Effects of powdery mildew on vine growth, yield, and quality of concord grapes

Vitis labruscana 'Concord' is a widely planted grape cultivar grown in the United States for processing into juice and other products. Concord fruit a re sporadically but sometimes severely damaged by the grape powdery mildew pathogen, Uncinula necator. The effects of powdery mildew on vine growth, y ield, and quality of Concord grapes at three levels of cropping intensity c ommonly found in commercial grape production were determined in vineyard st udies. Top-wire cordon-trained Concord vines were balance pruned, pruned to retain 80 nodes, or minimally pruned. Replicated plots of the foregoing we re then either protected from powdery mildew by regular fungicide applicati ons, or were inoculated and left unsprayed. Over a 4-year period, the effec ts of foliar infection on vine growth, yield, and juice quality of unspraye d vines were compared with vines that received a conventional protection pr ogram of four fungicide applications. Failure to control powdery mildew res ulted in a chronic reduction in wood maturity measured as the number of nod es on canes that developed periderm. The reduction in nodes did not reduce yield, possibly due to compensation in shoots produced from the remaining n odes. Powdery mildew did not affect bud survival or vigor, measured as the number of shoots produced per node on retained canes. The most significant effects of powdery mildew were on berry sugar levels and juice color and ac idity, which on the unsprayed vines were sometimes reduced below minimally acceptable thresholds for processed grapes. Significant reductions due to p owdery mildew in these parameters occurred in all three pruning treatments, but were most pronounced at higher cropping levels.