Interactive Effects of Deficit Irrigation and Crop Load on Cabernet Sauvignon in an Arid Climate

Regulated deficit irrigation (RDI) and crop-load adjustment are regarded as important viticultural practices for premium-quality wine production, although little is known about their interactive effects. Crop loads were altered on field-grown, own-rooted Cabernet Sauvignon grapevines exposed to RDI varying in severity and timing in the arid Columbia Valley (Washington) from 1999 to 2003. Following a dry-down period through fruit set to stop shoot growth, vines were irrigated at 60 to 70% of full-vine evapotranspiration until harvest. Other vines either received the same amount of water up to veraison, after which the irrigation rate was cut in half, or had their irrigation halved before veraison but not thereafter. Clusters were thinned within irrigation treatments during the lag phase of berry growth to achieve a target yield of 6.7 t/ha, compared with an unthinned control. The severity and timing of RDI had only minor effects on vegetative growth, yield formation, fruit composition (soluble solids, titratable acidity, pH, K+, color), and cold hardiness. The more severe water-deficit treatments slowed berry growth while the treatments were being imposed, but final berry weights were similar in three of five years. Although cluster thinning reduced yields by 35% and crop loads by 32%, crop load had little or no influence on vegetative growth and cluster yield components and advanced fruit maturity at most by three to four days. Very few interactive effects of RDI and crop load were observed, indicating that the crop load did not influence the response of vines to RDI.