Viticulture – General Viticulture Session

Pierre Davadant | Nataliya Shcherbatyuk | Lee A Kalcsits | James F Harberston | Markus Keller

Optimizing Nitrogen Supply for Winegrape Quality in Dry Regions

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)

Kiley Osterman | Patricia Skinkis

Changing Perceptions of Cluster Thinning in Willamette Valley Pinot noir Production

Kiley Osterman and Patricia Skinkis*
*Oregon State University, 2750 SW Campus Way, Corvallis, OR, 97331 
(patricia.skinkis@oregonstate.edu)

Cluster thinning is common in winegrape production and was conducted historically in Oregon Pinot noir to hasten ripening and ensure quality. Industry standard yield targets in the early 2010’s were 4.5 to 6.2 t/ha, and growers questioned those thresholds. To systematically evaluate the effect of cluster thinning on fruit composition and wine sensory perception, a research project was conducted by Oregon State University and >20 vineyard and winery companies from 2012 to 2021. The impact of this research was quantified using a series of surveys, interviews, and focus group meetings to gather information about individual, company, and industry-wide changes to yield management practices. Project collaborators increased yield targets, as there were no consistent differences in berry ripeness at harvest and a lack of differences in wine quality between crop thinning treatments. There were no vine health consequences of maintaining higher yields (dormant pruning weight or vine nutrient status). By the mid-point of the study, the industry began adjusting yield targets based on season variability, rather than a constant tonnage/ha. This was due to the observation that wine quality was more influenced by vintage variation and winemaker than by vineyard crop thinning practices. Most collaborators reported little-to-no difference in wine sensory perception between crop thinning levels. Although most project collaborators felt comfortable increasing yields by 1.24 to 7.71 t/ha without compromising quality, obstacles such as winery capacity and sales limited further adoption for some. Often, target yields were set to fit winery and sales capacity, leading to target yields that did not reflect vineyard yield-quality potential. Project collaborators improved management and vineyard knowledge and expressed a desire to share findings. The project had industry-wide impact with greater yields reported at the state level and greater tolerance of increasing yield targets by growers and winemakers alike.

Funding Support: Northwest Center for Small Fruits Research

Bhaskar Bondada

Diabetic Grapes: Motherlode of Sugary Metabolites, Yet Unsuitable for Winemaking

Bhaskar Bondada*
*Washington State University Tri-Cities, 2710 Crimson way, Richland, WA, 99354 (bbondada@wsu.edu)

Sugars are essential to grapevine growth and development and are therefore monitored under commercial growing conditions to ensure grapes accumulate optimum sugar levels for winemaking. Leaves produce sugars, then they are translocated as sucrose via the phloem, and unloaded in berries to a maximum accumulation of 25 Brix as hexoses at veraison. Any increase or decrease in this level reflects perturbations in the ripening process. In this study, the sugar levels in four cultivars trained to vertical shoot positioning (Syrah, Cabernet franc, Sangiovese, and Petit Verdot) were much greater than 25 Brix. The sugar level in Syrah was >45 Brix, whereas in the other cultivars, it ranged from 27 to 34 Brix. This study’s objective was to determine what caused such high sugar concentrations in these varieties and to provide plausible explanations. Afflicted and healthy clusters were sampled for chemical analysis of primary and secondary metabolites. The high concentration of sugars; other primary metabolites such as acids; and secondary metabolites such as anthocyanins, flavanols, and flavonols; resulted from a physiological disorder known as bunch stem necrosis (BSN). The nutritional status of the berries and the bunch stem was altered, reflected by the levels of macro- (N, P, K, Ca, Mg, S) and micro- (Fe, Mn, Zn, Cu, Mo, B) nutrients. The necrosis typically occurs on the rachis during ripening, causing dehydrated, raisin-type berries. Although BSN caused the raisining of the berries in the 2023 season, the necrosis of pedicels dominated the rachis necrosis. Cell viability tests revealed that most cells were dead. Tylose occluded most of the xylem vessels in the necrotic rachis. Although the occurrence of BSN has been known for several years, its causal factors remain obscure. This study proposes potential causal factors gleaned from the nutritional and compositional analyses.

Funding Support: N/A