In a red oaks growing in northern states, the symptoms usually occur 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 pathogen 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. The fungus causes the tree to produce chemicals in its effort to stop the spread of the fungus; which causes the brown vascular streaking.
White oak group trees are more tolerant to oak wilt. They may become infected but not die from the infection for many years. Left standing, they are a source for infection for any other oaks via insects and wildlife (ie squirrels) and via root grafts to nearby white oak group trees.
When two or more related oaks (in the same group - red or white) 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. Two to three months after the red oaks begin wilting, the fungus can be found in all or almost all parts of the tree. Generally after the leaves die due to infection, so do the smaller twigs. The fungus can not survive in these dead twigs. However the fungus does survive in the trunk and large branches of red oaks in the northern states for up to a year after the tree dies. The fungus may survive in the roots for three or four years.
Several months after defoliation, the fungus may produce pressure pads/mycelium mats under the bark of red oak group trees. The mats tend to attract sap-feeding beetles and wildlife that can help spread the disease. The squirrel is the main animal that may spread this disease between oaks in many areas. Sap-feeding beetles are also attracted to the mycelial mats and can carry spores to other oak trees. The insects feed on sap exuding from fresh wounds and introduce the spores to the wounded tissue. The fungus enters the water-conducting tissue of the tree (the xylem) through these wounds and grows in the sapwood. Plugging of the sapwood results in restriction of water movement. Thus, the tree dies from lack of water.
It is important that the root grafts be cut before the disease can spread from one tree to adjacent trees. Root grafts can be destroyed by mechanical means. Mechanical cutting is the trenching through the roots to a depth of three to four feet. JULIE (800-892-0123) 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. When destroying root grafts, consider two rings rather than one. The outer (made first) ring is about two thirds the height out from the infected tree. The inner ring is then made about one third of the height out from the infected tree. Related oaks inside the inner ring have a good chance of dying from roots that are still grafted to the infected tree. Trees between the outer and inner ring have a much better chance of surviving. Trees outside both rings should not be infected via root grafts if the root grafts dying and adjacent oaks were destroyed in time.
To reduce the spread of oak wilt via animals, which are often protected, the best control is to remove the infected tree or limb as soon as possible. After removing oak wilt infected trees, 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 the solution should sit on the tools for about a minute. To get very close to the desired 10 percent solution, use three parts water to one part bleach;when using this dilution the tools quickly become sterile.
Certain injectable fungicides can prevent above-ground infection on oaks. However, fungicides will not protect against infections via root-grafts.