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A disease with the potential to wipe out millions of our beloved oaks (Quercus spp.), oak wilt is an important pathogen that threatens oak trees across Michigan. To give your trees the best chance at surviving this disease, it’s important to familiarize yourself with the basic facts about the pathogen.
We’ll try to help by explaining the symptoms it causes, the manner in which it spreads, and what you can do to protect your trees.
Oak Wilt: Yet Another Disease Affecting Michigan’s Trees
Several devastating tree pests and diseases have spread through the United States over the course of the 20th and 21st centuries, leaving millions of dead trees in their wake.
In 1904, chestnut blight started spreading across the country, effectively killing all of our native chestnuts (Castanea dentata). Dutch Elm Disease followed about 20 years later, decimating many elm (Ulmus spp.) species, including the iconic American elms (Ulmus americana) that lined many streets across the Midwest. Around the turn of the century, the emerald ash borer was discovered in Michigan, and it quickly spread across large portions of the country, with countless ash trees (Fraxinus spp.) bearing the brunt of the invasive pest.
However, there is one other tree disease that has been in the background for much of this time, quietly spreading through much of the Upper Midwest. Called oak wilt, this disease was first documented in the 1940s, but it has become a more prevalent threat in recent years.
No one knows exactly where oak wilt originated, but some authorities suspect that Mexico or Central America is the primary reservoir of the fungus, given that this is the epicenter of oak species diversity.
What Is Oak Wilt?
Oak wilt is a devastating tree disease, caused by the fungus Bretziella fagacearum. It works in a surprisingly simple manner that’s similar to many other blights or wilts: The fungus invades the tree’s xylem (wood), blocking the vessels from conducting water up from the roots to the canopy. This effectively starves the canopy of water, leading to rapid wilting and — in many cases — death.
It is important to note that the disease plays out somewhat differently in different types of Michigan oaks.
Oak Wilt in Red Oaks
Northern red oaks (Q. rubra), black oaks (Q. velutina), scarlet oaks (Q. coccinea), pin oaks (Q. palustris), and other oaks in the red oak group are hit especially hard by oak wilt. They often begin displaying symptoms very quickly following infection, as they have virtually no resistance to the disease. Red oaks almost always die from oak wilt — typically during the initial season in which they become infected and sometimes in as little as four to six weeks.
Oak Wilt in White Oaks
Meanwhile, oaks in the white oak group, including white oaks (Q. alba), bur oaks (Q. macrocarpa), chinkapin oaks (Q. muehlenbergii), and swamp white oaks (Q. bicolor), are better compartmentalizers, have some (slight) resistance to oak wilt, and tend to suffer from a milder form of the disease. They may still die, but some do recover naturally or with the help of systemic fungicides.
How Does Oak Wilt Spread in Michigan?
Oak wilt can spread in several ways — that’s part of the reason it has become so widespread over time. Three of the most common ways it spreads include beetles, root grafts, and some human activities.
Oak Wilt Spread via Beetles
At least four different species of sap beetles in the family Nitidulidae (sometimes called picnic beetles) are attracted to the sap of oaks, as well as the fungal mats Bretziella fagacearum produces in red oaks. During the process of feeding on wounded oaks or these fungal mats, the beetles become contaminated with the fungal spores, enabling them to transfer the disease to uninfected trees.
Oak Wilt Spread via Root Grafts
Like many other trees, oaks often form interconnected root networks that allow the roots of different individual trees to become grafted together. The oak wilt fungus can then move from infected trees into the root systems of uninfected trees, which allows the disease to spread. In fact, root grafts are likely the primary way oak wilt spreads in Michigan.
Oak Wilt Spread via Human Activities
Humans also serve as a vector for the spread of oak wilt. For example, firewood harvested from an infected tree may be transported miles away, which can allow spores found in or on the wood to infect healthy trees far from the original source of the infection. Pruning can also allow the disease to spread, as the fresh wounds attract beetles that may have been exposed to the oak wilt fungus.
Oak Wilt Symptoms in Michigan
The specific symptoms oak wilt causes in a given tree may vary a bit based on the tree’s current health, stress level, and species. However, the following symptoms are among the most common:
- Sudden leaf wilting
- Leaf discoloration (especially around the leaf margin)
- Premature leaf drop
- Branch dieback
- Rapid canopy thinning
Additionally, ISA-certified arborists can check the tree for internal signs of oak wilt, such as the appearance of dark brown streaks in the tree’s sapwood. There is one other symptom that is only present on red oaks that have died from the disease: grayish-brown fungal mats under the bark, often called pressure pads. Vascular streaking and the presence of pressure pads are two of the most compelling signs that a tree has (or had) oak wilt.
Where Is Oak Wilt Found in Michigan?
Unfortunately, oak wilt is found across most of Michigan, including both the Upper and Lower Peninsulas. It is slightly more common in the northern portion of the Lower Peninsula, but it is also found in the southern portions of the state, including the Detroit Metro area. In the Upper Peninsula, it is most common around the Wisconsin border.
Nevertheless, it is important that homeowners take steps to avoid spreading the disease. While it is found across most of the state, that doesn’t mean it is present in every city, neighborhood, or property.
Oak Wilt Treatments
There aren’t many effective treatments for oak wilt, which is part of the reason the disease is so problematic.
