Late Season Fungicide Applications for Hop Powdery Mildew have Little Effect on Disease Suppression and Cone Quality Metrics in the Cultivar Nugget

Powdery of hop, caused by Podosphaera macularis, is managed in large part on susceptible cultivars by fungicide applications applied through the growing season to minimize disease on the cones, which are the harvested product. The impact of fungicide applications made after the juvenile stages of cone development are reported to have small or non-deductible effects on certain hop cultivars in certain environments. However, there is no information on the cultivar Nugget, a historically important cultivar in Oregon hop production regions, because until 2014 this cultivar was unaffected by powdery mildew due to R-gene mediated resistance. However, Nugget now routinely develops powdery mildew because virulent races of P. macularis are widespread in the Pacific Northwest. To guide Oregon growers’ management decisions on the cultivar Nugget, we conducted field studies in small plots in commercial hop yards during 2018 to 2020 to quantify the impact and value of late season fungicide applications typically used for powdery mildew control on cones. Under conditions of varying disease levels on cones, making one, two, or three additional fungicide applications after late July to early August did not improve control of powdery mildew (range 19.3 to 27.2% when analyzed over years); alpha-acids content, the hop storage index, or cone color also were unchanged relative to ceasing applications in late July. This finding should give confidence to Oregon producers of Nugget that late-season fungicide applications for powdery mildew can be omitted after the critical periods of juvenile susceptible in cones without compromising disease control or crop quality.