Effect of microclimate on Leveillula taurica powdery mildew of sweet pepper

Sweet pepper-Leveillula taurica microclimate relations were studied under controlled conditions and in commercial greenhouses. Conidial germination occurred at 10 to 37°C and was optimal at 20deg;C. Conidial viability declined as temperatures increased to 40°C for 6 h. Leaf colonization was optimal at 15 to 25°C. Severe leaf infections occurred at 15 to 20°C and conidiation was suppressed at 20 to 25°C. Highest germination rates were observed at 75 to 85% relative humidity (RH). Severity of leaf coverage by symptoms was high for plants which were subjected to longer periods of temperatures between 10 to 15°C and daytime RH between 85 to 95%, and positively correlated with nighttime RH. Disease severity was negatively correlated with lengthy periods of temperatures >25°C, day and night average temperatures, and average daytime RH. Conversely, leaf shedding was relatively high under conditions characterized by long periods of temperatures >20°C and <13°C, and positively correlated with average daytime temperatures and periods of RH <75%. Increasing nighttime temperatures by heating and daytime temperatures by closing the greenhouse side walls reduced disease in two commercial greenhouse experiments. A midseason shift from a cooler greenhouse climate to warm daytime climate halted epidemic development. Flower number and yield were reduced in infected crops.