Smith, F.A.G., Pool R.M., Dunst R.M.
Effect of Mechanical Thinning Minimally-Pruned Concord Grapevines on Fruit Composition,
To increase profitability, some "Concord" (Vitis labruscana, Bailey) growers in the Lake Erie Grape Belt have adopted machine pruning. This results in increased yield and decreased juice soluble solids; sometimes soluble solids levels fail to reach minimum commercial standards. Crop size can be reduced when berries are about 50% final weight using a mechanical harvester. LIttle is known on the effect of mechanical thinning on fruit quality, and specifically differences in composition of fruit at a given juice soluble solids level from different thinning levels. Crop estimations were made 30 days after bloom to a potentially over cropped vineyard in Forestville, NY, which had been machine pruned for more than 10 years. Four different thinning rates or beater speeds (0, 260, 280, and 315 rpm) were applied at this time. Duplicate fruit samples were taken from plots three days before mechanical harvest (130 days after bloom). One set of samples was used to determine juice soluble solids, total acid, pH, and absorbance at 520 and 430 nm for each treatment. Quality was determined with the second set of samples by removing individual berries and grouping into density (juice soluble solids) categories using a gradient of sugar solutions. Vines thinned at 315 rpm yielded 12.1 tons/ha (10.5 tons/ha lower than unthinned vines), with juice soluble solids at 17.8 degrees Brix (2.1 degrees Brix higher than unthinned vines). The results suggest that quality factors will increase with jiuce soluble solids as crop load is decreased. However, at a given juice soluble solid category, fruit from unthinned vines and from vines thinned had similar total acidity, pH, and color.