This is powdery mildew, a fungal disease. It's very common this time of year. Powdery mildew harms plants by reducing photosynthesis. Repeated infections stress plants. Powdery mildew over-winters on plant debris such as fallen leaves. During the following growing season, the fungus germinates and infects plants. Removing the debris (i.e., raking up leaves) reduces the amount of potentially infectious material. Improving air circulation in and around plants will help decrease mildew. Dense plantings create ideal conditions for mildew development because humidity is higher and air circulation low. Improve air circulation through thinning.
Fungicides are of little value now, as leaves will soon be dropping. Rake up leaves this fall. Thin plants next season. Minimize overhead irrigation, particularly late in the day, to keep humidity low in and around plants. For more information, visit the PlantTalk Web site and click on Powdery Mildew under Insects & Diseases.
