LEAF REMOVAL FOR CONTROL OF BOTRYTIS BUNCH ROT OF WINE GRAPES IN THE MIDWESTERN UNITED-STATES

The influences of leaf removal on canopy structure and Botrytis bunch rot were examined in two wine grape vineyards in Missouri. Leaf removal significantly reduced canopy density and increased evaporative potential in vines of hybrid grape cultivars Vignoles and Seyval blanc. However, the effectiveness of the practice in reducing disease varied with seasonal weather patterns and with vine support and trellis system. In the warm and dry growing season of 1991, grapes matured very early and no disease occurred in Vignoles with or without leaf removal. Disease levels also were low in Seyval blanc; however, leaf removal significantly reduced the incidence and severity by up to 47 and 79%, respectively, compared to vines without leaf removal. Application of iprodione provided no additional disease control. In the much wetter season of 1992, bunch rot occurred at both vineyards; and leaf removal significantly reduced the incidence and severity of bunch rot in both cultivars. However, iprodione applied to Seyval blanc also reduced disease incidence significantly in vines with or without leaf removal. Evaporative potential provided a simple means of measuring the degree of canopy opening and the drying conditions created by leaf removal.