Oak wilt is a destructive infection caused by the fungus Bretziella fagacearum. This fungus invades and disables the tree’s water-conducting system. Live oaks (Quercus virginiana) and red oaks (such as Spanish and Shumard oaks) are the most commonly affected species. Red oaks often die within a few weeks of showing symptoms, making early detection and management crucial to protect healthy trees.
Containing the Disease Spread
Controlling the spread of oak wilt requires a two-pronged strategy focused on interrupting both underground and above-ground transmission pathways. The fungus primarily moves from tree to tree through interconnected root systems, a problem especially common among live oaks that grow in dense, clonal stands. To halt this subterranean spread, a physical root barrier, or trench, must be installed to sever all root grafts between infected and healthy trees.
The barrier is created by digging a trench at least four feet deep, using specialized equipment like a rock saw or vibratory plow. The trench must extend a minimum of 100 feet beyond the outermost symptomatic trees to account for trees that are already infected but not yet showing signs of the disease. While the trench is immediately backfilled, its purpose is to sever root connections, isolating the disease center and preventing the fungus from traveling to adjacent trees.
The second transmission method is through insect vectors, specifically small sap-feeding beetles (nitidulids) that carry fungal spores overland. These beetles are attracted to the sweet, fruity smell of fungal mats that form under the bark of dying red oaks, which serve as spore production centers. The beetles pick up the spores and then fly to fresh wounds on healthy oaks, infecting them with the disease.
To prevent this spread, avoid non-emergency pruning or wounding of oaks from February through June, which is the period when the beetles and fungal mats are most active. Regardless of the time of year, any wound on an oak tree, including pruning cuts, stumps, or accidental injuries, must be immediately sealed with a wound dressing or paint. Applying paint within minutes provides a physical barrier and a deterrent, stopping contaminated insects from landing and transmitting the spores to the susceptible inner wood.
Fungicide Treatment Options
For high-value oak trees, a systemic fungicide can be used for both preventative and therapeutic purposes. This treatment involves the trunk injection of a fungicide containing Propiconazole, often sold as Alamo. The fungicide is absorbed and distributed through the tree’s vascular system, helping to suppress the fungus.
Because this procedure requires precise dosage and specialized equipment, application must be performed by a certified arborist or commercial pesticide applicator trained in macro-injection techniques. Dosage is calculated based on the tree’s diameter at breast height (DBH). Typical preventative doses are 6 to 10 milliliters of fungicide per inch of DBH, though a higher dose may be used for trees already showing signs of infection.
Fungicide injection is most effective when applied preventatively to healthy trees near an outbreak or therapeutically to live oaks showing less than 30% crown loss. While treatment will not reverse existing damage, it can significantly slow disease progression and extend the tree’s life. To maintain protection, injections typically need to be repeated every 12 to 24 months, depending on the tree’s health and local disease pressure.
Management of Infected Trees and Wood
Once an oak tree, especially a red oak, exhibits extensive canopy loss or is confirmed to be infected, removal is necessary. Red oaks rarely survive the disease and die quickly, and live oaks with severe symptoms are often too far gone for treatment. Prompt removal of dead or dying trees is essential to eliminate the source of fungal spores that can infect surrounding healthy oaks.
The wood from an infected red oak poses a particular risk because it can produce the spore-bearing fungal mats under its bark the following spring. To neutralize this threat, all infected red oak wood must be managed carefully, either through burning, chipping, or proper solarization. If burning is not possible due to local ordinances, the wood should be tightly stacked and completely covered with thick, clear plastic sheeting.
The edges of the plastic must be trenched or buried in the ground to create an airtight seal. This allows the sun’s heat to solarize the wood, killing the fungus and any trapped insects. The sealed wood should remain covered for a minimum of six months before it can be safely used or stored. For comprehensive maps of infection areas and specific regional recommendations for wood disposal, landowners should consult the Texas A&M Forest Service.