Reducing Sour Rot Spray Applications Initiated after Symptom Development Does Not Impact Disease Control

Goals: Previous sour rot research indicates the highest efficacy for sour rot treatments beginning prior to the onset of symptoms, yet many grape growers delay applications until after symptoms develop and continue weekly until harvest, which can exceed four sprays. This number of broad-spectrum insecticide applications is costly both financially and environmentally, and risks developing resistant populations of Drosophila fruit flies. The objective of this study was to determine the efficacy of a reduced number of spray applications beginning after symptom development, comparing weekly sprays totaling four applications to a reduced number of two to three applications based on Brix levels.

Key Findings:

• In both years of the trial, when chemical sprays were applied post-sour rot symptom development, more sprays did not equal more control. Reducing the number of post-symptom applications is a substantial cost-saving measure and is beneficial for environmental sustainability.

• In 2018, there were no significant differences between sour rot severity in both vineyard sites used in the trial. Incidence was significantly lower for the industry standard of weekly applications initiated at 15 Brix until harvest only in one vineyard site compared to applications at 16 and 20 Brix and applications at 16, 19, and 22 Brix.

• In 2019, there were no significant differences observed in disease severity and incidence between two sprays and weekly applications beginning at 15 Brix in both vineyard sites.

Impact and Significance: Many growers choose to initiate sour rot control applications until after symptoms develop. Weekly applications are financially and environmentally costly and the efficacy of timing applications initiated post-symptom development has yet to be researched. In this study, we show that a reduced number of chemical applications to control sour rot post-symptom development offers the same control as weekly sprays, consistent across both years of the study. Relying on weekly applications initiated only after sour rot symptoms develop does not significantly decrease incidence and severity at harvest. If sour rot symptoms are present, more spray applications do not offer better control.