Learn if your property is at risk, how to reduce the spread of oak wilt in a forested area, how to know if a yard tree has oak wilt and much more.
Oak wilt is widespread in southern Wisconsin but in much of northern Wisconsin it is still a new and uncommon disease.
View an expanded map of oak wilt detections in Wisconsin as of September 24, 2020.
Oak wilt is caused by a fungus, Bretziella fagacearum. The fungus invades areas inside the tree where water moves. Later, balloon-like bumps called tyloses are formed and they plug up the water's path through the tree. As water movement inside the tree is slowed, the leaves wilt and drop off the tree.
Oaks in the red oak group (black, northern red, northern pin and others with pointed leaf edges) get this disease most easily. Oaks in the white oak group (white, swamp white, burr and others with rounded leaf edges) are less susceptible.
Oak wilt spread underground
Most oak wilt moves from diseased trees to healthy trees through roots that have become interconnected (root grafts). Most root grafts form between oaks of the same species; grafts between red and white oaks are very rare. In general, red oak roots graft more than white oak roots.
Oak wilt spread overland
Oak wilt can also spread above ground by sap-feeding beetles. In the spring, fungal mats (small masses of Bretziella fagacearum) develop under the bark of some trees that have died from oak wilt the year before. These mats force the bark to crack open. The fungus produces a sweet odor that attracts sap-feeding beetles to the mats. The beetles then fly to healthier oaks to feed on sap flowing from fresh wounds, carrying the fungus on them and thus infecting healthy trees.
Oak wilt also spreads when firewood or logs from infected trees with fungal mats are moved. Fungal mats hide easily in firewood and often go unnoticed.
If the disease is allowed to progress, it will spread to healthy oaks that are connected by the roots (root grafts) to the diseased trees. In forested areas where oak is common and root grafting is widespread, an ever-widening pocket of dead oaks will form. Where oak is mixed with other species and is a minor part of the forest, oak wilt will spread slower and may actually stop where roots are not grafted. New pockets of dead oak may also be formed by sap-feeding beetles spreading oak wilt above ground.
Dead oak trees can serve as excellent den trees for wildlife. Oaks do not decay as quickly as aspen, birch and red maple, so they will provide shelter for wildlife for many years. Also, as oaks die, the site often becomes brushy for about 10 years. Warblers, grosbeaks, cuckoos, cardinals, grouse, rabbits, deer and shrews will be attracted to the brushy area. Brown creepers may nest under the bark falling off of dead trees. Dead trees will also supply insects for birds, and large dead trees may provide perches for raptors.
Are you thinking of building on a wooded lot? Learn how to protect your trees from oak wilt.
Symptoms and signs
It can be difficult to tell if a tree has oak wilt just by looking at it. Use the information below to do a preliminary assessment of your trees and then follow instructions for submitting a sample for testing.
Symptoms are the characteristics expressed by an infested tree, such as leaf discoloration and wilting and dropping of leaves. Signs are the physical evidence of the pest or disease itself, such as fungal mats beneath the bark of dead oak trees.
Oak leaves from infected trees show dull green or bronze discoloration around leaf margin while midrib and base are still green.
Infected leaves wilt rapidly and drop to the ground in summer. Unlike normal leaf fall, the leaves of infected trees drop while still partially green.
Fungal mats develop in the fall or spring following the death of a tree infected with oak wilt. These mats, formed underneath the bark, produce spores that are carried by sap-feeding beetles to fresh wounds or recently cut oak stumps.
Find out if your tree has oak wilt
If you think oak wilt might be causing problems for your tree, you can send in a sample for examination. There is a small fee for this service.
