For The Answer
Submitted by Calvin Finch, Ph.D., Manager, Conservation Division, Water Resources & Conservation Department, SAWS, and Horticulturist
Oak wilt has not received much attention lately. It is the disease that killed a few red oaks and thousands of live oaks in the Hill Country. Some trees were even killed in San Antonio.
The disease is still a threat to our area and it is time to be careful about fresh wounds on susceptible species. The fungal disease infects live oaks and red oaks like Schumardi, Spanish, Texas Red, and blackjack when sap beetles move from the fungal fruiting body on an infected tree to a fresh wound on the red or live oak. The easiest way to prevent the spread of the disease is to paint pruning and other wounds when they occur. Insist that companies cutting on oaks in your neighborhood paint the wounds. If you have oaks it is probably a good idea if you keep a spray can of pruning paint in your garage as well.
Once the disease infects a live oak, it will spread from tree to tree from the infection center at about 75 feet in radius every year through the root grafts. Red oaks do not root graft as readily, so individual trees are most likely to be affected. The only way to stop an infection center in live oak mottes is to cut the root connection by trenching—a very expensive, difficult job in a rural area. Imagine how difficult it would be to trench in an urban area! Prevention is easiest and best.
There is a fungicide, Alamo, that will protect most individual trees threatened by oak wilt. The fungicide needs to be injected into the tree. The operation may cost as much as $10 to $20/inch of tree diameter so is not something to do unless the tree is directly threatened by the disease. Before an Alamo treatment is considered, Mark Peterson, the regional forester with the Texas Forest Service, should be contacted at (210) 208-9306 to make a definitive diagnosis that the disease is a threat. The Alamo treatment protects single trees but does not stop the spread of the disease.
Recognize oak wilt in red oaks when an apparently healthy tree suddenly assumes autumn color and dies in early summer. The death occurs so fast that the leaves hang on the tree.
A live oak dies over several months. Major branches will die one at a time. The symptom is called flagging. An even more distinctive symptom is the leaf color. Leaves on live oaks infected by oak wilt show veinal necrosis. The veins will often be bright red or yellow while the area between the veins is green. The effect reminds me of the skeleton suits that children wear at Halloween; only the bones would be red and the rest of the suit green.
As bad as oak wilt sounds and can be, it is relatively easy to prevent. Its spread is not virulent and the wound painting protects most trees. If families and neighborhoods with oaks watch for symptoms, the disease can also be stopped early.
Other tactics to prevent the spread of oak wilt include only using dried firewood from the Hill Country and planting a diverse mix of shade trees. The white oaks (bur, lacey, Monterey, chinkapin) are not as susceptible to oak wilt. Cedar elm, Mexican sycamore, Montezuma cypress, Chinese pistache, and pecans are also not susceptible. Even with oak wilt live oaks and red oaks are some of our best shade trees, but it is good insurance to include other species in the mix. If your neighborhood is full of the susceptible oaks, plant the other species.
For more information on oak wilt visit the Texas Forest Service website at http://txforestservice.tamu.edu or plantanswers.com.