Powdery Mildew Severity as a Function of Canopy Density: Associated Impacts on Sunlight Penetration and Spray Coverage

Variable canopy densities and associated differences in sunlight distribution within the fruit zones on clusters were quantified and correlated with variable severities of powdery mildew within vineyards in the states of New York, Washington, and South Australia. Canopy density was measured with enhanced point quadrat analysis (EPQA), and the number of shading layers and the photon flux within the fruit zone of individual vines indicated that less disease developed on clusters with more exposure to sunlight. When clusters were categorized as heavily shaded (<= 10% photosynthetic photon flux), moderately exposed, or well exposed (>= 51% photosynthetic photon flux), vines with the least disease were also shown to have a significantly greater proportion of clusters in the well-exposed category relative to vines with the highest powdery mildew ratings. Consequently, these latter vines had significantly more heavily shaded clusters. The correlation remained strong and the relationship linear even with biweekly applications of either 2 kg/ha or 9 kg/ha of wettable sulfur during the growing season. Additionally, through the use of a fluorescent tracer and EPQA assessments, the deposit of spray materials on clusters was shown to be linearly related to their degree of exposure. Thus, canopy management practices designed to optimize sunlight exposure of grape clusters for fruit quality purposes should also significantly assist in the management of powdery mildew.