Integrating Knowledge of Disease Biology, Crop Management, and Fungicide Characteristics to Reduce Sprays on Grapevines

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 twoand 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. INTRODUCTION The economics of producing high-value horticultural crops typically rewards—if not demands—intensive management of infectious diseases. Grapes (Vitis spp.), like most other fruit crops, are no exception to this general rule. Cultivars of the European grape species V. vinifera, which represent the vast majority of worldwide production of this crop, are particularly vulnerable to diseases of North American origin, having evolved in isolation from these pathogens to which they are almost universally susceptible (Gadoury et al., 2011). Indeed, two such diseases, powdery mildew (Erysiphe necator) and downy mildew (Plasmopara viticola), threatened survival of large portions of the European grape industry upon their introduction to the continent in the 19 Century, until chemical agents