Spring is a season of peril for Texas' oak trees. That's when the tiny beetles that transmit the dreaded oak wilt disease are the most active and when the trees are at their most vulnerable. It's also a time when people need to refrain from pruning or otherwise wounding oaks because doing so creates an opening for the beetles to inflict their damage.
Oak wilt affects trees in 23 states, but Texas has been the hardest hit. The disease is present in more than 70 counties, most in Central Texas but a few in West Texas. Nationally the disease covers an area larger than Vermont, New Hampshire, Massachusetts, Rhode Island and Maryland combined. It can, and does, take out whole swatches of trees, leaving bare spots on the landscape or in neighborhoods. Meanwhile, the disease continues to spread to other trees, carried hither and yon by the tiny beetles.
Though oak wilt has become epidemic in Texas in recent years, it has probably been around for the better part of a century, maybe longer. Dave Appel, an oak wilt specialist with Texas AgriLife Extension Service, said he suspects that oak wilt has been in the Hill Country for a long time, citing descriptions by a Texas A&M University scientist of widespread oak mortality in the Austin area in the 1930s that sounds a lot like the work of oak wilt.
“The ecology of the Hill Country and the widespread predominance of live oak create the conditions for oak wilt to eventually 'explode,' much like wildfires can be exacerbated by certain conditions in a forest,” he said, adding that other conditions have contributed to the Hill Country epidemic
Oak wilt kills the tree by disabling its water conducting system. All oaks are susceptible to some degree, but red oaks, which include Texas oaks (sometimes called Spanish oak), Shumard oak, black jack oak and water oak are particularly susceptible. White oaks and live oaks are not as vulnerable, but live oaks are the most seriously affected by the disease because of their tendency to grow root sprouts and form vast, interconnected systems that allow a fungus to spread between adjacent trees.
Some infected red oaks produce fungal mats that attract the beetles. The beetles are attracted to the sweet, sticky texture of the mats, get the fungus all over them and transmit it to the tree through openings or wounds. With live oaks, because the disease is spread through the root system, whole sections of trees are affected, which accounts for the bare spots dotting Texas landscapes where live oaks and oak wilt are prevalent. Studies at Texas A&M show that the disease spreads an average of 75 feet per year.
Appel can't say that oak wilt is getting worse, nor can he make a case for a decline in the disease.
“We certainly have the tools necessary to control the disease,” he said. “Oak wilt management comes down to a matter of available resources. If the money is there, we can control the disease. But property values, site conditions, tolerance for environmental disruption and other circumstances dictate the outcome of any decision to control the disease.”
Generally, Extension and Texas A&M Forest Service identify three basic approaches to managing oak wilt and suggest using a combination of measures to develop an effective method of control. They include destroying diseased red oaks, handling firewood properly, and painting wounds on healthy oaks to prevent the beetles from dispatching the fungal spores into the tree.
The second method is trenching to disrupt root connections to keep the disease from spreading through the trees' root systems. Finally, injections of a fungicide into the root systems of individual, high-dollar trees can sometimes help extend the life of the tree. (There are several fungicides labeled for tree injection to help suppress oak wilt disease. The position of the Texas A&M Forest Service on these products and others like them is that the brand Alamo is currently the only fungicide that has research backed by Texas A&M University to prove that it is effective for injection and treatment of the oak wilt fungus.)
Also, red oaks that die in late summer or early falls should be burned, buried or chipped to prevent fungal mats from forming the following spring.
Most importantly, pruning or otherwise wounding the trees should be avoided from February through June when the beetles are active. All pruning equipment should be sanitized or sterilized and all wound wounds or fresh stumps should be painted with a wound paint. Oak firewood should be handled cautiously, and unseasoned oak woods from infected trees should not be stored near healthy oaks.
Control methods for oak wilt can be expensive and time consuming and Appel points out that oak trees die from a lot of other diseases that can be controlled without the costly measures required for oak wilt. The way the trees die can be a clue, he said. Red oaks die four to six weeks after infection and live oaks take three to six months to die.
For more information or for answers to oak wilt questions, go to texasoakwilt.org, or to the Texas Forest Service website, texasforestservice.tamu.edu. To find out how to have a tree tested for oak wilt, visit the Texas Plant Disease Diagnostic Laboratory at plantclinic.tamu.edu. Some cities and municipalities also have oak wilt programs in place.