Oak Wilt

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Oak wilt can attack all species of oak, but the red oak group is most susceptible. This group includes red oak, blackjack oak, and pin oak. The white oak group is more resistant to the oak wilt fungus, which is usually found in oak woodlots or in urban subdivisions that were built in areas where native oak stands were present.

From mid-May to early June, oak wilt symptoms become apparent. Infected trees show a bronzing or wilting of leaves on individual branches or on a portion of the tree crown. The leaves appear scorched on the outer half of the leaf, which then turns red or brown, and the inner half of the leaf, nearest the leaf stalk, remains green. In a few weeks, wilted leaves start to drop. Wilting spreads to other portions of the tree during the summer. Most red oaks are killed in one season.

If you suspect oak wilt, collect a sample of a wilting branch that is at least one-half inch in diameter and six inches long. Make sure the sample is not exposed to high temperatures, as the fungus is very sensitive to heat. Take the sample to your County Extension Office. Because oak wilt can be confused with other problems on oak, a sample is needed to positively identify the presence of the wilt fungus.

Since the fungus can pass from one tree to another by root grafts, and since oak trees within 50 feet of each other often have a common root system, oak wilt can quickly spread through a stand of oaks. Thus, to avoid transmission of the disease, control methods must concentrate on inhibiting the root-to-root contact. Trench to a depth of 3 feet between adjacent healthy and diseased trees. Also, oak wilt may be spread by insects, birds and small mammals, so avoid pruning in late spring, as the sap seeping from wounds might attract insects that could spread the disease.

A tree infected with oak wilt cannot be saved. Remove it and destroy the wood. The only way you can possibly use the wood for firewood is to split and stack it, cover the whole pile with 4-mil clear plastic tarp that has been sealed to the ground. There should be no tears or holes in the plastic, and the pile should be tarped all summer. Since the fungus is sensitive to high temperatures, this treatment will heat the wood sufficiently to destroy the oak wilt fungus.

Fungicide injections should only be considered for healthy oaks adjacent to oak wilt pockets. For more information about oak wilt, call your County Extension Office.

Dennis Patton, DPatton@oznet.ksu.edu
County Extension Agent, Horticulture
Johnson County, Kansas
Kansas State University Research and Extension