April 15, 2020
Contact: James Wieferich, 517-284-5866
Trees at higher risk of oak wilt now through mid-July
If you have oak trees – especially red oaks – now is the time to be wary of oak wilt spores carried by flying beetles.
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 a few weeks of infection.
“The guidelines against pruning oak trees during this period are a way to help prevent the spread of the disease,” said James Wieferich, forest health specialist in the DNR’s Forest Resources Division. “Unfortunately, many people learn not to prune or otherwise wound trees from mid-April to mid-July only after they lose their oaks to oak wilt.”
Fungus travels by insects, through root grafts
Once a tree is infected, the fungus also can move to neighboring red oaks through root grafts. Oaks within about 100 feet of each other – depending on the size of the trees – have connected, or grafted, root systems. Left untreated, oak wilt will continue to move from tree to tree, killing more red oaks 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.
Oak wilt was first identified in the 1940s and is now widespread across Michigan. Red oaks are most susceptible to the disease. These trees have leaves with pointed tips and include black oaks, northern red oaks and northern pin oaks. Trees in the white oak group have rounded leaf edges and include white oaks and swamp white oaks. They are less susceptible.
Symptoms most often appear from late June until 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.
Take steps to reduce risks to your trees
The high-risk period of infection occurs from April 15 to July 15, so it's important to avoid pruning or injuring oak trees during this time. If you have a tree that gets damaged during the risk period, immediately cover all wounds with tree-wound paint or latex-based paint.
Don’t move firewood, especially if it comes from oaks that may have died from oak wilt, as firewood can harbor the fungus. If you suspect your firewood is infected by 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-managed 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.
• Michigan State University’s Diagnostic Clinic also can verify oak wilt infection. Find instructions at PestID.MSU.edu/ 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.