Oaks (Quercus spp.) are medium to large deciduous trees commonly planted near buildings or in parks as shade trees. They produce acorns in the fall, which provide food for birds, squirrels, deer and other wildlife. The trees drop their foliage in late winter after most other trees are bare. A few leaf diseases affect oak trees, but they are not a major problem and usually do not require any treatment.
Oak Leaf Blisters
Taphrinia caerulescens is a fungus that infects the foliage of oak trees during wet cool springs and causes oak leaf blisters. Affected leaves turn slightly yellow and develop raised circular blisters on the upper surfaces, while the lower surfaces turn gray as the fungus grows on the leaf tissue. Leaves that have several blisters become distorted and may prematurely drop in the fall. The fungus survives in the crevices of the bark and on bud scales. An application of a fungicide approved for leaf blisters on oak trees may control the disease. It should be applied according to the manufacturer's directions before bud break, because treatment after bud break is not effective.
Common during rainy weather, the fungus Gnomonia quercina causes anthracnose on oak trees. Irregular areas of dead tissue form on the infected leaves and can spread to cover entire leaves. Although anthracnose may also infect shoots and small twigs, treatment is usually not necessary.
Numerous types of fungi cause leaf spots, which usually do not cause much damage, on the foliage of oak trees. The spots may be pale green, yellow, reddish-purple or brown and have a circular, oval or irregular shape. A fungus causes powdery mildew that forms a white coating on infected leaves during damp or humid weather. Although it is unsightly, powdery mildew normally is not a significant problem on oak trees. Sooty mold is a fungus that grows on the honeydew secreted by sucking insects, such as aphids. Insecticides will control the insects, and the sooty mold washes off the foliage with a strong stream of water.