The winter months are the ideal time to prune oak trees in Michigan if you don’t want to put them at high risk for oak wilt.
Oak wilt is a fungal disease that can weaken white oaks and kill red oak trees within just a few weeks. The fungus enters the tree through wounds that are often caused by pruning or storms.
Oak trees are at high risk for infection by the oak wilt fungus if they’re pruned between April 15 through July 15.
“Winter is considered low risk, but we’re dealing with nature, and winter weather is sometimes warm,” said Dr. Tara Bal. “Overland spread of oak wilt requires beetles to carry viable spores flying to fresh wounds at a time when trees are vulnerable. Research indicates this is unlikely to occur during winter months.”
How can you help a damaged tree?
If you have an oak tree that was damaged during the high-risk period you should cover all wounds with tree-wound paint or latex-based paint.
Painting tree wounds is not recommended for other tree species because it can reduce the effectiveness of the healing process, according to the DNR.
What is oak wilt? What are the symptoms?
Oak wilt is caused by the fungus Bretziella fagacearum.
Once a tree is infected, the fungus may fruit, creating gray to black fungal pads under the tree’s bark. Fungal spores can be picked up by sap-feeding nitidulid beetles, which are attracted by the pads’ yeasty odor. Sap beetles spread oak wilt when they visit other pruned or damaged oaks to feed on exposed sap.
Oak wilt was first discovered in the 1940s. It is widespread across Michigan. Officials said oak trees across areas of the Lower Peninsula, and in Dickinson, Iron, Marquette and Menominee counties in the Upper Peninsula, are dying from oak wilt.
Red oaks are most susceptible. Red oaks have leaves with pointed tips and include black oak, northern red oak, and northern pin oak. Trees in the white oak group have rounded leaf edges and include white oak, swamp white oak, and bur oak.
Oak wilt is usually first identified when green leaves turn yellow or brown on the edges and begin to drop during the summer.
- Symptoms usually appear from late June until September. Trees will wilt from the top-down, rapidly dropping leaves, which can be green, brown, or a combination of both colors.
- The fungus can move through root grafts to oaks about 100 feet away. Untreated, oak wilt will continue to move from tree to tree. As more trees die, more fungal spores are produced, which allows the beetle to carry infection to new locations.
Don’t transport infected firewood
Firewood cut from an infected tree can still harbor fungus. Don’t move firewood from one place in Michigan to another.
If you suspect your firewood has oak wilt, you can help slow the spread by burning it, chipping it, or debarking it before April.
Once the firewood has been dried for longer than a year and/or all the bark loosens, the firewood will no longer spread oak wilt.
How to report oak wilt infections
The DNR restricts cutting of red oak trees on state land between April 15 and July 15. The DNR is urging private forest landowners to use caution and delay harvesting activity in oak forests until after July 15.
You can use this interactive map to report infections. If you need more information you can reach out to a DNR specialist via email at DNR-FRD-Forest-Health@michigan.gov or by calling 906-203-9466.
Michigan State University’s Diagnostic Clinic also can verify oak wilt infection. Click here to view online instructions or call 517-355-4536.
Click here to learn more from an oak-wilt specialist. If you want to learn more about invasive species in Michigan, you can click here.