Michigan is the fourth largest producer of juice grapes (Vitis labrusca) in the United States, averaging more than 400,000 tons annually. Downy mildew (Plasmopara viticola) is an important pathogen of juice grapes in Michigan and throughout the grape growing regions of the United States. Downy mildew can damage leaves and fruit clusters, resulting in losses in fruit yield and quality. The objectives of this research were to: (1) study the effect of downy mildew on photosynthesis of 'Niagara' leaves, (2) study the effect of infection on dry weight and carbon partitioning in 'Niagara' vines and, (3) evaluate the ability of P. viticola to cross-infect different grape cultivars and species. In field studies, 'Niagara' leaves infected with downy mildew showed a reduction in total carbon assimilation with increasing disease severity. On potted 'Niagara' vines, inoculated leaves showed a significant reduction in photochemical efficiency, carboxylation efficiency, maximum rate of photosynthesis, and the stomatal limitation to photosynthesis before symptoms appeared. In field studies, downy mildew reduced total net dry weight in vines and affected the rate of carbon translocation, but had no effect on carbon partitioning. Cross infection studies carried out with isolates of P. viticola from different host species and isolates in Michigan showed that 'Niagara' leaves were resistant to infection by isolates of P. viticola obtained from several other host species.
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