Symptoms: Common on many plants and easily recognized, powdery mildew is a fungal disease found throughout the United States. It is caused by a variety of closely related fungal species, each with a limited host range. (The fungi attacking your roses are unlikely to spread to your lilacs). Low soil moisture combined with high humidity levels at the plant surface favors this disease.
Symptoms usually appear later in the growing season on outdoor plants. Powdery mildew starts on young leaves as raised blister-like areas that cause leaves to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surface; unopened flower buds may be white with mildew and may never open. Leaves of severely infected plants turn brown and drop. The disease prefers young, succulent growth; mature leaves are usually not affected.
Fungal spores overwinter inside leaf buds and other plant debris. Wind, water and insects transmit the spores to other nearby plants. Zucchini, roses and zinnia are especially susceptible.
Control: Plant resistant cultivars in sunny locations whenever possible. Prune or stake plants to improve air circulation. Keep the fallen and diseased foliage picked off the plant and from the ground. Make sure to disinfect your pruning equipment (one part bleach to 4 parts water) after each cut. Use a thick layer of mulch to cover the soil after you have raked and cleaned it well. The mulch will prevent the fungus spores from splashing back up onto the leaves. Washing foliage occasionally in mid-morning may disrupt the daily spore-releasing cycle. Water only in the morning so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry. Apply sulfur or copper-based fungicides weekly to prevent infection of susceptible plants. Green Cure, produced from 85% potassium bicarbonate, is a contact control agent that can be used to establish control once the disease is present.
Photo Credit: University of Maine Cooperative Extension