- Powdery mildew is a widespread fungal disease. Most species of plants are affected by powdery mildew, which is host-specific. While many types of powdery mildew fungi exists, the symptoms are similar in all infected plants. Roses, apple, pear, cherry, crabapple, dogwood, viburnum, lilac, azaleas and privet are among the many plants, trees and shrubs susceptible to infection of this disease. The severity of disease depends on weather conditions, temperature and health of the host plant. Powdery mildew does not require water to germinate on plants, making it most common in warm, dry climates.
- White patches on plant leaves, stems and fruits are the primary symptom of powdery mildew. This fungi eventually forms fruiting bodies that are tiny spherical structures that turn from white to yellow, then brown and eventually black. These spherical structures are fungal spores that spread the fungi to other plants. Once powdery mildew becomes established, it can cause distorted leaves, yellow leaves and premature leaf drop.
- Preventing powdery mildew on your shrubs and trees is best accomplished by choosing varieties that are resistant to powdery mildew fungi. Some highly susceptible plants such as roses and vegetables have a variety of powdery mildew-resistant cultivars from which to choose. If no resistant cultivars are available, avoid planting in low-lying areas or areas with heavy shade. Do not use overhead watering systems for shrubs with powdery mildew and remove all infected plant debris to prevent the spread of disease. Avoid applying nitrogen-rich fertilizers to your shrubs, as these fertilizers promote rapid new growth, which is favored by powdery mildew fungi.
- If cultural controls fail to keep powdery mildew under control, chemical products may be necessary. Fungicides with active ingredients such as potassium bicarbonate, sulfur and neem oil are often effective ways to control powdery mildew. Apply fungicides every seven to 14 days until the desired level of control is achieved.