Q. My neighbor has lost several red oaks to oak wilt. I have three red oaks that really make my backyard. I would hate to lose them. What should I do to protect my seemingly healthy trees?
A. It is wise to contract with a certified arborist for an accurate diagnosis of oak wilt and the safe removal of dead trees. An arborist can also recommend fungicide injections that can protect healthy trees growing near infected ones. Fungicides such as Alamo (propiconazole) injected every two years have been shown to protect uninfected trees growing near those killed by oak wilt. Even trees that have begun to show symptoms can be successfully treated with Alamo if less than 30 percent of the crown is affected. Since Alamo does not kill the fungus present in an infected tree's roots, such trees should be treated every year. These products are not available to home gardeners. Check the Yellow Pages to find a certified arborist in your area.
Trees of related species growing in proximity to one another often have their roots grow together as though they had been grafted. Vascular diseases such as oak wilt are easily transmitted from infected trees to healthy ones through shared tissue in these grafts. This is the most common method of transmission in such a localized outbreak. If your neighbor's trees are within 50 feet of your trees, it is likely their roots have grafted together.
Those root grafts must be severed BEFORE your neighbor's trees are removed. This can be done by digging a three-foot deep trench between infected and healthy trees, or by killing the grafts chemically with a soil fumigant. If infected trees are removed before the root grafts are broken, fungus-laden sap from the infected trees can "backwash" into your trees' vascular systems.
Infected trees should be removed as soon as possible once the root grafts have been broken. There is no chemical control for oak wilt once symptoms are apparent in more than 30 percent of the crown. Prompt removal of infected oaks is important to protect those trees not yet infected.
Destroy the wood immediately, including the stump, by burning, burying or removing the bark so that it is not attractive to the insects responsible for the spread of oak wilt. Do not stack the wood for firewood, or transport logs with intact bark, since insects in the infected wood can leave and carry the fungal spores to healthy trees.
Perhaps the most important way to prevent healthy trees from being infected with oak wilt is to prune oaks only when they are dormant, from late November to mid-April. If oaks are damaged in a summer storm, some pathologists recommend cleaning up the damage and painting the pruning cuts with wound dressing to avoid attracting the bark and sap beetles that spread oak wilt fungi. (Pruning paint is rarely recommended because it can interfere with the tree's natural ability to heal pruning cuts).
Oak wilt occurs in the United States in an area delineated by Minnesota, Texas, Pennsylvania, and South Carolina. It is found west of the Susquehanna River in Pennsylvania. Although all oaks can be infected, those in the red oak group are most susceptible. These include northern red oak (Quercus rubra), pin oak (Quercus palustris), black oak (Quercus velutina) and scarlet oak (Quercus coccinea).
Chestnut trees are also susceptible, including American (Castanea dentata), Chinese (Castanea mollisima), European (Castanea sativa) and bush chinquapin (Castanea pumila). Susceptible trees can die within a few weeks of infection.
Species in the white oak group are less susceptible. These include white oak (Quercus alba), swamp white oak (Quercus bicolor), bur oak (Quercus macrocarpa), and post oak (Quercus stellata). When trees in this group are infected, they may decline over a two to three year period, or the disease may go into remission and they recover.
Oaks in the red oak group are most susceptible to Oak Wilt. These include northern red oak, pin oak, black oak and scarlet oak.
Oak wilt symptoms vary according to oak species and geographical area. Generally for red oak group trees in our area, symptoms start near the top of the tree and progress downward. Leaves on infected trees turn dull green, then bronze, and browning is frequently evident at the leaf tips or margins. Sometimes the leaves droop and curl lengthwise. Browning may also occur along the veins.
Leaves at the ends of branches begin to fall soon after symptoms become noticeable, and often drop while they are still green. Twigs and branches die, and you may be able to see brown streaks in the sapwood of infected trees, but this symptom is not always apparent. Leaf discoloration and defoliation continue throughout the crown of the tree for several weeks until the tree is dead.
Symptoms in the white oak group are similar, but advance much more slowly, and do not cause the sudden defoliation and death seen in the red oak group. The symptoms are often confined to a single branch, and can look like typical fall coloration.
Oak wilt is caused by the fungus Ceratocystis fagacearum. The disease kills infected trees by clogging their vascular systems until they are unable to transport water and nutrients throughout the tree. It is transmitted from infected trees to healthy ones by oak bark beetles and sap beetles.
Oak bark beetles lay their eggs in infected trees. The adults emerge from egg laying covered with spores of the fungus and transmit the disease to healthy trees when they feed.
Sap beetles are attracted to the fungal mats produced by the disease because of their fruity odor. They also become covered with disease-causing spores and transmit oak wilt to healthy trees by feeding. They are particularly attracted to fresh pruning wounds.