Problem: Powdery Mildew (Rose)
This common plant disease is caused by a fungus (Sphaerotheca pannosa var. rosae). It is one of the most widespread and serious diseases of roses. The powdery covering consists of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps plant nutrients, causing distortion, discoloration, and often death of the leaves and canes. Powdery mildew may occur on roses any time during the growing season when rainfall is low or absent, temperatures are between 70° and 80°F, nighttime relative humidity is high, and daytime relative humidity is low. In areas where there is high rainfall in spring and summer, control may not be needed until the drier months of late summer. Rose varieties differ in their susceptibility to powdery mildew.
Apply a fungicide labeled for this disease, following label directions. Rake up and destroy leaves in the fall.