Foliar urea applications can increase berry yeast-assimiliable N in wine (Vitis vinifera L.) grapes.

Wine grapes (V. vinifera L.) in western North America commonly have deficient yeast-assimilable N concentrations (YANCs), necessitating the addition of diammonium phosphate to the must prior to fermentation. To examine strategies for alleviating this condition, N treatments were applied three times, centered around veraison, in replicated trials on seven commercial vineyards in the Okanagan Valley of British Columbia, Canada. In 2010, treatments included: 1% or 2% (w/v) foliar urea sprays; urea-N applied to the soil at the same rate as the 2% foliar spray; and a combination treatment in which half the N was applied as a foliar spray and half was applied to the soil. In 2011, an efficiency-enhanced soil N product was also applied. Treatments had few effects on fruit yield or berry quality but grape juice YANCs were significantly increased by the 2% foliar spray at six of the seven sites in both years. After 2% foliar spray application in 2010, grape juice YANCs approached but did not exceed the 140 mg N/L threshold generally considered sufficient to avoid stuck fermentation. After 2% foliar spray application in 2011, however, grape juice YANCs exceeded this threshold at four sites. The 2% foliar spray also reduced the ratio of proline:arginine (an indicator of N quality) in grape juice at most sites. Soil appli¬cations of N were infrequently effective at improving grape juice YANC at the rates applied. Ongoing work is using 15N-labelled urea to determine which grape juice amino acids are enhanced by N application treatments.