What oak wilt is
Oak wilt is a disease caused by a fungus, Ceratocystis fagacearum, that gets into an oak's water-conducting system and shuts it down. It is one of the most destructive tree diseases in the United States. The Texas A&M Forest Service reports it killing trees across 76 Texas counties, and it has reached epidemic levels in central Texas.
How it spreads
Live oaks: through the roots
Live oaks grow in groups, called motts, with roots joined underground. The fungus moves tree to tree through those roots, so one sick live oak puts its neighbors at risk. Infection centers grow about 75 feet a year on average, and up to 150 feet in any one direction.
Red oaks: fungal mats and beetles
Red oaks, including Spanish oak, Shumard oak and blackjack oak, are extremely susceptible and seldom survive. Dying red oaks can grow fungal mats under the bark, mostly in spring. The mats smell like fermenting fruit and draw small sap beetles, which can carry spores to a fresh wound on a healthy oak.
White oaks, such as post oak, bur oak and chinkapin oak, are less susceptible and rarely die from oak wilt.
Signs to look for
- On live oaks, leaves with yellow veins that turn brown. This is called veinal necrosis, and it's the most reliable leaf sign.
- Most infected live oaks drop their leaves and die within 1 to 6 months of the first symptoms.
- Red oaks often die within 3 to 4 weeks. In summer, a sick red oak can stand out with fall-like color among green trees.
- Patches of dead and dying live oaks next to each other.
Symptoms can look like other problems, so a lab test is the way to confirm it. A county extension agent, a Texas A&M Forest Service forester or a trained arborist can take samples.
Prevention: the rules that matter most
- No pruning or wounding oaks from February through June. The safest times are the coldest days of midwinter and long hot spells in mid to late summer.
- Paint every wound right away, in any season. That includes pruning cuts, fresh stumps and damaged surface roots. Wound paint or latex paint both work.
- Remove diseased red oaks quickly. Cut them down and burn or bury them, before fungal mats can form.
- Be careful with firewood. Never store unseasoned wood from diseased oaks near healthy ones. Buy wood dried for at least a year, and cover wood of unknown origin with clear plastic, edges buried.
Treatment options
Trenching
A trench at least 4 feet deep cuts the root connections between live oaks. It goes at least 100 feet beyond the trees that show symptoms, because the fungus is in the roots before the leaves show it. Trees inside the trench line may need to come down to strengthen the barrier.
Fungicide injection
Propiconazole is injected into the root flares at the base of the tree. It is the only fungicide tested and proven to protect live oaks, and it works best on healthy trees before they're infected. Once more than 30% of the crown shows symptoms, injection is unlikely to help. It does not stop spread through the roots, so it's used with trenching or to protect single high-value trees.
Who should do the work
The Forest Service guidance is to have injections done by a trained, licensed arborist who holds a certified applicator's license. When the tree pro calls you back, ask about that license before anything is injected, and ask how they plan to place a trench if one is needed.
Sources
- How to Identify and Manage Oak Wilt in Texas, USDA Forest Service, updated by the Texas Forest Service (now Texas A&M Forest Service).
- Prevent the spread of oak wilt in Texas, Texas A&M Forest Service, February 2024.
- texasoakwilt.org, the Texas oak wilt information site the Forest Service points to.