Charles Obiero* and Markus Keller
*Washington State University, IAREC, Prosser, 24106 North Bunn Road, Prosser, WA, 99350 (charles.obiero@wsu.edu)

The main approach to vineyard irrigation management in Washington is a regulated deficit irrigation program customized to fit either red or white varieties. However, growers report that many varieties show distinctive behaviors, making irrigation management ambiguous and sometimes resulting in over- or underirrigation of some varieties. This study explored some simple tools that can help growers irrigate different winegrape varieties. A field trial was conducted in 2021 and 2022 in a drip-irrigated research vineyard at WSU Prosser to evaluate the responses of 30 winegrape varieties to soil water deficit. Varieties were fully irrigated through bloom, then the soil was subjected to two drydown cycles to create gradual moisture stress. The first cycle began at fruit set and the second at veraison, following irrigation to replenish soil moisture to near field capacity. Findings showed approximately two-fold differences in canopy size among the 30 varieties, and the soil dried down faster under varieties with bigger canopies. Unexpectedly, all varieties were isohydric as the soil dried down, but became anisohydric once soil moisture declined below a relative extractable soil water (ESW) threshold of ~0.35. However, varieties responded differently, considering the ESW that remained at a midday Ψleaf of -1 MPa, indicating a transition from mild to moderate water stress. There were those with higher (0.15 to 0.2), medium (0.1 to 0.15), and low (<0.1) ESW thresholds. These findings suggest the following: (1) Canopy-based measurements (e.g., via remote sensing or pruning weight) can be used in irrigation scheduling for different winegrape varieties. Vigorous varieties may need more frequent irrigation once control of shoot growth has been achieved and during heatwaves; and (2) Growers can establish an ESW-based tool to similarly irrigate winegrape varieties with the same soil moisture threshold at which plant water status transitions from mild to moderate stress.

Funding Support: Washington State Department of Agriculture Specialty Crop Block Grant Program and Washington State Grape and Wine Research Program