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Microclimates of Grapevine Canopies Associated with Leaf Removal and Control of Botrytis Bunch Rot. J. T. English, Visiting postdoctoral scholar, Department of Plant Pathology, University of California, Davis 95616; C. S. Thomas, J. J. Marois, and W. D. Gubler. Postgraduate research associate, and associate professors, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 79:395-401. Accepted for publication 26 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-395.

Incidence and severity of Botrytis bunch rot are reduced significantly when leaves around grape clusters are removed. Disease reduction was found to be related to changes in microclimate brought about by this practice. Hourly measurements of temperature, atmospheric humidity, wind speed, and leaf wetness were made during the entire season in canopies of Vitis vinifera in which leaves had or had not been removed in 1986 and 1987. The contributions of these variables, individually and in combination, in distinguishing between canopy microclimates were evaluated by canonical discriminant analysis. Microclimates of these canopy types were not distinguished consistently by any single variable at three vineyards. Over the entire growing season, squared canonical correlations were less than 0.67 for any single variable. Microclimates were distinguished more completely when temperature, vapor pressure, wind speed, and leaf wetness were considered together. Squared canonical correlations generally were greater than 0.58. Of these variables wind speed was affected most by leaf removal. Average speeds in canopies in which leaves were removed were increased up to three or four times those in unaltered canopies. Microclimates of canopies were characterized and discriminated to greater extents as the period of sampling was decreased from the entire growing season to single days. Over the course of each day, canopy microclimates were distinguished most completely by wind speeds in the afternoon and evening. The impact of microclimate on bunch rot may be related to interactions between variables that were important in distinguishing canopies rather than any single variable alone.