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Effect of Variable Temperature on Infection Severity of Podosphaera macularis on Hops

First author: U.S. Department of Agriculture-Agriculture Research Service-Horticulture Crops Research Laboratory, 3420 NW Orchard Ave., Corvallis, OR 97330; second author: Cornell University, NYSAES, Department of Plant Pathology, Barton Laboratory, Geneva, NY 14456; and third author: Oregon State University, Department of Botany and Plant Pathology, Cordley Hall, Corvallis, OR 97331

The effect of variable temperature on the infection severity of Podosphaera macularis was investigated. Potted ‘Symphony’ hop plants were inoculated and exposed to different temperature regimes that included supraconducive temperatures (30 to 42°C) for varying periods of time (2 to 9 h). Infection severity (lesions per cm² of leaf area) was calculated 7 to 10 days after inoculation. Immediately exposing inoculated plants to 30°C for as little as 2 h significantly (P ≤ 0.05) reduced infection severity compared with exposure at a constant 18°C. However, exposure of inoculated plants to optimal conditions for 24 or 48 h prior to exposure to supraconducive conditions reduced this effect for plants exposed to 30, 33, or 36°C. Exposure to 39 or 42°C for 2 or more hours resulted in infection frequencies not significantly different from that of the uninoculated control regardless of prior exposure to favorable conditions. Exposure to simulated field temperatures programmed into growth chambers indicated that inoculation at 1700 or 2100 h resulted in significantly more disease than did inoculation at 0900 or 1300 h. Plants exposed to supraconducive or simulated field temperatures for 7 days prior to inoculation developed significantly lower disease severity than did plants maintained at 18°C for 7 days. The magnitude, length, and time of exposure to supraconducive temperatures in relation to time of inoculation plays an important role in the development of hop powdery mildew, and rules addressing these variables could be a useful addition to disease risk assessment models.

Additional keywords: Humulus lupulus, Sphaerotheca humuli, Sphaerotheca macularis.