Pierre Davadant,* Nataliya Shcherbatyuk, Lee A Kalcsits, James F Harberston, 
and Markus Keller
*Washington State University, 24106 N Bunn Road, Prosser, WA, 99350 
(pierre.davadant@wsu.edu)

Grapegrowers in dry regions face challenges with low nitrogen (N) in harvested grapes, affecting winemaking due to insufficient yeast assimilable nitrogen (YAN). An imbalance in N supply can decrease yield or increase vine vigor, impacting grower profits. Foliar N application at veraison may enhance grape and wine composition without inducing excessive vigor, yet its effect on phenolic compounds remains unclear. We conducted a three-year field trial and two separate pot experiments in arid eastern Washington to test whether foliar-applied N moves to the fruit and other plant organs, enhancing berry ripening and quality, and potentially replenishing the available N pool to support next year’s crop. In a field trial with own-rooted Syrah, we applied liquid urea ammonium nitrate (0, 22.5, 45, or 90 kg N/ha) to the soil at bloom and foliar urea at veraison. We also tested the effect of an extra foliar N spray at veraison (40 g urea/L) on potted Cabernet Sauvignon vines that had received four different rates of soil N at bloom (0 to 3.75g N per pot). Finally, using potted Riesling vines, we applied 15N-labeled urea at veraison on whole canopies, leaves only, or clusters only, to trace N partitioning at harvest.

Foliar N application at veraison significantly increased YAN in field-grown Syrah and potted Cabernet Sauvignon. Foliar N supply was particularly beneficial for vines with low N status. Higher YAN in harvested berries correlated negatively with skin tannins, but not seed tannins. N applied on Riesling fruit remained in clusters, while N applied on leaves was translocated to the perennial plant organs for storage.

Funding Support: USDA-NIFA Specialty Crop Research Initiative award number 2020-51181-32159, Washington State Grape and Wine Research Program Ste. Michelle Wine Estates (in-kind)