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Ascocarp Dehiscence and Ascospore Discharge in Uncinula necator. David M. Gadoury, Research associate, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; Roger C. Pearson, associate professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 80:393-401. Accepted for publication 29 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-393.

Cleistothecia of Uncinula necator were hemispherical with a basal concavity unless in contact with free water. Upon wetting, the cleistothecia swelled to become spherical within 2 min. Water potential of ascospore cytoplasm decreased during maturation and overwintering, presumably resulting in increased pressure potential when intact cleistothecia were wet. Water potential of ascospore cytoplasm decreased from ?670 KPa in September, 2 wk after detachment of cleistothecia from the mildew colony, to ?2,990 KPa in May, 8 mo after leaf fall. The mass required to fracture the ascocarp wall also decreased during maturation, from approximately 3.0 g in autumn to 2.0 g in winter to 1.5 g in early spring. The most rapid decrease in the strength of the ascocarp wall occurred during a 4-wk period in early spring just before dehiscence in the field. This decrease occurred without a commensurate decrease in cytoplasmic water potential. Water potential of cytoplasm decreased when young ascocarps were incubated at 20 C, but not at 4 C. Flexing of the ascocarp wall during repeated cycles of wetting and drying at 4 C did not significantly weaken the ascocarp wall. Cleistothecia dehisced circumscissilely at the junction of the concave and convex surfaces of the outer ascocarp wall, near the site of a thin zone in the ascocarp wall, immediately beneath the equatorially attached appendages. Transmission electron micrographs of cleistothecia collected at various times during overwintering showed decreases in the quantity and changes in the distribution of cytoplasmic lipids as water potential decreased during maturation. Reducing sugars were not present at detectable levels in cleistothecia in autumn, but were detected in spring. Following ascocarp dehiscence, ascospores were discharged through a slitlike rupture of the apex of the ascus. In laboratory tests, the proportion of cleistothecia that dehisced after 4 hr was directly proportional to temperature between 10 and 32 C. Some cleistothecia dehisced within minutes of wetting, but the greatest proportion of cleistothecia dehisced within 4 hr after wetting. There was little change in the number of dehiscent ascocarps between 24 and 48 hr after wetting. Ascocarp dehiscence was nearly completely suppressed at 4 C. Initial dehiscence and ascospore release in vineyards coincided with bud burst of grapevines and continued for approximately 5 wk. Ascospores of U. necator, collected by a volumetric trap in a vineyard for two seasons, were trapped between bud burst and bloom of grapevines only during or immediately following rains of more than 2.5 mm. The anamorph of U. necator is widely regarded as a xerophytic plant pathogen, and rain generally is considered to be deleterious to development of epidemics of grape powdery mildew. However, rainfall appears to be a critical event in the release of ascospores and the initiation of powdery mildew epidemics in areas where cleistothecia are sources of primary inoculum.

Additional keywords: cleistocarp, Oidium tuckeri.