Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2008
Publication Date: 3/3/2009
Citation: Peetz, A., Mahaffee, W.F., Gent, D.H. 2009. Effect of temperature on the sporulation and infectivity of Podosphaera macularis on Humulus lupulus. Plant Disease. 93:281-286. Interpretive Summary: This research was conducted to determine how sporulation of Podosphaera macularis (hop powdery mildew) was influenced by temperature. Several controlled environment studies examined the impact of constant and variable temperature on the number of spores produced and their potential to cause disease. The results indicated that temperature above 86 F (30 C) causes a reduction in the number of infective spores produced. Disease forecasting models need to account for this effect to further optimized fungicide applications.
Technical Abstract: Hop powdery mildew epidemics can result in complete crop loss and require numerous fungicide applications for effective management. However, it is likely that some fungicide applications during July and August are not necessary due to the impact of temperature on the production of Podosphaera macularis conidia and their potential to infect Humulus lupulus (hop). In order to assess the impact of temperature on the potential of conidia to infect, sporulating colonies, 10 d old colonies were exposed to 18, 30, 33, 36, 39, and 42°C for 6h then incubated at 18°C for 18h before conidia were harvested and inoculated onto hop plants, and lesions/cm2 leaf area were determined 10 days later. Disease was significantly reduced at temperatures >30°C. The effect of temperature on sporulation of P. macularis was examined using a custom spore trap in growth chambers programmed at constant temperatures of 5, 10, 15, 20, 25, 30, and 35°C and chambers programmed for 18°C before and after ramping to 18, 22, 26, 30, 34, and 38°C for 6 hours. The effect of constant temperature on sporulation was best described by R(T)=[0.9225(T-5.14)(1-e0.1986(T-34.5051)]2 (Pseudo-R2 = 0.812537, P = 0.0001) with maximal production occurring at 25°C. Similarly, exposure to fluctuating temperatures was best described by y = -1.11x2 + 59.39x – 636.10; adjusted R2 = 0.91, P = 0.0122 with maximal production at 25°C . These data indicate that inoculum availability is reduced when the temperature exceeds 30°C in the field and that modeling inoculum availability could help further refine disease forecasting models.