Imagine this: your beautiful oak tree, the pride of your property, suddenly starts dropping leaves in the middle of summer. Not fall. Not after a storm. Just... dropping. Fast. If this sounds familiar, you might be dealing with a serious threat: oak wilt.
Let’s dig into what it is, how to spot it, and how Safari Tree can help you save your trees before it’s too late.
It’s that time of year again—oak wilt season in Michigan. From April 15 through July 15, oak trees across the state are at high risk for one of the most destructive tree diseases around: Oak Wilt.
This fast-moving, deadly fungal disease—caused by Bretziella fagacearum—primarily targets red oaks and white oaks. Once infected, the fungus blocks the tree’s water-conducting vessels, causing leaves to wilt and discolor, branches to die, and, if left untreated, the entire tree can perish—sometimes within just a few weeks.
Oak wilt spreads throughout the root system of the tree. The water-conducting canals gradually become clogged. Water transfer eventually stops, causing the withering effects.
Oak Wilt is sneaky—and fast. It spreads in three ways:
That’s why timing matters—especially when it comes to pruning
3. Firewood transmission:
Among the oak species, red oak trees are most susceptible to oak wilt, often dying within 2-6 weeks of being infected. Red oaks are often the site of fungal spores, and their symptoms are not always as apparent. Once it spreads underground, it can infect multiple trees in a single season if not contained.
Arborists first discovered oak wilt in Wisconsin in 1944 and declared it a harmful disease. As of 6-5-25, there are confirmed oak wilt cases in over 61 Michigan counties per Michigan Department of Natural Resources.
Oak Wilt Mapper from MI Department of Natural Resources, 6-5-15; Red Dot = Confirmed Oak Wilt Case
Oak Wilt doesn’t waste time. Here’s what to look for:
If your oak is dropping leaves when it shouldn’t be, it’s time to take a closer look.
With the majority of red oaks unable to recover from oak wilt, the need for proactive measures is clear. Here are some simple yet effective ways to prevent the spread of this damaging disease:
Beetles tend to attack your trees not long after trimming or pruning. If they’re carrying oak wilt disease, they will spread it. So, avoid trimming or pruning your trees when the weather is warm. In Michigan, only prune or trim from November through March (the non-growing season).
If you do notice fresh wounds on your tree, you can paint over them (with latex paint) to prevent beetles from attacking. Also, space your trees far enough apart when planting. You’ll want to make sure your tree’s roots don’t touch the roots of another tree. As we mentioned, the roots can spread the disease.
Trimming away weak or dead branches from trees — otherwise known as pruning — is one of the most effective ways to keep them strong and healthy. Like other trees, only prune oak trees during the winter months. Not only is it easier to identify branch patterns during this time, but there is also less insect activity and subsequent infections to worry about.
In the case of sap beetles, these insects are most likely present between mid-April and mid-July. (Note that some types of beetles can be active during the summer and into the early fall.) Avoid pruning during these because the risk for infection is highest.
The Michigan Department of Natural Resources has additional information on its website. As does this brochure from Consumers Energy.
Avoid moving firewood many miles from its original location, whether it’s to a vacation property or a campsite. With this transport comes an assumed risk that the firewood may carry a disease that can harm subsequent trees.
If you’re cutting a tree infected with oak wilt to use for firewood, make sure it remains in one area to avoid transmission to healthy trees. And throw a tarp with no holes over the wood to cover it. This will keep beetles from feeding on the sap of the wood and picking up fungal spores.
When it comes to treating oak wilt, tree injections will not be able to save red oak group trees already exhibiting symptoms. We also recommend injecting non-infected oaks that are growing near the sick trees. This will help slow the disease’s spread.
It is also imperative that infected trees be removed. Do not use it for firewood unless the wood is sealed under a tarp during the warm season. It can be debarked, or the wood can be chipped.
At Safari Tree, we recognize that trees have unique needs and face unique threats. That’s why we’ve developed a comprehensive tree health care program that ensures trees can effectively resist and recover from any number of health issues (oak wilt included).
If you’re a Michigan homeowner in need of tree care, we encourage you to reach out to the Safari Tree team and request a FREE quote. Don’t expose your trees to the risk of fungal infection. For additional information on prevention or a free treatment estimate, contact Safari Tree today.