What is Oak Wilt?
Oak wilt is an infectious vascular disease caused by the fungus Bretziella fagacearum responsible for the deaths of hundreds of thousands of oaks across Texas. An infection starts when sap-feeding beetles pick up spores from infected red oaks and carry them to fresh wounds on other oaks. Once an oak tree becomes infected, this disease spreads on average about 75 feet per year to adjacent oaks through root connections. All oaks can be infected with oak wilt, but some species of oak are more susceptible than others. Red oaks, particularly Spanish oak (Quercus buckleyi), are very susceptible to the fungus and once infected, often die within 4-6 weeks. White oaks, like post oak (Q. stellata) and bur oak (Q. macrocarpa), are resistant to the fungus and rarely die from the disease. Live oaks (Q. virginiana and Q. fusiformis) are intermediate in susceptibility to oak wilt.
Oak Wilt Symptoms
The most common symptom on diseased live oaks is called veinal necrosis. Leaves often develop yellow veins that eventually turn brown and fall to the ground. Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree for months after defoliation. Foliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leaves simply wilt, turning pale green and brown, usually remaining attached for a period of time. Mature leaves develop dark green water soaking symptoms or turn pale green or bronze, starting at the leaf margins and progressing inward. This can begin on one branch and quickly engulf the entire tree.
Preventing Infection and Spread
There are multiple strategies that can be implemented to lessen the likelihood of your trees being infected, as well as stopping the spread of disease once you are seeing symptoms in Oak Trees. First, do not trim your oak trees during cool or mildly warm weather. The best periods for pruning are during the coldest days in winter and extended hot periods in mid to late summer, when the beetle that carries the fungus will be least prevalent. It is also best practice to paint any wound or cut made on an oak with tree sealant or paint. This will prevent the fungal mat from forming. Once infected, a common strategy is to stop the disease's spread through the roots. Measures can be taken to break root connections between live oaks or dense groups of red oaks to reduce or stop root transmission of the oak wilt fungus. The most common technique is to sever roots by trenching at least 4 feet deep with trenching machines, or rock saws. The trench should be placed a minimum of 100 feet beyond these symptomatic trees, even though there may be “healthy” trees at high risk of infection inside the trench. Trees that are infected can also sometimes be treated and saved by receiving fungicide injections from a licensed professional. Fungicide injection does not stop root transmission of the fungus; therefore, this treatment is used best in conjunction with trenching or to protect individual, high-value trees in situations where trenching is impractical.
Actions Taken at Blue Hole
Even after reading all of this information, identifying oak wilt in your trees is usually still very difficult to an untrained eye. Contacting an expert with the Texas A&M Forest Service (TAFS) to come out and evaluate your trees is a free and very valuable resource if you are seeing decline in your oak trees. The City of Wimberley has worked with them through the whole process of identifying the disease to coming up with a strategy to manage the issue at hand, and we are very grateful for the work they do. After walking our property and confirming the disease was present, TAFS drew a map of the infected area with an outline for where a roughly 1,500 ft trench would need to be dug. If successful, the trench will keep the disease on a small southern portion of the Blue Hole property, and prevent it from moving further in to the rest of the park. There are companies all over central Texas that specialize in trenching and tree injections. A link for those types of services can be found here: Oak Wilt Services