The cause of oak wilt

Oak wilt is caused by a fungus, Ceratocystis 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.

Trees that can get oak wilt

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 spreads both above and below ground

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.

Overland

Oak wilt can also spread above ground by 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 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.

Preventing oak wilt

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 DNR recommends that you avoid 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 Observations and unpublished research have shown that overland infection can occur after July, yet these mid-summer through early fall infections are not common.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

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 guide.

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.

Firewood

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.