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