| Epidemics of powdery mildew, arguably the most important disease of grapevines worldwide, are driven by moderate temperatures (21 to 30°C), but high temperatures can decrease growth and sporulation or eradicate the fungus. Using controlled environments, I investigated the response of 3 stages of Erysiphe necator's life cycle to 8 temperatures at 12 exposure times (¼ to 24 h). Isolates of E. necator from 4 California production regions were used to evaluate the effects of heat treatments in 2°C increments from 30 to 44°C on conidial germination, colony size, and conidiospore production. After a 24-h exposure to 30, 32, or 34°C, conidia continued to germinate and colonies grew and sporulated. Lethal effects started at 36 or 38°C; lethal exposure times decreased with increasing temperature. At 44°C, less than 1½ h was lethal. Sublethal exposure times that delayed colony growth were also detected. There were interactions among temperature, exposure time, and colony age, but not with source of isolate. The pathogen survived, grew, and reproduced at higher temperatures than previously reported; exposure time was as important as temperature in defining lethal and sublethal conditions. My results require verification with field studies to assess combined effects of repetitive exposure to high temperature and other environmental stresses. |
