Oak wilt
Bretziella fagacearum
A vascular fungus that can kill a red oak in a single summer and travels root to root between neighbors. The whole defense is one rule: never prune an oak in spring.
What you'll see
- Leaves browning from the tip and outer margin inward while the base stays green, then dropping while still half green — the diagnostic fall.
- A red oak going from full leaf to bare in four to eight weeks in midsummer.
- Wilting starting at the top of the crown and working down.
- Dark streaking in the sapwood when a wilting branch is cut across.
- Neighboring oaks going down one after another in a widening circle over successive years, which is the root-graft spread.
Why it happens
A fungus that plugs the water-conducting vessels of the tree. It moves two ways: sap beetles carry spores from fungal mats on a dead tree to a fresh wound on a healthy one, and — more importantly for a group of oaks — it runs directly through the natural root grafts between neighboring trees of the same species. Red oaks die fast; white oaks compartmentalize it and often survive for years.
Established across the midwest, Texas and the eastern states. The infection window is spring and early summer, when the beetles are flying and the wounds are fresh. A garden with a group of red oaks is at far more risk than one with a single tree, because of the root grafts.
Treating it
- There is no cure for an infected red oak. Have it removed and the wood chipped or burned before the following spring, while the bark is still tight.
- Sever the root grafts between the infected tree and its healthy neighbors — a trench or vibratory plow cut to a meter deep, five meters out — before or as the tree comes down.
- Do not store the logs for firewood in a way that lets the fungal mats form and the beetles find them; if you must keep it, seal it in plastic through the summer.
- Have a valued white oak nearby injected with propiconazole by an arborist, which does work as a preventive but not as a cure.
- Call a certified arborist early; this is one where the diagnosis and the trenching are genuinely beyond hand tools.
Keeping it away
- Never prune, wound or plant an oak between April and July. This one rule prevents most new infections and costs nothing.
- If an oak is damaged in that window by a storm or an accident, paint the wound immediately — the one situation where wound paint is right.
- Prune in the depths of winter, when neither the beetles nor the fungus is active.
- Watch a neighboring group of oaks and report a sudden midsummer wilt; the trench has to be cut before the fungus reaches you.
- Plant a mixture of species rather than a row of red oaks, so root grafting between susceptible trees is limited.
Who gets it
All oaks. The red oak group — red, pin, black, scarlet — die within weeks; white and bur oaks are far more resistant.
See also
The Cottage Garden Companion — the gardener's apothecary at cottagegardencompanion.com