Pete Jacoby,* Xiaochi Ma, and Jeremy Thompson  
*Washington State University, P.O. Box 646420, 289 Johnson Hall, Pullman, WA 99164 (jacoby@wsu.edu)

Subsurface microirrigation using plastic tubes to deliver water directly into the lower root zone was examined over two growing seasons (2015 and 2016) in the Red Mountain AVA of Washington State to determine advantages over surface drip irrigation in water conservation to maintain vineyards with season-long deficit irrigation. Irrigation was applied on the same schedule used to achieve production and quality goals in a commercial planting of Cabernet Sauvignon; however, total water applied via subsurface delivery was reduced to 60, 30, or 15% of the surface drip amount. Continuous and pulse delivery was compared for the direct root-zone (DRZ) applications, but no differences were attributed to either method. Additionally, no consistent differences correlated with a particular water delivery depth below surface. A randomized block design with three replicates was used to determine statistical differences in fruit production and chemical composition of harvested grapes. In 2015, the hottest and driest season on record, DRZ treatments receiving reduced irrigation season-long yielded from 90 to 70% of the grape production of surface drip application. In 2016, commercial production was 50% higher than in 2015, and DRZ plot production was slightly lower than in 2015. In 2015, clusters from each treatment and commercial block were collected during veraison and harvest weights per vine were made in late-September. Harvest weights were taken during commercial harvest in 2016 and replicated samples of DRZ and commercial plots were submitted to a private lab for a full grape analytical panel. Acidity diminished with increasing water reduction, while Brix, tannins, and anthocyanins increased progressively with water reduction. Results to date show promise for maintaining vineyard production using DRZ, with concomitant increase in grape quality while conserving water resources during years of limited supply.

Funding Support: Washington Winegrowers, Northwest Center for Small Fruit Research, WSDA Specialty Crop Block Grant Program