Prune oaks in winter to avoid oak wilt

Protecting oaks from wilt disease starts with restricting times trees are pruned to the cold weather season.

Oak wilt, Ceratocystis fagacearum, is a deadly disease in oak trees with the red oak group being more susceptible than white oaks. The disease is thought to be native to the United States and was first identified in the 1940s. Trees die from the disease as the fungus invades the vascular tissue that transports water throughout the tree. The growth of the fungus within the vascular tissue physically limits movement of water. Oak trees block their own vessels in an attempt to stop the spread of the disease, cutting off the flow of water and causing the tree to wilt and eventually die.

Oak wilt can be carried to the tree by sap-feeding insects such as picnic beetles. These sap feeding beetles are attracted to oaks with fresh wounds. Picnic beetles feeding on the sap of oaks can transmit this fungal disease.

Often the first symptom of oak wilt is browning leaves at the top of the tree which then spreads to lateral branches. With red oaks, this can happen in late spring to early summer. Inspection of leaves shows browning opposite the stem of the leaves and along the outer margins. As it progresses, the leaf continues to brown down towards its base.

Oak wilt
Oak wilt. Photo credit: C.E. Seliskar, Bugwood.org

Infected red oaks can wilt within several weeks. Branches that are wilting may show brown to black streaking in their vascular tissue, which can be seen when the bark is pulled away from the wood. Though symptoms are similar in white oaks, the wilting process is slowed by the presence of tyloses, which blocks vessels and reduces susceptibility of the disease.

As the trees begin to die, a mass of fungal growth may be found under the bark of the tree. These fungal mats cause the bark to crack and a fruity odor produced by the fungus attracts the sap-feeding insects back to the tree where they pick up the fungus, starting the cycle all over.

Fruiting bodies
Oak wilt fruiting bodies. Photo credit: John N. Gibbs, Forestry Commission, Bugwood.org

Oak wilt is not easy to diagnose just by visual inspection. Wilting and scorched leaves can be the result of other problems. During the summer it is common to see browning leaves on an oak due to scorch or another fungal disease. Because it is confused with other disorders, it is best to have the trees inspected by a certified arborist and it may be necessary to send samples to a diagnostic laboratory. Getting a diagnosis early is critical to reducing the spread of the disease. Michigan State University’s Diagnostic Services recommends taking a sample from a partially wilted branch that is an inch in diameter. Cut the branch into 6-to 8-inch lengths and place in plastic bags to be delivered or overnight shipped to the lab. Fresh samples are critical in testing for the oak wilt fungus.

Oak wilt can also spread from one infected tree to another through root grafts with surrounding oaks. The roots from one tree grow into another, creating a connection. Once diagnosed, the spread of the disease by graft can be stopped through the use of trenching equipment that cuts the root grafts. Once oak wilt has infected a red oak tree, there is little chance of saving it. Injections are a management option, but these should be directed to healthy trees that are not showing symptoms of the disease. For an in-depth explanation of management options from fungicides to trenching, read the Michigan State University Extension bulletin E-3169, “Oak Wilt in Michigan’s Forest Resource.”

The most important management strategy is to avoid pruning of oaks during the growing season. Plant pathologist David Roberts recommends that if pruning on oaks is necessary, it should be restricted to the winter when sap feeding beetles are not active. Whether a commercial pruner or a homeowner, this recommendation is critical in preventing the spread of the fungus. Many of our oak trees are over 100 years old and cannot be replaced. Keep this in mind as winter draws to a close in Michigan. Do not make your oaks a target for the main vector of the oak wilt fungus by pruning these trees during the growing season.