NORTHERN MICHIGAN – Oak Wilt is devastating Michigan’s oak forests, and it’s up to the public to minimize the spread.
Avoiding oak tree pruning at the high-risk period is an important way to prevent oak wilt. One way that oak wilt is spread is by sap beetles which are attracted to the sap on tree wounds and pruning cuts. During the growing season, primarily spring and early summer, these beetles feed on fresh mycelial mats of the oak wilt fungus (Bretziella fagacearum) where they pick up spores. The spores enter healthy oaks when a contaminated beetle feeds on a fresh oak tree wound. In the winter, beetles are less active, and mycelial mats with viable spores are not common, so risk of infection is low. Pruning between April 15th and July 15th should be strictly avoided, and wounds to oaks should be sealed with pruning sealer to prevent infection. Spring is when beetle activity and viable spores are highest, so risk of infection is at its peak.
Oak wilt can also be spread when wood from oaks killed by oak wilt is moved into new areas. Mycelial mats that attract sap beetles can be produced on oak wood from trees killed by oak wilt. Dried and completely de-barked wood does not contain enough moisture for the oak wilt fungus and is safe to move. Go to the Michigan Oak Wilt Coalition website to read about how to tarp freshly cut oak wood from oak wilt killed trees to avoid additional disease spread (Michigan Oak Wilt Coalition). It is best to avoid moving firewood to new locations.
Another mode of oak wilt disease transmission is from tree to tree via shared tree roots. Once an oak tree is infected, it can pass the oak wilt to healthy oaks nearby. The most common and effective way to halt this kind of disease spread is to sever the root connections between infected and healthy oak trees. This is done with trenching and then removal of infected trees. Sometimes a buffer of healthy oak trees will need to be removed as well. Protecting healthy oaks in the infected root-zone area can be achieved with fungicide injections of propiconazole to the root flares. Other methods of halting tree to tree infection are being explored and are used when trenching is not possible. It is important to have a professional arborist trained in oak wilt management to draw up a management plan before any tree removal is done.
Be aware of what oak wilt looks like to help control disease spread. Oak trees in the red oak family will show leaf wilt symptoms often starting at the top of the tree, followed by rapid leaf loss. Symptomatic leaves can range in color from tan to green and trees die within weeks. A dead oak surrounded by oaks with these leaf symptoms may indicate that the disease has been spreading for 1 or more years. Because several other pests and diseases can cause problems in oaks, it is critical to confirm the symptoms are being caused by oak wilt, and not something else. Confirmation of oak wilt disease requires a positive lab test unless a mycelial mat is located under the bark of a dead oak tree. For information on how to collect a sample to test for oak wilt, read The Best Sampling Procedures for Accurate Oak Wilt Testing. If oak wilt is confirmed, consult a qualified arborist with oak wilt management training. The Michigan Oak Wilt Coalition maintains a directory of professionals experienced in the diagnosis and treatment of oak wilt disease posted on their website.
Know the signs of oak wilt and the steps to preventing it. These steps include avoiding pruning activity on oaks during high-risk periods, managing disease outbreaks effectively, and not moving firewood to new areas. These actions will help protect oak trees and the countless benefits they provide to people, wildlife, and our communities.
