Eric K. Hand,
Christopher N. Janousek, and
Walter D. Gubler, Department of Plant Pathology, University of California–Davis, Davis 95616
Epidemics of powdery mildew, the most important disease of grapevines in California, are driven by moderate temperatures (21 to 30°C). High temperatures can delay or stop the growth and sporulation of Erysiphe necator. Using controlled conditions, we investigated the response of the pathogen's colony growth, conidiospore production, and germination to eight temperatures (30 to 44°C) at 12 exposure times (0.25 to 24 h). The pathogen survived, grew, and reproduced at higher temperatures than previously reported; exposure time was as important as temperature in defining lethal and sublethal effects. Lethal effects started at 36 to 38°C. Lethal exposure times decreased with increasing temperature. Based on this new information on the pathogen's biology, we tested revisions to the high-temperature threshold of the UC Davis Powdery Mildew Risk Index in field settings. No differences in the total number of fungicide applications resulted from the use of either the original or the revised models. However, use of the 38°C for 2 h threshold consistently showed equal (leaves) or better (fruit) disease control compared with the original model, as well as equal disease control compared with a calendar schedule, but with five fewer applications over the 2 years of the study.