What is Killing Our Trees?

Oak Wilt on the Increase

This is the third in a six-article series called "What Is Killing Our Trees." The series will explore some of the lesser-known diseases that affect Michigan trees and potentially impact wildlife.

Oaks are among the most widespread and important trees in Michigan. The two major groups—red oaks and white oaks—include species that grow in a wide variety of habitats. Northern red oaks and white oaks often dominate on well-drained soils, while pin oaks (a member of the red oak group) and burr oaks (white oak group) are common in lowlands, even those subject to flooding. More than a dozen oak species and cultivars have been planted routinely in urban parks, along streets, and in yards throughout the state. Oaks are used for nesting by birds, and their acorns are choice foods for deer, turkeys, squirrels, and many other species of wildlife.

Researchers and foresters have long kept a close watch on diseases that could threaten this enormously valuable resource, and their concern about oak wilt, a fungal disease that is fatal in the red oak group, has heightened recently. Oak wilt, which is caused by the fungus Ceratocystis fagacearun, has been around for more than 50 years, but is just now invading new areas in the Great Lakes states. It is now widespread in Minnesota where it has killed thousands of oaks, and it has increased dramatically in the last six years in Pennsylvania. Over the past two decades there have been new outbreaks of oak wilt in forested areas in both peninsulas of Michigan as well as in Bloomfield Hills and other cities.

The only good news is that, so far, oak wilt has not been as devastating as a similar fungal infection, Dutch elm disease. (See previous article on Dutch elm disease.) Both are exotic diseases caused by wilt fungi (although a different type attacks elms). In both elms and oaks the fungi move up and down the tree with the flow of water and quickly reach the roots. The tree reacts by secreting substances that unfortunately stop the flow of water and nutrients to the branches and leaves. ÒWilting,Ó and eventually loss of leaves, follow. As with elms, there is substantial root grafting among oaks that grow close together, and the fungi move from tree to tree via that root complex. Both diseases can be spread long distances by bark beetles, but oak wilt is more often spread in Michigan by sap-feeding beetles. Humans also can spread both diseases by moving infected logs and firewood.

Dutch elm disease pretty much eliminated large native elms in our state after a new and faster-spreading strain of fungi developed. Hopefully, a new strain of fungi will not quicken the spread of oak wilt disease or prove deadly to white oaks, which are much less susceptible than red oaks to the strain now found in the Great Lakes region. There is some evidence that white oaks often recover from oak wilt with relatively little long-term damage.

The best prevention is to avoid pruning or injuring oaks during the growing season. Trimming between April through October is the biggest cause of oak wilt infections. If storm damage to oaks occurs during the warm season, the best means of oak wilt protection is to prune the branch at least 12 inches back from the injury and seal the cut edge with pruning paint. Winter storm damage can usually be ignored or trimmed up the next winter—not during the warm months.

Where oak wilt is discovered in groups of high value trees, as in parks, it may be cost-effective to have professionals dig trenches and/or install root barriers between healthy and infected trees. But the trenches need to be at least 3-4 feet deep and cost around $10 per linear foot to establish. Fifty feet or more of trench may be needed between just one infected and one healthy tree. Saving a small grove of oaks where multiple trees have been infected may require more disturbance than is compatible with site uses.

State government has not seen a need to restrict oak wood movement, but it might make sense for communities with particularly valuable oaks to consider ordinances to limit movement of oak firewood and regulate tree and limb disposal. A single tree infected with oak wilt can cost a homeowner $5,000-$10,000 when removal/clean-up costs are added to the tree value loss. So, all residents with oaks on their properties should be concerned with preventing spread of oak wilt disease.

Dr. Patrick J. Rusz
Director of Wildlife Programs