunshine than the average, have been suggested as potential stimuli for development of cleistothecia (1,2); neither of these conditions had occurred prior to my observations of cleistothecia on C. pepo, S. fuliginea most likely is heterothallic, as is the case with several other powdery mildew fungi. The few recent reports from climatically and spatially diverse regions of North America suggest that the less common mating type is becoming more widespread and an environmental stimulus is not required for development of cleistothecia. The occurrence of the sexual stage is noteworthy because of the potential for increased genetic diversity and overwintering ability.

References: (1) L. F. Grand. Myeologia 79:484, 1987. (2) W. R. Jarvis and K. Slingsby. Plant Dis. 68:536, 1984.