Sunlight'S Influence On Grapevine Powdery Mildew: Direct Effects On Pathogen Development And Attendant Consequences Of Canopy Management And Vineyard Variability
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Sunlight inhibits powdery mildew development through at least two mechanisms, i.e., UV radiation's damaging effects on the exposed conidia and thalli of the pathogen and through elevating the temperature of irradiated tissues to a level supraoptimal or inhibitory for pathogen development. Furthermore, these effects are synergistic at temperatures near the upper threshold for disease development. Improved understanding of the role UV-B exposure, surface temperature, and their interaction have on powdery mildew development may assist in better management of this disease through chemical and cultural means. Variability of sunlight distribution within vineyards was quantified via the enhanced point quadrat analysis technique (EPQA). Using EPQA the number of canopy shading layers and the fruitzone photon flux within individual vines were shown to have a significant correlation with fruit disease severity for those vines, i.e., less disease developed on clusters with more exposure to sunlight. Additionally, through use of a fluorescent tracer and EPQA assessments, deposition of spray materials upon 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 significantly assist in the management of powdery mildew as well. These results underscore the fact that viticultural practices targeted primarily at general vine growth and crop quantity/quality issues such as; vine vigor management, pruning level, training system, basal leaf removal, and irrigation regime, can also significantly affect the development of powdery mildew.