Central Texas live oaks still battling oak wilt  

By MINDY POEHL | Central Texas Edition

Renee Burks with the Texas Forest Service spoke about how damaging the oak wilt fungus can be to oak trees., at MCC’s Highlander Ranch, August 31. Burks holds an i.v. that sprays a fungicide treatment into oak trees to help eliminate the disease.  --Photo by Mindy Poehl

Sept. 8, 2005 - All across the state of Texas, people enjoy their oak trees. Oak trees add to the beauty of landscapes, as well as provide shade to lawns and homes to animals. They can also be the home to the oak wilt virus, which can destroy oak trees.

�� Oak wilt is a lethal disease caused by the fungus Ceratocystis fagacearum. The fungus invades and disables the water-conducting system in oak trees. Oak wilt is primarily located in Central Texas, but is also found in Lubbock, Amarillo, San Angelo, and Houston.�

�� Oak wilt spreads quickly through the root grafts that connect trees of the same species growing near one another. It can also be carried by insects, like the beetle.

Renee Burks, with the Texas Forest Service, spoke about oak wilt control and prevention at McLennan Community College Highlander Ranch on Aug. 30. Burks warned that the oak wilt fungus can spread from already diseased trees to non-diseased trees, "so make sure to use a disinfectant, such as Lysol, on any tools that touch the tree."

Different oak tree leaves are wilted and show veinal necrosis as an indication of the fungus. --Photo by Mindy Poehl

Live oaks grow in large, dense groups with connecting roots. Root transmission is the only proven way of spreading the fungus in live oaks. As a result, patches of dead and dying trees, also called infection centers, are formed. 

"Infection centers in live oaks expand about 50 feet per year, " Burks said. "You can identify oak wilt in live oaks by veinal necrosis in the leaves. The veins on the leaves are discolored and pop out. "

Oak wilt begins in the canopy and works its way down, progressing in a six week to six month period. Some live oaks can survive the fungus. 

Red oaks play a key role in the establishment of new infection centers. "One-in-ten red oaks produces oak wilt. Oak wilt looks different on red oaks and is less distinct. In early spring, young leaves wilt and turn a pale green and brown. Mature leaves develop dark green water soaking symptoms on the margins and progresses inward, Burks explained. "Humans can spread the oak wilt fungus by moving wood from infected red oak trees to other locations."

The disease progresses in seven-to-10 days and almost no red oaks can survive the fungus.

Fungal mats are also indicators that a tree has oak wilt. Fungal mats can be found by looking for unobtrusive narrow cracks in the bark of dying red oaks that lead to hollow areas between the bark and wood. They often have a distinctive odor that is similar to fermenting fruit, which attracts the fungus-spreading beetles. Fungal mats can be exposed for inspection by chopping away the loose bark. 

Three primary forms of disease management exist for treating the oak wilt fungus. The first approach is to prevent the formation of a new oak wilt virus infection centers by eliminating infected red oaks, handling firewood properly and painting wounds on healthy oak trees. 

The second approach involves disrupting the root connections by trenching. Trenching breaks up the root connections between live oaks or red oaks to reduce or stop the transmission of the oak wilt fungus. Burks said she severs roots by digging five-foot-deep trenches 100 feet around the infected trees and nearby trees.

�The third approach involves injections of a fungicide into the tree.�

"It's just like hooking a big i.v. up to the tree," Burks explained.

Fungicide injections do not stop root transmission of the fungus, therefore, injections work best when used with trenching. 

"You should inject a tree with foliar symptoms of oak wilt, as well as any non-symptomatic trees adjacent to a tree with symptoms," Burks said. 

Infected red oaks that die in late summer or fall should be cut down and burned or buried soon after they are found to prevent insects from transmitting spores from fungal mats that may form on these trees. If the tree cannot be burned or buried, the trees should be injected with herbicide or stripped of all bark two to three feet above the soil line.