Oak wilt is a fungus that blocks the water-carrying vessels inside an oak until the tree dies of thirst with its roots in wet soil. Central Texas has the worst of it anywhere in the country, and the reason is live oaks. They grow in motts, connected underground by grafted roots, so the fungus moves tree to tree without ever crossing open ground. A single infection can take out a whole street.
If oaks near you are browning in a spreading pattern, send us photos of the leaves quickly. Timing changes what can be done.
What it looks like
On live oak, the tell is veinal necrosis: the veins in the leaf turn yellow or brown while the tissue between them stays green, so the leaf looks like a road map. Trees drop leaves heavily out of season and thin from the top down. Death usually takes three to six months.
On red oak, Spanish oak and Texas red oak, it is faster and more brutal. The whole canopy browns from the top in a matter of weeks and the tree can be dead in a single season. Red oaks also produce fungal mats under the bark, which is how the disease travels to new areas by beetle rather than by root.
The pattern matters as much as the symptom. Oak wilt spreads outward through a group of trees, so a widening circle of dying oaks is the signature. One sick oak surrounded by healthy ones is more often drought or root damage.
The pruning window
Do not prune oaks in Central Texas between February 1 and June 30. Sap-feeding beetles are active in that window, they are attracted to fresh wounds, and they arrive carrying fungal spores from infected red oaks. This is the single most preventable cause of new infections in Austin.
Paint every wound on an oak, immediately, whatever the month. Ordinary latex paint is fine. This runs against the standard advice for other species, where wound dressings do more harm than good, and in oak wilt country it is the exception that matters. If a storm breaks an oak limb in April, paint the break the same day.
Our own rule: we do not open oaks between February and June, and we carry pruning paint on every truck year round. More in can trimming an oak tree kill it.
What can actually be done
Trenching is the main defence and the only thing that stops root spread. A machine cuts a trench four to five feet deep between the infected trees and the healthy ones, severing the grafted roots. It has to be placed well beyond the visibly affected trees, which means sacrificing some that still look fine, and that is the part people struggle with. Expect $8 to $15 a linear foot, so $4,000 to $12,000 for a residential block.
Fungicide injection with propiconazole is preventative for high-value trees not yet infected, or occasionally for one showing early symptoms. It is injected into the root flare and lasts around two years. Roughly $300 to $1,200 a tree depending on size. It protects that individual tree and does nothing to stop the spread.
Removal and disposal of infected red oaks matters, because those are the ones producing spore mats. Cut wood should not be stored or moved as firewood. We take it away rather than leaving it stacked.
What does not work: injecting a live oak that has already lost most of its canopy. By then you are spending money on a tree that is going to die anyway, and it would be better spent on trenching to protect the rest. We will say so.
Questions about oak wilt
How do I know if my oak has oak wilt?
Look for veinal necrosis on live oak leaves, where the veins go yellow or brown and the tissue between stays green. Then look at the neighbours. Oak wilt spreads through connected roots, so a widening group of declining oaks is the real signature. A single sick oak among healthy ones in Austin is more often drought damage.
Can a tree with oak wilt be saved?
Sometimes, if it is a live oak caught very early and injected with propiconazole before much canopy is lost. Red oaks essentially never survive. Once a live oak has lost more than about a third of its canopy, money is better spent trenching to protect the healthy oaks around it than treating that one.
When can I prune my oak in Austin?
July through January. The safest months of all are the coldest, January and December, when the beetles that carry the fungus are dormant. Avoid February 1 to June 30 entirely. Whatever the month, paint every cut on an oak immediately, which is the opposite of the rule for most other species.
How much does oak wilt treatment cost?
Fungicide injection runs $300 to $1,200 per tree and lasts about two years. Trenching, which is the only thing that stops root-to-root spread, costs $8 to $15 a linear foot, so a residential block often lands between $4,000 and $12,000. Trenching is usually shared between neighbours, since it protects all of them.
Can I burn the firewood from an infected oak?
Burning it is fine. Storing it is the problem. Cut red oak can grow fungal mats under the bark and beetles will carry spores from that woodpile to healthy trees. Never move infected oak firewood to another property. If you must keep it, seal it under plastic until it is thoroughly dry.
Where we treat oak wilt
Across the oak wilt belt: Austin, Lakeway, Round Rock, Cedar Park, Georgetown, Leander, Pflugerville, Hutto, Kyle and Buda.