Oak Wilt Maintenance
Calvin R. Finch, Ph.D., County Extension Agent-Horticulture
Texas Cooperative Extension, Bexar County

To minimize the threat of oak wilt, do your oak maintenance work in late December, January and early February and always paint wounds.

Our South Texas oak trees don't need much pruning, they are most attractive and healthiest when they are left to grow in a natural shape. As with everything in the landscape, however, we compromise with nature on the oaks. We remove lower branches that have been shaded to the point that they cannot support leaves, we remove dead branches, and we remove hazardous branches. They are least susceptible to infection with the oak wilt fungus if pruned in January and February.

Oak wilt is a vascular disease caused by the fungus Ceratocystis fagacearum. All oaks can be infected by the pathogen; although red oaks are most susceptible and white oaks least susceptible to the disease. Live oaks fall in between the other oaks in vulnerability. The disease is spread by two routes: (1) bark beetles can pick up the spores from an infected red oak (Spanish oak, Shumard oak, and blackjack oak) and carry them to a newly inflicted wound on another oak and/or (2) the disease organisms can spread through interconnected root systems from one tree to another. The usual scenario in the Hill Country is for the disease to be spread from an infected Spanish oak by the beetle to a live oak and then from oak to oak by roots in the live oak thicket.

The disease stops the flow of nutrients and water from the roots to the leaves. Once symptoms appear, the Spanish oak may be dead in two weeks. A live oak can survive longer; two months to a year.

Veinal necrosis is a widely accepted, but not foolproof, symptom on live oaks. The interveinal areas on the leaves stay green while the areas directly adjacent to the veins die. Prior to the brown coloring, the dying area often is bright yellow or orange.

Red oaks infected with the disease will turn a brilliant red in early summer and lose all their leaves. On some infected Spanish oak trees, a fungal mat forms under the bark. It is this mat that attracts the bark beetles which carry the spores on to open wounds on other oak trees.

Oak wilt has appeared in Bexar County. It is not a major problem, but it has the potential of becoming one. Researchers have developed the chemical Alamo® to treat threatened trees and trees in the early stages of infection. It is an expensive and difficult process. Prevention is still best.

There are a number of effective measures that can be taken to minimize the threat of oak wilt.

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