Disease name: Oak wilt
Pathogen name: Bretziella fagacearum (fungus)
Hosts: All oaks are susceptible to oak wilt. The red oak group, which includes species whose leaves have sharp-pointed lobes on their leaves (like red, black, Hill’s, pin, and scarlet oaks) is more susceptible to oak wilt than the white oak group, which includes species whose leaves have rounded lobes on their leaves (like white, bur, English, swamp white, and chinkapin). Trees in the red oak group usually die quite rapidly, often within weeks or months after infection. Trees in the white oak group typically develop symptoms more slowly, showing branch dieback for years before dying. In some cases, white and bur oaks recover after one year of infection.
Oak wilt, caused by the fungus Bretziella fagacearum, has become a serious disease threat to oaks in the eastern and central United States, southward into Texas. While the spread of oak wilt has not been rapid, it can be found in all counties of Illinois. The estimated range of oak wilt runs from southern Michigan through central Pennsylvania along the Appalachian Mountains south to Georgia, westward to the Great Plains and including much of Texas, then northward into Minnesota.
Symptoms
The fungus that causes oak wilt invades the water conducting tissues (xylem) and induces the tree to clog its own vessels, preventing the normal flow of water. This causes the foliage to discolor and die. As the disease progresses, limbs die back and eventually the entire tree can die.
In the red oak group, the spread of the disease can be rapid with symptoms starting at the top of the tree and progressing inward and downward on the lateral branches within a few weeks. Complete leaf drop usually occurs by the middle of summer. Leaves may be grayish-green, brown, or brown at the tip and margin when they drop. Other symptoms include curling leaves that become stiff, a yellow or bronze leaf color, and profuse suckering at the base of the tree. Brown streaking may be present under the bark of infected branches. A branch cut in cross-section may show a brown ring in the sapwood..
In the white oak group, symptoms will be similar to those in the red oak group, but spread more slowly and appear localized on individual branches. Complete defoliation does not occur. Trees infected for two years or more develop isolated dead branches in the crown, creating a stag-head appearance. Similar dieback symptoms can result from other causes, such as two-lined chestnut borer, construction damage, soil compaction, changes in soil grade, cankers, and root rot.
Disease Cycle
Oak wilt survives as spores in fungal mats that form under the bark of an infected tree, in the fall or spring following the death of that tree. As a tree in the red oak group begins to die, the oak wilt fungus produces fungal mats between the sapwood and bark of the trunk or large branches of a tree. These fungal mats push apart weakened bark and produce a fruity odor and sticky sap that attracts sap-feeding insects, especially picnic beetles (Nitidulidae) and bark beetles (Scolytidae). When the beetles visit the mats, they pick up spores on their bodies. Overland spread of oak wilt occurs when these beetles move from infected trees to feed on healthy trees, carrying spores from the fungal mats to the fresh feeding wounds.
Oak wilt can also spread from infected trees to healthy trees through root grafts between nearby oaks. Root graft transmission is responsible for the vast majority of new oak wilt infections. Trees of the same species, and sometimes the same group, growing within 50 feet of one another, may graft together and share the same vascular system. This network of roots allows the disease to move freely from one tree to the next, usually within a one-to-six year period, causing a whole stand of trees to become infected.
Another way to prevent oak wilt spread is to stop the movement of infected logs and firewood. New pockets of the disease occur when beetles visit transported logs that contain fungal mats. Logs from the red oak group should not be transported unless all bark is removed. Trees in the white oak group do not form mats.
Management
Oak wilt is difficult to diagnose in the field and a lab diagnosis is needed. Samples should be sent to a diagnostic laboratory as soon as oak wilt is suspected. The University of Illinois Plant Clinic will confirm oak wilt for a nominal charge. Some arborist companies have their own labs or have access to other labs and can assist with obtaining a proper diagnosis.
Cultural Management
Avoid any unnecessary wounding of oaks. Oaks should NOT be pruned between mid-April and mid-October. Fresh wounds made during the growing season can attract the beetles that transport oak wilt spores. These beetles are most active during the growing season. If pruning is necessary during this time period, sterilize tools between each cut and paint the wound with nontoxic tree wound dressing. The safest time to prune oaks is from early November until very early April.
Dead or infected red oaks should be removed from the site before they produce fungal spore mats. Time is of the essence for success. Nearby trees of the same species may also need to be removed because of the disease moving through root grafts. Root grafts can be severed with mechanical trenches or chemical methods. Consultation with a certified arborist is recommended. Although mechanical barriers are generally more effective than chemical barriers in stopping the spread of oak wilt, chemical barriers are the best option when space is limited by the presence of sidewalks, driveways, utility lines, or other obstructions.
Chemical Management
Systemic fungicide treatments that can be injected into infected trees are available. These need to be applied by licensed arborists. These products are most effective when used preventatively on valuable trees that are not yet infected or are in the very early stages of infection.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).