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Integrating knowledge of disease biology, crop management, and fungicide characteristics to reduce sprays on grapevines

Wilcox, W.F.
Acta horticulturae 2011 no.916 pp. 141-150
Erysiphe, Uncinula, biological resistance, canopy, crop management, disease severity, field experimentation, fungi, fungicides, grapes, growth habit, host-pathogen relationships, inorganic salts, leaves, pathogens, pesticide application, powdery mildew, small fruits, solar radiation, temperature, topical application, ultraviolet radiation, vines, vineyards
Powdery mildew, caused by the fungus Erysiphe (Uncinula) necator is a grapevine disease of nearly universal importance, often requiring 10 to 15 annual fungicide applications for its control. In the field, disease severity on fruit and leaves provided good sunlight exposure was typically two- and sevenfold lower, respectively, than on those in the shade. Controlled-environment studies showed that these responses are due primarily to the deleterious effects upon the pathogen of UV radiation and elevated tissue temperatures under high sunlight-exposed conditions. In vineyard experiments, increasing sunlight exposure via an open canopy training system and fruit zone leaf removal reduced cluster disease severity by 35% relative to the control treatment on unsprayed vines; when sprays were applied, deposits were tripled on those clusters most versus least exposed. Grape berries are highly susceptible to infection for 2 to 3 weeks following the start of bloom, but quickly develop age-related resistance thereafter; consequently, fungicides and leaf removal treatments designed to protect berries against disease are most effective when applied during this period. The superficial growth habit of E. necator makes it susceptible not only to UV radiation but also to topical applications of numerous relatively non-toxic materials, such as inorganic salts. By varying inoculation and application timings in the glasshouse, we determined that these materials exert their activities in a curative mode, with little or no residual, protective activity; subsequent field trials exploiting this knowledge showed that frequency of application, more than dosage, was key to their successful use. These examples illustrate how studies to illuminate pathogen biology, host-pathogen interactions, and fungicide activities can be utilized to better manage disease and limit the need for conventional pesticide use.