The Plant Clinic had its first case of oak wilt for the season this past week. The disease was confirmed by isolations of the causal fungus from an infected tree in Peoria County. We do not usually see the disease symptoms this early in the season, but disease problems are a couple of weeks ahead of the average this year. This fact should be a reminder to the rest of us not to prune oaks until at least the middle of summer, especially in areas known to have oak wilt. Pruning actively growing trees results in sap flow, attracting the beetles that may carry the fungal pathogen.
Oak wilt is caused by a fungus (Ceratocystis fagacearum) that enters the water-conducting vessels of the sapwood and causes them to become plugged. 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 drops all its leaves. In some years, we have seen red oaks progress from scorched foliage to total defoliation in as little as 3 weeks. This year’s confirmed case was on an oak in the red–black oak group.
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. An affected tree in this group may die in one season but is much more likely to survive for many years with a stagheaded appearance. Recent appearances of anthracnose on white oak have caused concern among many tree specialists who fear oak wilt. Anthracnose causes brown spotting scattered over the leaves and may cause slight leaf cupping.
Other problems can mimic oak wilt, including construction damage, soil compaction, changes in the soil grade or water table, lightning damage, nutritional disorders, insect and animal injuries, chemical damage, cankers, and root decay. None, however, has the distinct vascular discoloration found with oak wilt.
To detect the discoloration, peel the bark back with a knife. The sapwood of a healthy tree is white or tan. An oak wilt suspect shows brown and white streaking of the wood. Samples without streaking do not yield the oak wilt fungus even if the fungus is present elsewhere in the tree. Therefore, the disease can go undetected if not properly sampled. There is a slight brown streak to healthy wood as the air comes into contact with the sapwood. The distinct discoloration from oak wilt is visible as soon as the bark is peeled back and does not intensify as the wood dries. Sometimes the discoloration is visible just under the bark, and other times it is deeper in the wood and visible only when viewed from the end of a cut branch.
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 6 to 8 inches long, about thumb thickness, alive but showing symptoms, and must contain vascular discoloration. It takes about 7 days for the fungus to develop in the lab to the point that a positive confirmation can be made. Oak samples submitted for oak wilt testing should be sent on disposable ice packs to prevent killing the fungus with high temperatures before it can be isolated in the lab.
Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds and a beetle vector, and it can spread by root grafts between trees. You cannot save the infected tree, but you may be able to save surrounding trees; so a positive diagnosis is important in many cases. Pruning of oaks should be done only in the dormant season if at all possible. Refer to Report on Plant Disease (RPD) no. 618 for more on oak wilt. You can obtain this report on the Web (http://www.ag.uiuc.edu/~vista/horticul.htm) or from your local Extension office.