Infected red oaks are essentially doomed, and rarely (if ever) recover from the disease. Accordingly, these trees should generally be removed and the wood disposed of properly. However, when diagnosed at an early stage in the disease progression, white oaks may be treatable with a synthetic fungicide called propiconazole.
This fungicide helps limit the fungus’ spread and allows the tree to compartmentalize (seal off) the area. But because the fungus is found in the tree’s vascular system, it must be administered via a process known as trunk injections. This involves a certified arborist injecting the compound directly into the tree’s wood tissue, where it will be absorbed and hopefully reach the infected vessels.
Propiconazole provides about one to two years of protection, so repeated treatments are required.
Protecting Trees from Oak Wilt in Michigan: What Can Homeowners Do?
Because treatments for oak wilt are somewhat limited, it is important to be proactive and do everything possible to prevent your trees from contracting the disease. A few of the most effective preventative measures homeowners can employ include:
- Remove and dispose of oak-wilt-infected trees immediately.
- Protect oaks from mechanical damage.
- Avoid pruning oaks during the active season.
- Sterilize pruning equipment and tools between trees.
- Paint all stumps and branch stubs following pruning or removal.
- Use care when handling oak firewood.
- Install trenches around infected trees (or clusters of trees).
- Consider having your trees treated proactively with propiconazole.
Oak Wilt in Michigan: FAQ
Have additional questions about oak wilt in Michigan? Want to learn more about protecting your trees from this disease? We’ve rounded up a few of the most common questions we hear from homeowners and answered them below.
Is there oak wilt in Michigan?
Yes, oak wilt has been documented in most portions of Michigan. However, that doesn’t mean it is present in your city or zip code, so it is still important to do everything possible to prevent further spread.
Can a tree recover from oak wilt?
White oak trees can recover from oak wilt at times, especially if they’re treated with propiconazole at the earliest stages of the disease. However, red oaks virtually never recover, regardless of whether or not they’re treated.
Should you cut down a tree with oak wilt?
Tree removal is generally warranted for red oaks suffering from oak wilt. White oaks may require removal in some cases, but you’ll generally want to have them treated first to see if they will recover.
When should you not cut oak trees in Michigan?
Always avoid pruning oaks in Michigan between April and August, when sap beetles are most active. If you have to remove a tree during this time, be sure to dispose of the wood properly and cover any remaining stump with wound-sealing paint (or remove the stump entirely).
Does oak wilt spread from tree to tree?
Yes, oak wilt spreads from tree to tree via root connections called grafts. It can also spread indirectly through the feeding activity of sap beetles or human activity (such as not sterilizing pruning tools between trees).
What does the beginning of oak wilt look like?
Sudden leaf discoloration, wilting, and premature leaf drop are all common early signs of oak wilt. In red oaks, leaf discoloration tends to start at the top of the canopy and spread rapidly, while it often takes the form of “flagging” (individual branches turning brown) in white oaks and progresses more slowly.
Can I treat oak wilt myself?
Generally speaking, no — homeowners can’t treat oak wilt themselves. Propiconazole is available for general landscaping use, but the formulations and equipment required for tree injections are typically restricted to licensed professionals. It may even be illegal for homeowners to carry out tree injections in some cases. Given these factors, it’s generally wisest to leave oak wilt treatments to the professionals.
Get Your Oaks the Help They Need!
Whether you suspect that your existing oaks are already suffering from wilt or you want to get ahead of the disease and protect your trees, the professionals at Longtree Tree Service can provide the kind of tree care service you need.
With more than 200 5-star reviews, an A+ rating from the Better Business Bureau, and 70 years of combined tree-care experience, we’ve built a rock-solid reputation for being the arborists homeowners and commercial property managers across Southeast Michigan can depend on.
Contact us now to schedule your free on-site consultation and learn more about how our certified arborists can treat infected trees and prevent the spread of this troubling disease.
Sources & References
- New Phytologist – “Title: [Study on oak wilt and forest impacts]”
Peer-reviewed scientific research examining oak wilt disease dynamics, transmission, and impact on oak populations.
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.14773 - Michigan State University Extension – “Oak Wilt Disease in Michigan”
Provides an overview of oak wilt identification, spread, and best practices for homeowners and landowners in Michigan to prevent and manage the disease.
https://www.canr.msu.edu/news/oak_wilt_disease_1 - University of Minnesota Silvics Library – “Oak Wilt Treatments: Michigan’s Upper Peninsula & the Lower Great Lakes Region”
Discusses treatment methods, regional considerations, and long-term management approaches for oak wilt in northern climates.
https://silvlib.cfans.umn.edu/oak-wilt-treatments-michigans-upper-peninsula-state-michigan - MichiganOakWilt.org – “Oak Wilt Information & Resources”
A resource hub offering detailed information about oak wilt disease, spread, prevention, tree care recommendations, and local outbreak maps.
https://www.michiganoakwilt.org - ArcGIS – “Oak Wilt Outbreak Map – [Interactive Dashboard]”
Provides a geographically detailed, interactive map showing oak wilt cases and outbreaks, helping homeowners and professionals assess regional risk.
https://experience.arcgis.com/experience/9b2ab59dd0994b49a5de5331bca5a8fb
