Fungicide Physical Mode of Action: Impacts on Suppression of Hop Powdery Mildew

Understanding of the physical mode of action of fungicides allows more efficient and effective application and can increase disease control. Greenhouse and field studies were conducted to explore the preinfection and postinfection duration and translocative properties of fungicides commonly used to control hop powdery mildew, caused by Podosphaera macularis. In greenhouse studies, applications made 24 h before inoculation were almost 100% effective at suppressing powdery mildew, regardless of the fungicide evaluated. However, percentage control of powdery mildew based on the number of pathogen colonies per leaf varied significantly between fungicides with increasing time from inoculation to application, ranging from 50 to 100% disease control depending on the fungicide. Fluopyram or fluopyram + trifloxystrobin was particularly efficacious, suppressing nearly all powdery mildew development independent of application timing. In translocation studies, fluopyram and flutriafol were the most effective treatments in each of two separate experiments, resulting in zones of inhibition of 1,036 and 246.3 mm2, respectively, on adaxial leaf surfaces when a single droplet of each fungicide was applied to the abaxial surface of leaves. In field experiments, all fungicide treatments provided nearly complete control of powdery mildew infection when applied before inoculation. Levels of disease control decreased with time depending on treatment, showing trends similar to those observed in greenhouse studies. In the 2017 field experiments, high levels of disease control (>75%) were observed at postinoculation time points for all treatments tested, whereas the same fungicides were more sensitive to application timing in a different year. Findings from this research indicate that differences in efficacy between fungicides are small when applications are made preventively, but postinfection activity and translaminar movement of certain fungicides may render some more effective depending on application coverage and preexisting infection.