|Mahaffee, Walter - Walt|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2003
Publication Date: 12/1/2003
Citation: Mahaffee, W.F., Turechek, W., Ocamb, C. Effect of variable temperature on the infection frequency of Podosphaera macularis on Humulus lupulus. Phytopathology. 2003. v93 p. 1587-1592. Interpretive Summary: Hop Powdery mildew was introduced into the Pacific Northwest in 1996 and continues to threaten the hop industry. This research is part of ongoing efforts to develop a sustainable management system for hop Powdery mildew. This research examines the effects of short exposures to high temperature on disease development of hop powdery mildew. Temperatures above 86 F for as little as 2 h can reduce disease development by 50% or more and exposure to temperatures above 90 F, common in pacific Northwest hop yards, appears to cause colony death. These data are being used to develop a disease-forecasting model for hop powdery mildew so that growers can more effectively manage the disease and reduce pesticide applications.
Technical Abstract: The effect of variable temperature on the infection frequency of Sphaerotheca macularis (sym. S. humuli) on Humulus lupulus L. (Hops) was investigated. Potted 'Symphony' hop plants were inoculated and exposed to different supra-optimal temperature regimes for varying amounts of time. Infection frequency (lesions/cm2 leaf area) was calculated 7-10 days after inoculation. Exposure of inoculated plants to 30 C for as little as 2h significantly reduced infection compared to exposure to optimal conditions, constant 18 C. However, pre-exposure to optimal conditions for 24 or 48 h prior to exposure to supra-optimal conditions reduced this effect for plants exposed to 30, 33, or36 C. Exposure to 39, or 42 C for 2 h or more hours resulted in infections frequencies not significantly different from the uninoculated control. Exposure to simulated field temperatures, ,hourly recorded field temperatures programmed into growth chambers, indicated that inoculation at 17:00 hr or 21:00 hr resulted in significantly more disease than inoculation at 9:00 hr or 13:00 hr. Plants exposed to supra-optimal and simulated field temperatures for 7 days prior to inoculation had significantly lower disease than plants maintained at 18 C for 7 days. The effect of supra-optimal temperatures in relation to time of spore movement could be a useful addition to disease risk assessment models.