The fungus, Ceratocystis 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.