Oaks (Quercus sp.) are commonly planted for shade and structure in gardens. They are also an integral part of some natural woodlands and forests. Although oaks are considered strong, long-lived trees, they are susceptible to fungal and bacterial diseases. Different diseases attack different plant parts, from the roots to the leaves. Most diseases in the bark cause cankers–dead areas with raised edges. The cankers are vulnerable to secondary infections and insects. Diseases can spread to other trees by wind, water, animals or humans.
Bacterial wetwood breaks down affected wood and produces a sour-smelling fermented liquid. When the liquid breaks through the bark over the affected area, it runs down the trunk and leaves discolored streaks on the tree. Other bacterial or fungal diseases can cause slimy growths in the exposed liquid. There is no control for bacterial wetwood, but the disease is usually minor. A 10 percent solution of bleach and water can be used to remove the stains from the bark.
The foliage on twigs and small branches affected by Diplodia cankers turn brown and fall off. Diplodia cankers attack stressed trees, such as new transplants, trees that are poorly maintained, and trees that are growing in compacted soil.
Mats of Hypoxylon fungi grow under the bark of the infected tree. The dead bark falls off, forming a canker that reveal strips of tan to silvery gray fungal matting. The matting hardens and blackens with age. The affected sapwood is tan to light brown with a black border. The foliage may turn yellow and wilt due to the stress caused by the canker.
Sudden Oak Death
The fungus Phytophthora ramorum causes sudden oak death. Reddish-brown to black cankers that ooze dark sap form on the trunks of affected trees from the soil line to approximately 6 feet high. The leaves turn brown over a period of two to four weeks. There is no preventative treatment or control for Sudden Oak Death. Affected trees die within one to two years after the initial infection.