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Ascospore Dispersal and Infection of Grapes by Guignardia bidwellii, the Causal Agent of Grape Black Rot Disease. Donald M. Ferrin, Graduate Research Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824, Present address of senior author: Yoder Bros. Inc., Ft. Myers, FL 33901; D. C. Ramsdell, Associate Professor of Plant Pathology, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824. Phytopathology 67:1501-1505. Accepted for publication 24 May 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1501.

Airborne ascospores of Guignardia bidwellii were trapped with a Burkard 7-day recording volumetric spore trap located in two Michigan vineyards during the 1974 and 1975 growing seasons. Ascospore discharge was best correlated with duration of rainfall (r = 0.6738), and they were trapped continuously from within 1 hr of the onset of rain to 8 hr after the rain ceased. As little as 0.03 cm of precipitation caused ascospore release. Peak ascospore catches were obtained from prebloom (late May) to midbloom stages (mid-June). Artificial field inoculations with ascospore suspensions (5 104 spores/ml) resulted in maximum leaf infection beginning when shoots were 10-20 cm long and continuing through early berry development. Maximum berry infection occurred at midbloom. Ascospores did not germinate in atmospheres of 98 or 100% relative humidity, but did germinate in free water on Concord grape leaves within 6 hr at 10, 20, and 30 C. By 24 hr, percentage germination was identical at all three temperatures. When ascospores were incubated on dry leaves, percentage germination decreased with increasing incubation period. No germination occurred after incubation for 48 hr on dry leaves. Infection of Concord grape leaves developed most rapidly at 27 C, but less so at 10, 16, and 21 C; no symptoms developed at 32 C.

Additional keywords: epidemiology.