The Plant Clinic has already confirmed a case of oak wilt for 1999, so keep an eye out 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. 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 of its leaves. We have seen red oaks move through initial symptoms to total defoliation in as little as three weeks.
The white and bur oak group generally shows symptoms on scattered branches of the crown. Leaves 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 trees are much more likely to survive for many years with a stag-headed 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 as well.
Other problems, 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, can mimic oak wilt. None, however, has the distinct vascular discoloration found with this disease. To detect this discoloration, peel the bark back with a knife. The sapwood of a healthy tree is white or tan. A suspected oak wilt tree 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. Therefore, the disease can go undetected if the tree is not properly sampled. There is a slight brown streak to healthy wood as air comes into contact with the sapwood, but 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; 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 seven days for the fungus to develop in the lab to the point that a positive confirmation can be made.
Occasionally, we hear from clients who thought oak wilt was present when the tree was sampled, but the clinic did not isolate the fungus. The fungus is slow to grow but it is not tricky to isolate in the lab. Assuming a proper sample was taken, the problem may be the sensitive nature of this fungus. Many states report that oak wilt fungus in sampled wood does not survive high mail-truck temperatures. Many southern states require oak wilt samples to be shipped on ice. If you have had this problem in the past, consider shipping samples with ice packs in the package and using overnight shipments. We will do our best at this end to get oak wilt suspects in culture as soon as possible.
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. Prune oaks only in the dormant season if at all possible. Refer to Report of Plant Disease No. 618 for more on oak wilt. This serious disease has been found in all parts of Illinois over the last decade. We have not, however, found that it is spreading more quickly than it has in the past, and we do not expect to see an epidemic of oak wilt.