Pierre Davadant, Nataliya Shcherbatyuk, Ryan Doyle, James Harbertson, and Markus Keller*
*Washington State University, IAREC 24106 n Bunn Road, Prosser, WA, 99350 (mkeller@wsu.edu)

This study aims to determine if foliar-applied urea at veraison can supplement or substitute for early-season soil-applied nitrogen (N) to maximize berry quality and increase yeast assimilable nitrogen (YAN) for red wine grape production in an arid climate. Previous studies showed that foliar application of urea at veraison did not impact vigor, increased grape and wine aroma composition and YAN, but the response of phenolics was contradictory. We hypothesized that foliar urea application at veraison increases berry phenolic compounds and YAN in red grapes and wine relative to soil-applied N. A field trial was conducted in a commercial, drip-irrigated, own-rooted Syrah vineyard in arid eastern Washington in 2021 and 2022. We compared four treatments of soil-applied liquid urea ammonium nitrate (0, 22.5, 45, 90 kg N/ha), split at the five to six leaf stage, bloom, and fruit set, with a 15 kg N/ha foliar urea treatment, split into three applications at veraison. Berries were analyzed and wine was produced from all treatments. Results were consistent across the two vintages. Berry weight, cluster number, yield, juice total soluble solids, pH, titratable acidity, and malic acid were similar among N treatments. However, while must YAN increased with increasing soil-applied N, the foliar N treatment was much more effective at increasing YAN. The phenolics concentration in the juice at the beginning of fermentation was similar among treatments. Tannin concentrations were low in wines at pressing and were lowest in the foliar N treatment, contrary to our hypothesis. The fertilizer treatments did not affect wine anthocyanins and total phenolics. Results from plant tissue and berry analysis will be presented once they have been completed. The application of N at veraison increased YAN in the juice and decreased tannins in the wine in the eastern Washington Syrah vineyard.

Funding Support: USDA-NIFA SCRI, Washington State Grape and Wine Research Program, Ste. Michelle Wine Estates (in-kind)