Collect three twigs (about 1/2-inch in diameter and four inches in length) from three different branches with wilting leaves. Be sure these samples still have living wood and leaves. Scratch the sample branch with your fingernail. If the wood under the bark is a light color (white to green), the sample is fresh. If the wood is brown or dark, it is too old to be sampled. Wrap fresh samples in wax paper and keep them cool until you mail them to:
Plant Disease Diagnostics Clinic
University of Wisconsin - Madison
1630 Linden Drive
Madison, WI 53706-1598
In urban/residential areas
Oak trees are most easily infected by overland spread in the springtime from bud swelling until two to three weeks past full leaf development. The Wisconsin Department of Natural Resources recommends avoiding pruning, cutting or wounding oak from April through July in urban areas. Observations and unpublished research have shown that overland infection can occur after July, yet these mid-summer through early fall infections are not common. To take a very cautious approach, do not prune or otherwise wound oaks from April to October.
In some years, spring comes much earlier. If daytime temperatures begin to reach the 50-degree mark, stop pruning oak at that time, even if it is still the middle of March.
If you must prune when temperatures are above 50 degrees or between April and October, cover the wound surface with tree wound paint immediately. Tree wound paint can actually slow the natural closing of the wound, so limit the use of wound paint to the situation described above.
In forests,site-specific oak harvest guidelines are used. An interactive guide is also available; answer a few questions about your specific forest site to get the most appropriate guidelines for that type of site.
The oak harvesting guidelines state specific situations as Exceptions and Modifications. If any of the Exceptions or Modifications apply to your stand, you may not need to implement the seasonal harvesting restriction due to oak wilt. For important details by site, refer to the interactive or print guide.
View a map of Wisconsin showing counties in the North and South zones.
Options for managing oak wilt
Managing oak wilt by disrupting common root systems
The most common way oak wilt spreads is through connected roots (root grafts) of different trees. If healthy oaks of the same species are near an infected tree, removing the infected tree will not control the spread. In fact, the fungus may move more quickly through the root grafts if infected trees are removed.
The best control measure is to create a root graft barrier which disconnects the shared root systems between healthy and diseased trees. The most successful way of doing this is to physically cut roots with a vibratory or cable plow or trencher. The barrier must be located correctly to be successful.
Often, nearby healthy looking oaks may already be infected but are not yet showing symptoms. A forest pest specialist, forester or consultant trained in oak wilt management should work with you to plan the location of the barrier.
Diseased oak wood
After creating root graft barriers, diseased wood may be removed and used nearby for firewood or other projects. Trees that have died from oak wilt can still have the fungal mats so if this wood is moved, the fungal mats are moved and the disease may spread into new areas.
Any trees that have died from oak wilt and have bark that is tightly attached to the wood could be hiding fungal mats. This wood must receive special treatment before being moved. Once the bark has naturally become loose and falls off the wood, the mats are no longer a concern and no special treatment is necessary to safely move the wood.
There are two ways to treat wood to prevent the overland spread of oak wilt in firewood.
- Debarking (removing the bark from the wood) will prevent the fungus mats from forming. Debarking must be done before fungal mats form, so it should be done in the late summer, fall or winter following tree death.
- Cutting, splitting, stacking and covering the wood with a four mil or thicker plastic will also prevent overland spread. All sharp edges or stubs of the wood should be cut so the plastic is not punctured or torn. The entire pile must be sealed (seal the bottom by covering it with dirt and logs or other heavy objects). If the wood is not burned the winter following tree death, leave the tarp on through the next growing season (until October) or until the bark is loose.
Other wood products
Wood from infected trees may be sold to a sawmill or chipping facility--preferably one several miles away form the nearest red oak. Tell the purchaser that the infected trees with tightly attached bark must be used over the coming winter.
The oak wilt fungal mat does not survive well when it is dried out, exposed to other adverse conditions or put in competition with other wood decay fungi. Thus, wood chips from infected trees are highly unlikely to be a source of disease and can be used for landscaping.
Saws and diseased wood
There is not much research on this topic. Experience has shown that it is not likely oak wilt would spread by saws. At this time, it does not seem necessary to disinfect saws.
Both native and non-native oaks are susceptible to a range of insects, diseases and abiotic issues. Some examples include bur oak blight, anthracnose, leaf tatters, leaf and twig galls and two-lined chestnut borer. Review the following links to learn more about these other issues that may be impacting oak trees on your property.