Help prevent the spread of oak wilt: Avoid pruning or wounding oak trees between April 15 and July 15

Contact: James Wieferich, 517-284-5866
Agency: Natural Resources

April 14, 2021

Leaves of a tree suffering from oak wilt are partly green and partly brown. Oak trees have a reputation for being mighty, but these majestic trees need our help in spring and early summer to prevent the spread of a microscopic killer.


From April 15 to July 15, oak trees are at high risk for oak wilt, a serious fungal disease that can weaken white oaks and kill red oak trees within weeks of infection. During this time of year, flying beetles can carry oak wilt fungus spores from tree to tree and the fungus can infect trees through wounds left by pruning or storm damage.

"The guidelines against pruning oak trees during this time can help prevent the spread of the disease," said James Wieferich, forest health specialist in the DNR's Forest Resources Division.

The fungus can move from an infected oak to neighboring oaks through root grafts. Depending on tree size, adjacent oaks may be connected to other trees, or grafted, through root systems. Roots of large trees can reach 100 or more feet. Left untreated, oak wilt will continue to move from tree to tree, killing more red oak over an increasingly larger area. As more trees die from oak wilt, more fungal spores are produced, which allows the beetle to carry infection to new locations. Trees in the white oak family are more likely to survive an infection because they are capable of compartmentalizing the fungus. 

Oak wilt was first identified in Michigan in 1951 and this invasive species now is widespread across the state. An invasive species is one that was introduced to Michigan's environment from elsewhere and can cause harm to the environment, economy or human health.

Red oaks are vulnerable; can die within weeks of infection

A vibratory plow is used to dig trenches to separate oak roots and stem the spread of oak wilt Trees in the red oak family are most susceptible to oak wilt. These trees 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 and swamp white oak. They are less susceptible.


Symptoms most often appear from late June through September. Affected trees will suddenly begin to wilt from the top down, rapidly dropping leaves, which can be green, brown or a combination of both colors.

If you have a tree that gets damaged during the risk period from April 15 to July 15, immediately cover all wounds with tree-wound paint or latex-based paint.

Firewood can harbor the fungus, so don't move firewood from place to place in the state. If you suspect your firewood is infected with oak wilt, you can help slow the spread by burning it, chipping it or debarking it before April. Once the firewood has been dried over a year and/or all the bark loosens, the firewood can no longer spread oak wilt.

To minimize the risk of oak wilt infection caused by logging damage, the DNR restricts cutting of red oak trees on state land between April 15 and July 15. The DNR recommends private forest landowners exercise caution during this period and, whenever possible, delay harvesting activity in oak forests until after July 15.

What to do if you suspect oak wilt:

Report infections at Michigan.gov/ForestHealth using an interactive map.

Contact a local DNR forest health specialist for more information at DNR-FRD-Forest-Health@Michigan.gov or 517-284-5866.

Contact your local Forestry Assistance Program forester for a site visit in select counties.

Seek verification from Michigan State University. Find instructions at canr.msu.edu/pestid or call 517-355-4536.

Get help from an oak-wilt qualified specialist. Visit MichiganOakWilt.org for a listing and more information.

Learn more about invasive species in the state at Michigan.gov/Invasives.