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Botrytis Bunch Rot of Grapes: Influence of Trellis Type and Canopy Microclimate. S. D. Savage, Former graduate research assistant, presently assistant professor, Colorado State University, Orchard Mesa Research Center, 3168 B Road, Grand Junction 81503; M. A. Sall, assistant professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 74:65-70. Accepted for publication 11 June 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-65.

Grape clusters grown on vines trained to grow on a cross-arm style of trellis developed 47% (P <0.05) more infection by Botrytis cinerea than vines trained to a two-wire vertical trellis in test plots under typical Napa Valley, California, summer drought conditions in 1981. To test the hypothesis that disease differences were related to canopy microclimate, temperature and moisture parameters were recorded within the two trellis regimes. Temperature and moisture measurements for vines maintained on the two trellis types were similar, particularly the duration of periods of low water vapor stress favorable to fungal development. Cluster temperature under the two regimes differed as much as 2 C, although this was relatively infrequent. Characteristic, subtle, diurnal temperature difference patterns between the two regimes were consistent. During each day, there were four distinct trend periods of differential temperature change between the two trellis types. Correlations of these trends with ambient wind speed measured 300 cm above ground level indicates the possibility of differential wind penetration of the canopy types.