Oak wilt is a fungal disease that moves through oak trees and, in the red oak group, can kill a mature tree within a season. It is present in Michigan, and the way people most often spread it is by pruning at the wrong time.
How it spreads
Sap beetles are drawn to fresh wounds on oaks. They pick up fungal spores from infected trees and carry them to the cut. The disease also moves underground between oaks whose roots have grafted together, which is why it tends to work outward through a group of oaks rather than appearing at random.
The timing rule
Michigan State University Extension and the Michigan DNR advise against pruning oaks during the high risk window in spring and early summer, roughly mid April through mid July, when beetles are most active. The safest time to prune an oak here is in the dormant season: late fall through winter.
If a storm breaks an oak limb during the risk window, that is a different situation. The wound already exists. Making the cut clean and sealing it is generally recommended in that case, which is the one time sealing a pruning cut is advised.
What it looks like
- Leaves browning from the outer edge inward, often starting at the top of the canopy.
- Rapid leaf drop in summer, including leaves that are still partly green.
- In red oaks, a canopy that goes from healthy to bare within weeks.
White oaks tend to decline more slowly and often survive, which is one way to tell the groups apart in practice.
If you suspect it
Do not prune anything in the area, and do not move the wood. Firewood from an infected red oak can carry the fungus. Get the tree looked at, and be aware that managing oak wilt sometimes involves severing root grafts between neighboring trees to stop it spreading underground.
Practical advice for Macomb County yards
Plenty of properties around Armada, Romeo and Washington Township have mature oaks, often several in a row along a fence line. Those root systems are likely connected. Plan oak pruning for winter, keep an eye out for sudden summer browning, and deal with problems promptly rather than seasonally. Our pruning and shaping work is scheduled with this in mind.
Common Questions
When is it safe to prune an oak in Michigan?
The dormant season, roughly late fall through winter. Michigan State University Extension advises avoiding the high risk window in spring and early summer, around mid April to mid July.
A storm broke my oak in June. What now?
The wound already exists, so make the cut clean and seal it. This is the one situation where sealing a pruning cut on an oak is recommended.
Can oak wilt spread to my other oaks?
It can, through root grafts between nearby oaks as well as by beetles. That is why a diseased oak in a row of oaks is treated as a group problem.
Get a Free Estimate
Not sure whether that is oak wilt or ordinary stress? We will take a look. Tip Top Timber Co. is based in Armada and works throughout Macomb County, St. Clair County and the surrounding area. Call (586) 651-5488 or send us a message for a free estimate. If a tree is already down, our 24 hour line is (586) 651-4552.
