This disease might be confused with sudden oak death, but in name only. Sudden oak death was described in issue no. 1 of this newsletter. It may kill oaks; and infected trees have an oozing canker on the trunk. Sudden oak death has not yet been found in Illinois, whereas oak wilt can appear in any part of the state. In fact, you need to start looking for symptoms now. Infected trees may die in one season, do not have oozing cankers on the trunk, but show scorching of the leaves, which quickly moves down the tree. The vascular tissue of infected wood is stained a dark brown that gives the wood a streaked appearance when bark is peeled off or appears as a brown ring in branches when cut and examined end-on.
Red and black oaks die quickly, usually in one season. Oaks in the white oak group may decline over many years. No oak species is immune. Remember, quick action may save nearby oaks. Look for vascular discoloration to help diagnose the presence of oak wilt. If you think your tree is infected with oak wilt, the Plant Clinic can prepare cultures from the wood and detect the fungus when it is present. Samples should be 8 to 10 in. long, about thumb thickness, alive but showing symptoms, and must contain vascular discoloration. It takes about 7 to 10 days for the fungus to develop in the lab to the point where a positive confirmation can be made. The processing time cannot be shortened. Oak samples submitted for oak wilt testing should be sent on disposable ice packs to prevent killing the fungus (in mail trucks) with high temperatures before it can be isolated in the lab.
If you have oak wilt in your area, do not prune oaks now. Pruning when trees are actively growing results in sap flow, attracting the beetles that may carry the fungal pathogen to your tree. Both the beetles and the fungus are now active. If oak wilt is present in your area, try to leave pruning of oaks until at least after midsummer. The dormant season would be an even better time for this task.
Oak wilt symptoms vary depending on the oak species involved. Generally, oaks in the red-black group develop discolored and wilted leaves at the top of the tree or at the tips of the lateral branches in late spring and early summer (now). The leaves curl slightly and turn a dull pale green, bronze, or tan, starting at the margins. Usually by late summer, an infected tree has dropped all its leaves. In some years, we have seen red oaks progress from scorched foliage to total defoliation in as little as 3 weeks.
The white and bur oak group generally shows symptoms on scattered branches of the crown. The disease is often confused with general dieback and decline. Leaves on infected white oaks become light brown or straw-colored from the leaf tip toward the base. The leaves curl and remain attached to the branches. This tree group may die in one season but is much more likely to survive for many years with a stagheaded appearance. Anthracnose may produce some look-alike symptoms. Anthracnose causes brown spotting scattered over the leaves and may cause slight leaf cupping as well. A tree infected with only anthracnose will produce healthy new leaves as temperatures turn warm. If oak wilt infects a section of a tree, the new leaves will not appear healthy.
Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds by a beetle vector, and it can spread by root grafts between trees. The infected tree cannot be saved, but you may be able to save surrounding trees, so a positive diagnosis is important in many cases. Refer to Report on Plant Disease, no. 618, for more on oak wilt. You can obtain this report on the Web (http://www.ag.uiuc.edu/%7Evista/horticul.htm) or at local Extension offices.