There are four primary approaches used for oak wilt management in Texas. Successful control usually depends on an integrated program incorporating measures from all four approaches. The first approach attempts to prevent the formation of new oak wilt infection centers by eliminating diseased red oaks, handling firewood properly, and painting wounds on healthy oaks. The second approach involves trenching or other measures to disrupt root connections responsible for root transmission of the pathogen. The third approach is the injections of the fungicide propiconazole into individual, high-value trees help reduce crown loss and may extend the life of the tree. Finally, consider planting other tree species to create diversity in the landscape and to mitigate the impact of oak wilt. These measures will not cure oak wilt, but can significantly reduce tree losses.
Preventing New Infections
Infected red oaks that die in late summer, fall or early winter should be cut down and burned, buried, or chipped soon after discovery to prevent fungal mats that may form on these trees the following spring. If this is not possible, the trees should be injected with herbicide or deeply girdled with an ax and stripped of all bark 2 to 3 ft above the soil line. Drying of the wood before spring discourages formation of fungal mats.
All wounding of oaks (including pruning) should be avoided from February through June. The least hazardous periods for pruning are during the coldest days in winter and extended hot periods in mid- to late summer. Regardless of season, all pruning cuts or other wounds to oak trees, including freshly-cut stumps and damaged surface roots, should be treated immediately with paint to prevent exposure to contaminated insect vectors. Any type of paint (latex, oil-based, spray-on, brush-on, or wound dressing) will suffice.
Transporting unseasoned firewood from diseased red oaks is a potential means of spreading the oak wilt fungus. Oak wilt cannot be transmitted by burning infected firewood; however, fungal mats may form on unseasoned oak firewood in storage. Presently, no vectors have been proven to transmit the fungus from live oaks to other oak trees, but diseased wood from any oak species should never be stored near healthy oak trees unless precautions are taken. It is best to purchase wood that has been thoroughly dried for at least one full year. If firewood from diseased trees is stored near healthy oak trees, it should be covered with clear plastic with the edges buried to prevent insects from leaving the pile.
Stopping Spread through Roots
Measures can be taken to break root connections between live oaks or dense groups of red oaks to reduce or stop root transmission of the oak wilt fungus. The most common technique is to sever roots by trenching at least 4 ft deep with trenching machines, rock saws, or ripper bars. Trenches more than 4 ft deep may be needed to assure control in deeper soils.
Correct placement of the trench is critical for successful protection of uninfected trees. There is a delay between colonization of the fungus and appearance of symptoms in the crown; therefore, all trees with symptoms should be carefully identified first. The trench should be placed a minimum of 100 ft beyond these symptomatic trees, even though there may be “healthy” trees at high risk of infection inside the trench. Trees within the 100-ft barrier, especially those without symptoms, may be uprooted or cut down and removed to improve the barrier. Tree removal should be initiated after trenching, starting with healthy trees adjacent to the trench and gradually working inward to include symptomatic trees.
Oak wilt infection centers are more easily suppressed when detected early, before they become too large. The untreated trees immediately outside the treated area should be closely monitored for several years. If the pathogen appears to have crossed the barrier, the same measures (new trenching and treatment of trees within the barrier) should be repeated while the diseased site is still small.
Propiconazole is the only fungicide scientifically tested and proven effective (when properly applied prior to infection) for use as a preventative treatment to protect live oaks. Limited success also may be achieved in trees treated with therapeutic injections during the earliest stages of infection. The fungicide is injected into the tree’s water-conducting vascular system through small holes drilled into the root flares at the base of the tree. Treatment success depends on the health of the candidate tree, application rate, and injection technique. Injection should be done only by trained applicators.
Fungicide injection does not stop root transmission of the fungus; therefore, this treatment is used best in conjunction with trenching or to protect individual, high-value trees in situations where trenching is impractical. Healthy live oaks at high risk of infection in advance of an expanding infection center are preferred candidates for injection. Foliar symptoms can be used in selecting trees as candidates for preventative or therapeutic treatments. A tree with foliar symptoms of oak wilt, as well as any non-symptomatic oak tree immediately adjacent to a tree with symptoms, should receive a therapeutic treatment. If symptoms are observed in more than 30 percent of the crown, it is unlikely a fungicide injection will be effective. Injections of non-symptomatic oak trees at greater distances from symptomatic trees (75 to 150 ft) will yield the best results for preventative treatments.
There are several steps in the injection process that require careful attention following tree selection. Mixing the fungicide solution, exposing and drilling holes in the flare roots, connecting the injection apparatus to the tree, and monitoring uptake must be done according to label specifications and directions. Treatment may take several hours. Information and training are available through Texas A&M AgriLife Extension Service or Texas A&M Forest Service offices. The services of a professional arborist or other experienced person may be required to assure proper injection.
Plant Diverse and Resistant Trees
Planting trees that are resistant to oak wilt can lessen the impact of the disease. It is best to plant trees that are native and adapted to your local environment to decrease the need for supplemental watering and fertilizing. Native trees also provide habitat for local bird and wildlife species. Avoid planting a monoculture. Plant a diverse selection of tree species to defend against any future forest health issues.