Powdery mildew is a common fungal disease found in a wide range of plants, that causes cosmetic damage but would not result in plant death. Symptoms occur on the leaves as powder sugar or "fuzz". When the infection starts, distinct circular areas of white mildew are formed. As infection progress, the foliage can be covered with powdery mildew.
Powdery mildew is the disease name, or the generic term we use to refer to the symptoms. However, the fungi that cause powdery mildew are different for every host plant or family, meaning the powdery mildew you observed in your roses will not cause disease on trees or shrubs.
Powdery mildew control in the home garden after symptoms appear is challenging. Powdery mildew varieties of winter squash and pumpkins, as well as annual and perennial ornamentals, that are resistant to powdery mildew are available (see additional resources, below).
Planting resistant varieties is an effective way to minimize this disease in vegetables and ornamentals. Other important cultural practices include planting in full sun, increasing aeration and avoiding excess fertilizer use. Overhead irrigation is not recommended as it can contribute to the development of other diseases.
For susceptible varieties, fungicides are available but are mainly protectant, not curative. Fungicides can only help when applied prior to the infection. Fungicides available for the home garden include:
- chlorothalonil products (such daconil or Echo)
- biofungicide (like Serenade)
- sulfur products (such Safer Garden Fungicide; may burn plants if label directions are not followed carefully)
- some oil sprays
All sprays will have little to no effect if applied in late summer after symptoms appear.
The fungal pathogens that cause powdery mildew are capable of surviving the winter. Therefore late fall sanitation is recommended to help reduce the amount of the pathogen present in the garden next year.
Iowa State University
- ISU Plant & Insect Diagnostic Clinic
- ISU Extension and Outreach
- Integrated Pest Management for Home Gardens and Landscapes
- Ornamental varieties with resistance to powdery mildew (Purdue Univ)
- Winter squash resistant or tolerant varieties (Cornell Univ)
- Pumpkin resistant varieties (Cornell Univ)
- Powdery Mildew on Ornamental Plants (Ohio State Univ)
- Powdery Mildew on Vegetables (UC-Davis)