Crop load adjustment is widely accepted as an important vineyard management tool for premium-quality wine production. However, little information is available on its effectiveness under warm, dry climatic conditions. Crop loads were altered on three own-rooted winegrape cultivars grown in a mature, deficit-irrigated vineyard in the arid Yakima Valley (Washington) over a five-year period (1997 to 2001). Thinning consisted of preferentially removing clusters on shoots not arising from nodes deliberately retained at pruning, either one month after bloom or at veraison, to achieve target yields for Cabernet Sauvignon (6.7 t/ha), Riesling (9.0 t/ha), and Chenin blanc (11.2 t/ha). Removing, on average, 39% of the clusters from Cabernet Sauvignon, 30% from Riesling, and 38% from Chenin blanc reduced yields by 36%, 17%, and 20%, respectively. Average crop loads varied from 6.7 to 14.8 kg fruit/kg pruning weight for nonthinned vines and from 4.4 to 9.2 for thinned vines, and average yields varied from 7 to 25 t/ha for nonthinned vines and from 5 to 16 t/ha for thinned vines, depending on cultivar and season. Cluster thinning and its timing had little or no influence on shoot growth, leaf area, pruning weight, berry number, berry weight, and fruit composition (soluble solids, titratable acidity, pH, color) in both the current and subsequent seasons. Differences in vegetative growth, yield formation, and fruit composition within cultivars were mostly due to season (including weather and soil moisture) rather than to yield or crop load.
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