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.