The Plant Clinic has had a confirmed case (i.e., confirmed by isolating the causal fungus) of oak wilt already this year, so keep an eye out now for this oak disease. It is caused by a fungus (Ceratocystis fagacearum) that enters the water-conducting vessels of the sapwood and causes them to become plugged, much like Verticillium does with Verticillium wilt. Symptoms of oak wilt vary, depending on the oak species involved. Generally, oaks in the red and 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. We've seen red oaks move through the phases of initial symptoms to total defoliation in as little as three weeks in central Illinois.
The white and bur oak group generally shows symptoms in a more subtle fashion, and initial diagnosis is often blamed on other factors. In this group, symptoms occur on scattered branches of the crown. Their leaves become light brown or straw-colored from the leaf tip toward the base. The leaves curl and remain attached to the branches. The tree may die in one season but is much more likely to survive for many years with a stagheaded appearance. Anthracnose on oak has been fairly common and severe this year, as it was in 1997, and often causes concern among homeowners who fear oak wilt. Anthracnose will cause brown spotting on leaves and may cause slight leaf cupping as well.
Other problems that mimic oak wilt include 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 of these causes show the distinct vascular discoloration found with oak wilt, although cankers can cause a localized browning that could lead to misdiagnosis. To detect the discoloration caused by oak wilt, peel the bark back with a knife. The sapwood of a healthy tree is white or tan. A tree suspected of having oak wilt will show 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, which means the disease can go undetected if the tree is not properly sampled. On healthy wood, a slight brown streak appears when the air comes into contact with the sapwood. The distinct discoloration from oak wilt is apparent as soon as the bark is peeled back and does not intensify as the wood dries.
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 eight to ten inches long, about thumb thickness, alive but showing symptoms, and must contain vascular discoloration. Samples that do not meet these criteria will not be cultured. It takes about seven days for the fungus to develop in the lab to the point at which a positive confirmation can be made.
Oak wilt is particularly threatening because there is no complete control or cure once infection occurs. The fungus infects through fresh wounds and 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.
Refer to Report on Plant Diseases No. 618 for more on oak wilt. This devastating disease has been found in all parts of Illinois over the last decade. We have not, however, found that it is spreading any more quickly than it has in the past, and we do not expect an epidemic.