Pietro Previtali,* Thomas Giagou, Luis Sanchez, and Nick Dokoozlian
*E. & J. Gallo Winery, 1541 Cummins Drive, Modesto, CA, 95358 (pietro.previtali@ejgallo.com)

Extreme heat and drought events are becoming increasingly frequent and constitute a threat to grape production. Heatwaves are defined as two or more days with maximum temperature >38°C, and are known to negatively affect vine performance, yield, and wine quality. We conducted a meta-analysis to link historic climate, yield, and grape composition data across five ranches in the Napa region to approximate heatwave effects at the commercial scale. Cultural practices are needed to effectively mitigate heatwaves in the vineyard; in this case, increased irrigation during heatwaves was studied in Cabernet Sauvignon. Irrigation intensity (50% or 100% more than the control) and timing (zero, one, or two days prior to the heatwave) were evaluated. Experimental treatments were compared to the control: deficit-irrigated at 80% crop evapotranspiration through the heatwave. The trial was conducted in California at two sites, Lodi and Sonoma. The 2022 season was characterized by two postveraison heatwaves, one in mid August (four to five days, Tmax = 40.2°C) and one in early September (six to nine days, Tmax = 46.5°C). Across the two sites, additional irrigation effectively improved plant water status and physiological activity. Berry dehydration rates were significantly decreased by the most irrigation (9%) compared to the control (up to 22%), resulting in larger yields at harvest. Additional irrigation mitigated rapid total soluble solids increases observed in the control (up to 6 Brix/week). Incremental increases in irrigation levels during heatwaves resulted in wines with reduced levels of heat damage. The concentration of negative aroma compounds was greater in heat-affected wines, and that of positive fruity esters was less. Phenolic compounds were greatly affected by heat, which translated directly into lower color intensity, higher hue, and lower concentration of mouthfeel-related phenolics.

Funding Support: E. & J. Gallo Winery