Trees and bushes form the backbones of the home landscape. They supply the garden with structure and leafy texture, with many offering colorful blooms. Sometimes, however, trees and shrubs are affected by diseases that mar their beauty and health. Among them are a white fungus known as powdery mildew. Proper plant care and occasional chemical treatment usually control this common affliction.
Powdery mildew is a group of related fungi that causes white mildew, powdery in appearance, to form on the tops and bottoms of plant leaves. Spores are carried on air currents and spread from plant to plant.
Powdery mildew grows best in humid conditions when days are warm and nights are cool. The leaves do not have to be wet for spores to germinate; high humidity is sufficient for germination. Damp conditions--whether from dew, rain, humidity or watering--encourages powdery mildew.
The damage to trees and shrubs from powdery mildew is rarely lethal. In many cases, it is considered a cosmetic blight. In the most severe cases, mildew can cover the leaves so completely that photosynthesis is hindered, causing plant decline and possibly death. On some species, the leaves curl up and sometimes drop. Damage is usually confined to the leaves, but sometimes appears on flower buds and tender, green stems.
Given favorable conditions, powdery mildew affects a multitude of trees and shrubs. Affected trees include magnolia, dogwood, hawthorn, willow, sycamore, oak, crepe myrtle and many fruit trees. Shrubs include euonymus, cotoneaster, privet, rose, rhododendron, azalea, lilac and some fruiting bushes, such as blueberries. The larger the plant, the less likely it is powdery mildew will severely damage the plant. Some smaller shrubs, such as roses, Rhododendron spp. and blueberries can be severely damaged.
Plants in full sun are less likely to be severely affected by powdery mildew. Give plants plenty of room and prune out crowded branches to increase air circulation. Irrigate at the base of plants instead of overhead watering. Treat affected plants before the mildew spreads to other plants.
Severe or unsightly infestations of powdery mildew can be treated with a fungicide. Look for sprays containing triadimefon (Bayleton, Strike), triforine (Funginex), thiophanate-methyl (Domain) or propiconazole (Banner). Spray the tops and bottoms of leaves thoroughly, as well as flowers, buds and stems. Follow the instructions on the label.