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Effect of Leaf Wetness Duration and Temperature on the Infectivity of Guignardia bidwellii on Grape Leaves. Robert A. Spotts, Assistant Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center and The Ohio State University, Wooster, OH 44691; Phytopathology 67:1378-1381. Accepted for publication 21 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1378.

Leaves of potted grape plants were sprayed with suspensions of Guignardia bidwellii conidia (50,000/ml) to study the effects of selected combinations of leaf-wetness duration and temperature on infection of American bunch grapes and French hybrid cultivars. Plants then were exposed to postinoculation treatments in which leaf-wetness duration and temperature were variables. Infection occurred after 6 hr leaf wetness at 26.5 C but 24 and 12 hr of leaf wetness were required at 10 and 32 C, respectively. Disease was significantly less severe with either increasing or decreasing temperature regimes during the infection period than with constant temperature. Infection occurred after rewetting inoculated leaves that had been kept dry for up to 2 days, although a 1-day postinoculation dry period caused a significant reduction in severity. This information on effects of leaf wetness duration and temperature on infection was reliable for determination of when foliar-infection periods had occurred during two seasons in a commercial vineyard.

Additional keywords: epidemiology, Vitis spp.