Oak Wilt

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Oak wilt, caused by the fungus Bretziella fagacearum, has become a serious disease threat to oaks in the eastern and central United States. While the spread of oak wilt has not been rapid, it can be found in all counties of Illinois and 18 other states. 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.

All oaks are susceptible to oak wilt. However, the red oak subgenus (red, black, Hill’s, pin, and scarlet) is more susceptible to oak wilt than the white oak subgenus (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.

The fungus, Bretziella fagacearum, 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 wilt and, frequently, tree death. Oak wilt can spread from infected trees to healthy trees through root grafts between nearby oaks and by insects that carry spores of the fungus from one tree to another.

Root grafts. Root graft transmission is responsible for the vast majority of new oak wilt infections. Trees of the same species, and sometimes same subgenus, 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. Insects. As a tree in the red oak subgenus begins to die, the oak wilt fungus produces green-gray patches (called fungal mats) between the sapwood and bark of the trunk or large branches of a tree. This occurs during the spring following tree death. 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).

Overland spread of oak wilt occurs when beetles move from infected trees, carrying spores from the fungal mats to fresh wounds on healthy trees.

Red oak subgenus. In red oaks, the spread of the disease can be rapid with wilt 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. Few other diseases cause premature leaf drop. Other symptoms include curling leaves that become stiff, a yellow or bronze leaf color, and profuse suckering at the base of the tree.

When an infected branch is cut in cross-section, brown streaking may be present in the outer ring of sapwood.

White oak subgenus. Symptoms of oak wilt on this subgenus are similar to those on red oaks, 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. Remember that 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.

Because oak wilt symptoms can be confused with other stresses or diseases, a positive identification requires laboratory testing. For the best chance of success, samples should be collected from live wood and show streaking. The fungus is heat sensitive, so samples should be kept cool and submitted as soon as possible. The University of Illinois Plant Clinic is a diagnostic laboratory that will confirm oak wilt for a fee. Samples submitted for diagnostic testing should be taken from recently wilted branches. Wrap several branch samples, about one-half-to-one-inch in diameter and at least six-to-ten-inches long, in a plastic bag to prevent samples from drying out. Phone: 217-333-0519.

Depending on the situation, various management options are available.

Minimize Pruning. Avoid any unnecessary wounding of oaks. Oaks should NOT be pruned between April 15 and October 15. Open wounds attract beetles that are most active during this period, increasing the likelihood of infection. 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 after the first hard frost, or November 1 until April 1. Remove Infected Trees. Time is of the essence for success. Dead or infected red oaks should be removed from the site before they produce fungal spore mats the following spring. Nearby trees of the same species may also need to be removed because of the disease moving through root grafts. Consult a certified arborist about trees to be removed.

Sever Root Grafts. Root grafts can be severed with mechanical trenches or chemical methods. Again, consultation with a certified arborist is recommended. Mechanical trenching equipment can be used to cut through the roots around an infected tree, creating a barrier 31/2 to 4 feet deep. Do not trench too close to the trunk of a tree. Several barriers will provide the best protection.

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 for trenching by the presence of sidewalks, driveways, utility lines, or other obstructions. Most chemicals recommended for treating oak wilt have a label restriction that requires a licensed pesticide applicator to purchase and apply the chemical.

Remove Infected Wood. 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 and can be used safely as firewood. Fungicide injections labeled for oak wilt management on both red and white oak groups under certain conditions must be applied by a professional. Consult with a licensed, knowledgeable Certified Arborist in your area. This option is practical only for high value trees.