Suppression of Hop Downy Mildew as Influenced by the Timing of Selected Fungicides

Abstract

Downy mildew of hop is a disease that is controlled by the implementation of cultural practices along with the application of fungicides, as there are few cultivars with high levels of resistance to this disease. We conducted studies in western Oregon to elucidate the effect of using synthetic fungicides of various modes of action early versus late in disease management programs when rotated with a copper-based fungicide. When analyzed over the four years of this study, there were independent effects from both the specific fungicide mode of action and application timing on disease control. The use of cymoxanil + copper hydroxide or mandipropamid in a program reduced area under the disease progress curve by 66.3 or 59.7 units (17.7 or 19.6%), respectively, as compared to phosphorous acid. Using synthetic fungicides in the first two applications in a program provided better disease control than when using the same fungicides in the third and fourth applications, reducing area under the disease progress curve by 47.8 units (14.4%). By simply applying modes of action found to be more efficacious earlier in the growing season, the severity of downy mildew was reduced as much as 32% without increasing the total number of applications made.