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.

  • OAKS ARE SAFEST IF PRUNED IN THE WINTER OR LATE SUMMER. Any wound on an oak tree, when the bark beetle is active, is likely to get visited by the beetle. If the wound and the visit occur during the period of early February through May, when the fungus is also active, there is a chance that the tree will be inoculated with the disease. Prune unsafe branches as they occur.

  • COVER ALL WOUNDS ON OAKS WITH PRUNING PAINT. The period of susceptibility is very short, perhaps only three days after the wound occurs, but it is safest to get in the habit of covering the wounds. Remember that pruning is only one of the ways trees get wounded. Automobile bumps, lawn mowers, string trimmers, wind and ice breakage, and bulldozers on roots are other common examples of wounds.

  • MANAGE FIREWOOD TO AVOID INFECTION. It is not necessary to reduce the use of the fireplace in order to stop the spread of oak wilt. Smoke from infected wood burning is NOT a threat! The fungus is destroyed by heat and will not even survive in dry firewood. If you utilize red oak firewood, try to purchase wood from trees that have not been infected or killed from oak wilt. Only wood that has been cured for an entire summer should be stored in the vicinity of uninfected red or live oaks. If you bought oak firewood for this winter and are unsure of its age or origins, use it up before spring.

  • DIVERSIFY YOUR SHADE TREE PLANTINGS. Large blocks of red and live oaks are susceptible to the spread of the disease. Plant cedar elm, chinkapin oak, bur oak, Chinese pistache, and bald cypress in addition to red and live oak. Parson's Archive has a list of outstanding landscape plants for all areas of Texas that suggests other alternatives.

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