Oak trees are common woodland trees and popular landscape trees. Most are large, up to 100 feet tall or more, with a broad canopy. Some small oak trees have a shrubby irregular form. Oak trees can be grown for shade, screening or wildlife habitat. They are susceptible to numerous diseases, especially fungal.
Many fungal diseases cause cankers--dead areas of bark and wood--on the limbs, branches and trunks of affected trees. Cankers destroy the beauty of trees, as well as their structural integrity. There are no chemical preventatives or controls for most fungal diseases. Consult the local county extension office or other reliable source for information on selecting, planting and caring for oak trees.
Several types of bacterium cause bacterial wetwood, which is also known as slime flux. The bacterium lives in soil and water and enters oak trees through wounds in the bark and wood. The bacterium ferments and produces slimy ooze that breaks through the bark and runs in streaks down the trunk. Other symptoms of bacterial wetwood are curling and wilting leaves and dieback of limbs and branches.
The fungus Botryosphaeria dothidea causes sunken cankers that can girdle and kill limbs, branches and trunks of oak trees. Infected areas turn black and foliage on affected plant parts wilts. Botryospaeria is dormant in the winter. The spores are spread by the wind in the spring.
Cankers caused by the fungus diplodia usually affect limbs and branches smaller than 1 inch on oak trees. Other symptoms include browning and loss of foliage. Newly transplanted and poorly maintained trees and trees planted in compacted soil are more susceptible to diplodia cankers.
Oak trees weakened by high temperatures, drought or root injury are susceptible to cankers caused by the fungus Hypoxylon atropuntatum. The bark on the trunk peels off in strips several yards long. Tan patches of fungal mat grow on the exposed wood. The mats darken and turn black as the fruiting bodies mature. Other symptoms are wilting and yellowing leaves and twig dieback.
The nectria fungus causes target cankers around wounds or branch stubs on oak trees. New callus tissue grows around the outer edge of the canker each year. Over time, the ridges create a target shaped canker. Target cankers can girdle and kill limbs and branches or even trees.