Powdery mildew, Microsphaera penicillata.
Symptoms appear as a white, mealy growth on the leaves.
This disease is favored by hot, humid weather and usually develops late in the season. Chemical control of this disease is usually not necessary. Many new cultivars are available which are resistant to powdery mildew. Water-soaked spots develop on leaves and young stems. These gradually enlarge into irregular brown lesions.