Oak wilt is a lethal systemic disease caused by a fungal pathogen. This disease is found throughout the eastern and central U.S. as far as Texas. It is responsible for killing many landscape and forest trees. In the 1940’s it was first identified in Wisconsin. Since then, the number of incidences has fluctuated but it is still a slow and sporadic threat to local populations of oak trees. This disease is found more commonly in the eastern third of the state.
How does this disease kill a tree?
The fungus Ceratocystis fagacearum, grows in the water conducting tissues of the tree in the xylem. The tree responds to the infection as well by plugging up those xylem vessels to stop the fungal spread. This prevents the upward movement of water causing the tree to wilt and die. Both the fungus and the tree’s natural defenses cause eventual mortality.
What oaks are hosts for the disease?
Oak wilt can attack all species of oak, but the red oak group is most susceptible. This group includes northern red, shingle, shumard, blackjack, and pin oak. The white oak group is more resistant to the oak wilt fungus bur, chinquapin, post, and white oaks, may survive several years of infection. In Kansas, oaks are usually found woodlots or in urban subdivisions that were built in areas where native oak stands were present.
Signs & Symptoms
During the late spring to early summer (mid-May to early June), symptoms become apparent. Infected trees show a bronzing or wilting of leaves on individual branches or on a portion of the tree crown. Usually the top of the tree will be affected, followed by the lateral branches. The leaves appear scorched turning a bronzed brown color at the tips and outer margins while the inner portion of the leaf, nearest the leaf stem, remains green. You can see a distinct line between green healthy tissue and discolored.
In a few weeks, wilted leaves start to drop. The wilting will spread to other portions of the tree during the summer.
Most red oaks are killed in one season. Symptoms are more variable on white oaks, often beginning later and progressing slower. In addition to the leaves, sometimes the outermost ring of sapwood can turn black and appear as streaks when the bark is peeled on a branch.
Disease cycle and spread
Oak wilt can spread two ways, connecting root grafts of nearby oaks (these can naturally form with oaks of the same species up to 50 feet apart) and insects carrying the spores. Root systems of dead red and black oaks can transmit the fungus through root graphs to healthy trees up to three years. This usually forms patches of infected trees with dead ones in the center.
As a tree in the red oak group dies, the fungus will grow opening long cracks in the bark. Under the bark will be spore-bearing fungal mats. This growth occurs during spring and fall, when rain is more characteristic. These fungal mats produce a fruity smell to sap-feeding insects and can attract beetles, which can then carry the fungus to another host tree.
Preventative Management of Spread
Since the fungus can pass from one tree to another by root grafts, oak wilt can quickly spread through a stand. To avoid transmission of the disease, control methods must concentrate on inhibiting the root-to-root contact. This requires trenching to a depth of 3 feet between adjacent healthy and diseased trees. In addition, because oak wilt may be spread by insects, birds and small mammals, 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. Recommendations are similar to Dutch Elm Disease, in that diseased trees should be removed and destroyed. The only way you can possibly use the wood for firewood is to split and stack it – loosening the bark, cover the whole pile with 4-mil clear plastic tarp that has been sealed to the ground. This means there should be a large amount of excess to cover the edges with soil to keep it from lifting. 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 local district forester or county extension agent.
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.
This disease can spread rapidly among oaks in the red oak group and move long distances if recently dead trees are used for firewood or lumber. The disease is regulated in some states and in international markets.