Q. A disease called oak wilt has been identified in my neighborhood. I have three red oak trees that make my backyard a shady haven and I would hate to lose them. How can I protect my trees?
Oak wilt occurs in the United States from Minnesota to Texas and Kansas to Pennsylvania, where it is found east of the Susquehanna River.
Although all oaks can be infected, those in the red oak group are most susceptible. These include northern red oak (Quercus rubra), pin oak (Q. palustris), black oak (Q. velutina) and scarlet oak (Q. coccinea).
Chestnut trees are also susceptible, including American (Castanea dentata), Chinese (C. mollisima), European (C. sativa) and bush chinquapin (C. pumila).
Susceptible trees can die within a few weeks of infection.
Species in the white oak group are less susceptible, including white oak (Q. alba), bur oak (Q. macrocarpa), and swamp white oak (Q. bicolor). When trees in this group are infected, they may decline for two or three years, rather than dying quickly.
On occasion, the symptoms go into remission and the tree survives. It is possible that the same characteristic that makes white oak useful for whiskey barrels and wine casks helps protect them from oak wilt. When trees in the white oak group are wounded or infected, small plugs called tyloses block off the sapwood and wall off the damage. These tyloses make the wood impervious to water and appear to inhibit the fungus from moving throughout the tree.
Oak wilt symptoms vary according to species and region. 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 or tan. This 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 branch ends 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.
Leaf discoloration and defoliation continue throughout the crown of the tree for several weeks until the tree is dead. It is not always possible to isolate the oak wilt fungus from samples in the laboratory; trees that are already dead yield no results at all, so oak wilt cannot be determined after the fact. Diagnosis is primarily based on symptoms.
Symptoms in the white oak group are similar but advance much more slowly, and they 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. Two species of beetles -- oak bark and sap -- are responsible for long-distance transmission of the disease.
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, which also transmit oak wilt by feeding, are attracted to the fungal mats produced by the disease because of their fruity odor. They are particularly attracted to fresh pruning wounds. Prune oak trees during winter dormancy (November through mid-April) to minimize infection via this route. Oaks are most susceptible in spring and early summer as new growth is occurring.
Another way that oak wilt is spread is through naturally occurring root grafts. This is the most common method of transmission in a localized outbreak. Trees of related species growing in proximity to one another -- within about 50 feet -- often have their roots grow together as if they had been grafted. Vascular diseases such as oak wilt are transmitted from infected trees to healthy ones this way.
These root grafts must be broken to protect uninfected trees. This can be done by digging a trench between infected and healthy trees or by killing the grafts chemically. Root grafts should be broken before removing an infected tree. If infected trees are removed prior to breaking the grafts, infected sap can "backwash" into the healthy tree. After breaking the root grafts, inject healthy trees with propiconazole (Alamo).
Infected trees should be removed as soon as possible. There is no chemical control for oak wilt once symptoms are apparent in more than 30 percent of the crown. Prompt removal is important to protect those trees not yet infected. Destroy the wood immediately, including the stump, by burning, burying or debarking so that it is not attractive to insects. Do not stack the wood for firewood, or transport logs with intact bark because insects in the infected wood can leave and carry the fungal spores to healthy trees.
It is wise to contract with a certified arborist for an accurate diagnosis of oak wilt and the safe removal of dead trees. If less than 30 percent of the crown is affected, injections of Alamo (propiconazole) can put the disease into temporary remission.
Any root grafts between the infected tree and neighboring oaks should be broken because Alamo does not kill the fungus present in the tree's roots. An arborist can also advise you about protecting uninfected oaks with injections of Alamo every other year. To find a certified arborist near you, go to the International Society of Arboriculture's website, www.isaarbor.com/faca/findArborist.aspx.
Send questions to Sandy Feather by e-mail at email@example.com or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington Ave., Pittsburgh 15208.