Suraj Kar, Joseph DeShields, Ricky Clark, Mariana Stowasser, and Alexander Levin*
*Oregon State University – Southern Oregon Research and Extension Center, 569 Hanley Rd., Central Point, OR, 97502 (alexander.levin@oregonstate.edu)

Initiating irrigation is a critical annual management decision that has cascading effects on grapevine productivity and wine quality in the current and subsequent growing seasons. A multi-site trial was begun in 2021 to optimize irrigation initiation timing using midday stem water potential (ψstem) thresholds characterized as departures from non-stressed baseline ψstem values (Δψstem). Plant material, vine and row spacing, and trellising systems were concomitant among sites, while vine age, soil type, and pruning systems varied. Five target Δψstem thresholds were arranged in an RCBD and replicated eight times at each site: 0.2, 0.4, 0.6, 0.8, and 1.0 MPa (T1, T2, T3, T4, and T5, respectively). When thresholds were reached, plots were irrigated weekly at 70% ETc. Yield components and berry composition were quantified at harvest. Across sites and years, consistent linear trends were found for several key production and fruit composition variables. Compared to T1, vine yields decreased by 7, 15, 22, and 30% for T2, T3, T4, and T5, respectively, when averaged across years and sites. These reductions were driven by reductions in berry weight, but in the second year of the study, cluster numbers per vine were also reduced. Comparatively, basic berry chemistry varied little among treatments. Juice total soluble solids consistently decreased from T1 to T5 across sites and years, but the trend was mild and not significant, and there were no consistent responses of juice pH and titratable acidity. Fruit anthocyanins increased linearly from T1 to T5, but the rate of increase (1% per 0.1 MPa of delay) did not match rate of yield loss (-4% per 0.1 MPa of delay). Because producers are paid by the ton and contracts stipulate a target maturity level, results after two years suggest that there may not be an economic incentive to delay irrigation initiation, regardless of vineyard site.

Funding Support: Oregon Wine Board Oregon Department of Agriculture Northwest Center for Small Fruits Research