Final Report: 2017-2021 Funding Cycle

Principal Investigator: Markus Keller
Washington State University, Irrigated Agriculture Research and Extension Center
24106 N. Bunn Rd., Prosser, WA 99350, (509) 786 9263, mkeller@wsu.edu
Cooperator: James Harbertson, WSU Wine Science Center, Richland WA 99352

Vineyard management for optimum wine quality and low labor input is a top industry priority in Washington and elsewhere. This includes tailoring irrigation practices to produce premium-quality fruit according to winery specifications. Irrigation management is the most powerful tool available in arid eastern Washington to maximize fruit quality, but there is little knowledge regarding optimum irrigation amounts and timing for white wine grape varieties. This is in part because most irrigation trials have been conducted with relatively few red varieties. We studied the response of Riesling wine grapes to different irrigation practices, namely regulated deficit irrigation (RDI), partial rootzone drying (PRD), as well as full (no stress) irrigation. Full irrigation was based on replacing 100% of the water evaporated from the vineyard during the growing season. A key outcome from this study is the recommendation to avoid excessive water deficit in the production of white wine grape, and especially aromatic grapes like Riesling. Imposing RDI between fruit set and veraison tended to result in smaller canopies that were associated with high sun exposure of the fruit. By contrast, PRD from fruit set through harvest led to similar, or occasionally more, shoot growth compared with full irrigation and showed little difference in fruit composition. Thus PRD may be an interesting alternative irrigation strategy considering its potential for higher white wine quality compared to RDI despite similar irrigation water savings. The irrigation scheduling decision for PRD was easy to make based on soil moisture on two sides of a vine: irrigation was initiated on the drying side of the rootzone and withheld on the wet side whenever the soil moisture of the drying side fell below 12%. The “switching threshold” could be adapted to different soil types for integration in automated irrigation decision-support tools. Our results are being communicated to industry end users and other stakeholders during field days, workshops, industry meetings (e.g. WWA, WAVE), and other means as appropriate. This work will place valuable information in the hands of grape growers, enabling them to make better, science-based decisions regarding application and conservation of limited irrigation water. It will also lay the foundation for the development of more effective irrigation strategies for the wine industry, which will permit improvements in fruit and wine quality through the judicious application of deficit irrigation strategies with a focus on white wine grapes.

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Irrigation //