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 degrees C and was optimal at 20 degrees C. Conidial viability declined as temperatures increased to 40 degrees C for 6 h. Leaf colonization was optimal at 15 to 25 degrees C. Severe leaf infections occurred at 15 to 20 degrees C and corridiation was suppressed at 20 to 25 degrees 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 Ion,er periods of temperatures between 10 to 15 degrees 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 degrees 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 degrees C and < 13 degrees 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.