OAK WILT: FACT AND FICTION

By Shane Kiefer, Senior Wildlife Biologist, San Antonio Region

Oak wilt is a very real and very important concern among Texas landowners. Oak wilt, caused by the fungus Ceratocystis fagacearum, is one of the most destructive tree diseases in the United States. The bad news is that it occurs in over 60 Texas counties and will never completely disappear. The good news is that with some basic knowledge about how and why oak wilt spreads, you can take steps to help prevent infections on your land.

The oak wilt fungus kills by clogging the water conducting system of the tree. All oaks are susceptible to oak wilt, but white oaks, such as post, bur, and chinkapin oaks are very resistant to the disease. Red oaks such as Texas, pin, and blackjack oaks are extremely susceptible and often die within a few weeks of apparent symptoms. Red oaks act as breeding grounds for the fungus and serve as sources of new infections. Under the right conditions, fungal mats may form under the bark of a red oak. These mats often create cracks in the bark and emit a fruity odor. Tiny, sap-feeding Nitidulid beetles are attracted to these mats and feed on them, gathering fungal spores as they do. When the beetles find a fresh wound on another oak they feed on the sap and transmit the spores, infecting the tree. This is how new infection centers are created in live oaks.

Live oaks are probably the most obvious victims of the disease because they die in the largest numbers. This is due to the common growth habit of live oaks in which they form motts with interconnected root systems that allow the rapid spread of the fungus from tree to tree once an infection occurs. Infected trees often die within 6-12 months.

Red oaks often do not exhibit any clear symptoms. Typically, the leaves will pale and brown during an unusual time of year. The tree dies shortly thereafter. The presence of fungal mats is a reliable indicator of oak wilt in red oaks. In live oaks, veinal necrosis and veinal banding are good diagnostic indicators of oak wilt. The area around the leaf veins will darken and then turn brown while the rest of the leaf remains green. Diagnosis can be confirmed through isolation of the fungus in a laboratory.

Since the disease only spreads in two ways and new infections are only established by beetles feeding on wounded oak trees, by taking precautions you can help prevent the spread of oak wilt. Fungal mats tend to form during mild, spring-like weather, when beetles are most abundant. You should avoid pruning oaks during these times to reduce the chance of infection. Winter and summer are good times to prune your trees, while spring is the worst time for these activities, even though it may be the nicest time to work outside. No matter when you prune, you should always immediately paint your cuts or any wounds on your oak trees with pruning paint or standard latex paint. Contaminated beetles may be present at any time and painting will prevent them from feeding on the sap from fresh wounds. Tree health, age, and size have no impact on susceptibility to oak wilt so you should always paint cuts on oaks, no matter how healthy they are.

Good land stewardship is the best defense against long-term damage from diseases and the best way to ensure its beauty and function in the future despite any type of disturbance. Oak wilt is not an imported disease. It is a native fungus that has been in North America and Texas for a long time. It actually serves a purpose, much like other seemingly destructive agents such as fire. Pockets of dead trees provide foraging habitat for woodpeckers and nesting cavities for songbirds. A healthy and diverse shrub layer and a well distributed age structure in the woodlands meant that there was always something to replace oaks that succumbed to oak wilt or other diseases. Imagine the damage that could be done if the only woody plants on your property were live oaks. If you walk through the woods and see no young oaks, hackberry, cherry, or walnut, imagine how long it will take to recover if oak wilt or another natural destructive agent strikes. Controlling deer numbers to prevent damage to young oaks and other species, brush management, and encouraging diversity in your woody plants by planting native shrubs and trees other than oaks can help your land support a more diverse and resistant habitat.

Talk to your Plateau biologist about how to improve the health of your woodlands to ensure that even if disaster strikes, your land will recover and continue to provide quality wildlife habitat and enjoyment for you and your children.

Information in this article was derived from various publications of the Texas Forest Service. For more information, including pictures and diagrams see texasoakwilt.org.