A major concern during grape ripening is sunburn, which can be a significant problem for winegrape production worldwide and in eastern Washington. Commercial grapevine cultivars differ in heat tolerance; this study characterizes tolerance to sunburn among red (Merlot, Syrah, Cabernet Sauvignon, Grenache, Sangiovese, and Cabernet Franc) and white (Riesling, Chardonnay, and Gewurztraminer) cultivars by analyzing skin characteristics and fruit composition including Brix, pH, TA, malic and tartaric acids, glucose and fructose, potassium, YAN, and phenolic compounds including total anthocyanins, anthocyanin profile, and tannins. The symptomatology of sunburn varied between red and white cultivars: in red cultivars, color development was poor, while brown lesions appeared on the skins of white cultivars.  In all cultivars, the exposed surfaces lost the crystalline structure of epicuticular wax, resulting in a shiny surface. The wax platelets were degraded and transitioned into amorphous masses, creating a rough surface morphology. The flesh cells incurred loss of cell viability. Generally, sunburned berries of all cultivars dehydrated, increasing Brix, but acids were lost to respiration. The degree of quality deterioration due to sunburn varied and was a function of cultivar, which is linked to berry structure and physiology.

Funding Support: Washington State University