Influence of Leaf Removal, Fungicide Applications, and Fruit Maturity on Incidence and Severity of Grape Powdery Mildew

Abstract

Vineyard plots were established in 1988 and 1989 to examine the effects of basal leaf removal, timing of fungicide applications and fruit maturity on incidence and severity of powdery mildew on Vitis vinifera cv. Chardonnay. In plots receiving a single prebloom fungicide application, basal leaf removal reduced disease severity (the percentage of infected fruit per cluster) from 15.3% to 4.2% in 1988 and from 46.1% to 21.0% in 1989. When basal leaf removal was combined with one prebloom and two postbloom fungicide applications in 1988 and three postbloom fungicide applications in 1989, disease incidence (the percentage of infected clusters per vine) and severity was reduced to levels similar to adjacent commercial plots receiving 12 and 11 fungicide applications in 1988 and 1989, respectively. In 1989, significant levels of rachis infection were observed in all plots except those receiving 11 fungicide applications. Healthy fruit removed over the course of the season was inoculated in the laboratory under controlled conditions. A negative exponential relationship was observed between susceptibility of fruit and accumulation of soluble solids, with fruit above 7°Brix rarely infected.