Oak Wilt in Wisconsin: Biology and Management

What is the distribution of oak wilt?

Oak wilt has probably been a part of our forests in Wisconsin for 100 years. Oak wilt is widespread throughout the southern Wisconsin oak resource.

What causes this disease?

Oak wilt is caused by a fungus, Ceratocystis fagacearum. The fungus invades water-conducting vessels and induces the formation of balloon-like projections called tyloses which also plug the vessels. As water movement within the tree is slowed, the leaves wilt and drop off the tree.

Which trees are susceptible?

Oaks in the red oak group (black, northern red, northern pin and others with pointed leaf edges) are most susceptible. Oaks in the white oak group (white, swamp white, burr and others with rounded leaf edges) are less susceptible.

What are the symptoms?

These trees drop their leaves rapidly (usually within a 3-week period) most often in late June and throughout July and August. Some lose a portion of their leaves in September then rapidly lose all their leaves just after they come out in the spring.

These trees drop their leaves on 1 to several branches several years in a row. Trees in the white oak group do not always die; they may survive an infection.

How does this disease spread?

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; red oak roots graft more commonly than do white oak roots, and grafts between red and white oaks are very rare.

Some movement of oak wilt is overland via sap-feeding beetles. In the spring, fungal mats (small masses of Ceratocystis 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 on the mats. The beetles then fly to healthier oaks to feed on sap flowing from fresh wounds, thus infecting healthy trees.

Overland spread can also occur when firewood or logs from infected trees harboring fungal mats are moved.

How do I confirm my tree has oak wilt?

Collect 3 twigs (about 1/2" in diameter and 4" in length) from 3 different branches with wilting leaves. Samples must still have live tissue. 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 the sample in wax paper and keep it cool until you mail it. Mail the sample to:

Brian Hudelson
Plant Disease Diagnostics Clinic
Department of Plant Pathology
University of Wisconsin - Madison
1630 Linden Drive
Madison, WI 53706-1598
(608) 262-2863
There is a $15.00 fee for this service.

How do I prevent oak wilt?

How do I manage oak wilt?

Spread through grafted root systems is the most common way that oak wilt spreads. If healthy oaks of the same species are near an infected tree, removing the infected tree will not control the spread. In fact, movement of the fungus through the root grafts may be hastened by prompt removal of infected trees. The best control measure is to install a root graft barrier which disrupts the common root systems between healthy and diseased trees. The most successful methods of doing this involve physically severing roots with a vibratory or cable plow or trencher. Locating the barriers correctly is crucial to their success. Often adjacent healthy appearing 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 barrier location.

What should I do with diseased wood?

After installing root graft barriers, diseased wood may be removed and utilized for firewood or other projects. Trees that have died from oak wilt can harbor the fungal mats so if this wood is moved, the fungal mats are moved and the disease may spread into areas currently unaffected. Any trees that have died from oak wilt and have bark that is tightly attached to the wood could harbor fungal mats. This wood must receive special treatment. Once the bark has become loose and falls off the wood, the mats are no longer viable and no special treatment is necessary; movement of the wood is no longer a concern.

Two methods of wood treatment are effective in preventing overland spread via firewood.

1. Debarking (removing the bark form the wood) the wood will prevent the fungus mats from forming. Debarking must be conducted before fungal mats form, thus it should occur in the late summer, fall or winter following tree death.
2. Cutting, splitting, stacking and covering the wood with a 4mm or thicker plastic will also prevent overland spread. All sharp edges or stubs should be cut to eliminates the possibility of puncturing the plastic. The entire pile must be sealed all around. Seal the bottom by covering it with dirt and logs or other heavy objects. If the wood is not burned over the winter following tree death, leave the tarp on through the next growing season (October 1) or until the bark is loose.

Wood from infected trees may be sold to a sawmill or chipping facility--preferable one which is several miles away form the nearest red oak. Advise the purchaser that the infected trees with tightly attached bark must be utilized 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 serve as a source of disease inoculum or spores and can be used for landscaping.

Should I disinfect my saw after cutting diseased wood?

The research on this topic is minimal, yet experience has shown that the likelihood of spreading oak wilt via saws is very low. At this time, it does not appear necessary to disinfect your saw.

What will happen if I do not actively try to stop oak wilt?

If you allow the disease to progress, it will spread to healthy oaks that are grafted through roots to the diseased trees. In stands where oak is common and root grafting prevalent, an ever-widening pocket of dead oaks will form. In forests where oak is mixed with other species and is a minor component, spread will be slower and may actually stop from a lack of root grafting. New pockets may also be formed via overland spread by sap-feeding beetles (see description of overland spread).

Dead oak trees can serve as excellent den trees for wildlife. Oaks do not decay as quickly as aspen, birch and red maple, thus 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 sloughing bark on dead trees. Dead trees will also furnish insects for birds, and large specimens may provide perches for raptors.

Read these publications for further information about oak wilt:
"Oak Wilt Management: What are the options?" (exit DNR)
"Are you thinking of building on a wooded lot? Protect your trees from oak wilt"

Forest Health Protection