Oak Wilt is a fungus. The pathogen is Ceratocystis fagacearum. The fungus attacks most oaks and has also been found in Chinese chestnuts. The trees in the red oak group are very susceptible and tend to die in one year. Occasionally a few trees in the red oak group may live into their second year before they die. Oaks in the white oak group are more resistant and the disease is often localized and therefore may last for many years. Oak wilt is spread via root grafts, animals, tools and insects.
The progression of the disease in a red oak is as follows. After infection, the tree usually starts to die from the top down. Leaves turn a dull green and wilt followed by bronzing or tanning along the edges towards the main vein. The leaf may droop, roll lengthwise and wilt. Or the leaf dies from the tip of the leaf down towards the petiole. The death from the tip down is almost a straight line across the leaf between green and brown. Other leaf symptoms sometimes occur. As the disease travels down the tree, upper leaves turn yellow and fall off. Occasionally a few green leaves will fall off too.
Brown streaks develop in the sapwood. The streaking is sometimes difficult to separate from the normal browning found in some oak vascular tissue. The streaking caused by the oak wilt fungus results in the vascular tissue being plugged by various chemicals produced by the tree.
When two or more oaks of related species are growing close to each other, there is the possibility that roots from both trees will cross and grow together (root grafting). If one of the trees becomes infected with oak wilt, the disease can spread from one tree to another through these root grafts, thus causing the death of many more trees. In situations like this, it is important that the root grafts be cut before the disease can spread from one to the other. Root grafts can be destroyed via chemicals or by mechanical means. Chemical control takes longer to kill the roots and may let the disease cross the root graft before the roots die.
Mechanical cutting of the roots is much quicker. Mechanical cutting is the trenching through the roots to a depth of three feet or more. JULIE should be called before this is done to avoid cutting telephone, electric, cable, gas, or any other lines. The breaking of root grafts is usually done about half way between the close growing oaks. Oaks are close growing if another oak is closer than the height of the largest oak. Whether chemical or mechanical methods are used to destroy root grafts, consider two rings rather than the normal one ring. The first ring is about two thirds the height out from the infected tree. The second ring is then made about one third of the height out from the infected tree. Other trees inside the second ring have a good chance of dying from roots that are still grafted. Trees between the first and second ring have a much better chance. Trees outside both rings should not be infected via root grafts if the root grafts were destroyed in time.
After removing oak wilt infected trees and pruning another oak, pruning tools should be sterilized properly to reduce the risk of infecting other oaks. Tools may be sterilized by using rubbing alcohol and letting it sit on the tools for about a minute or flaming the alcohol off. Another way is to use chlorine bleach. If a name brand bleach that contains about 37 percent chlorine is used and if nine parts water to one part bleach is the dilution, then tools should again sit with the solution on them for about a minute. To get very close to the desired 10 percent solution, use three parts water to one part bleach. Now the tools hardly need any time to become sterile.
Animals are another way to spread the oak wilt disease from one tree to another. There are many animals that may do this but in the Chicago area, the squirrel is the main animal that may spread this disease between oaks. Since the animals are protected, the best control to reduce the spread of this disease by animals is immediate removal of the infected tree.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.