Oak wilt, and when not to prune your oaks

What oak wilt actually is

It's a fungal disease that blocks the water-conducting vessels inside an oak. The tree effectively can't drink, and the leaves go with it. In red oaks it usually kills fast, over a matter of weeks. Live oaks tend to decline over months to years.

It spreads two ways, and the difference matters because it changes what you do about it. Above ground, sap-feeding beetles pick up spores from fungal mats that form on infected red oaks and carry them to fresh wounds on healthy trees. Underground, live oaks growing near each other graft their roots together, and the fungus moves through the shared root system from tree to tree.

Why February to June specifically

Two things line up in spring. The fungal mats on infected red oaks are most likely to form in that weather, and the sap beetles that visit them are most active then. A fresh pruning cut on an oak in April is an open, sweet-smelling wound at exactly the moment the insects carrying the disease are out looking.

Texas A&M Forest Service puts the window at February 1 through June 30. That's five months of the year when the honest answer to “can you come trim my oak” is: not unless we have to.

The exception, and how to do it properly

Storms don't check the calendar. If you've got a cracked limb over a bedroom in April, it comes down in April — the risk of the limb beats the risk of the fungus.

The way to do it: make as few cuts as you possibly can, and paint every single wound immediately. Not at the end of the day. Immediately, while you're still up there. Wound paint on an oak is normally unnecessary and slightly counterproductive; during oak wilt season on a fresh cut, it's the whole point.

That's also true of accidental wounds. If a mower barks an oak trunk in May, or a storm tears a limb out, go paint it that day.

How to tell if you already have it

The classic sign on a live oak is veinal necrosis — the veins of the leaf turn yellow or brown while the tissue between them stays green for a while, so the leaf looks like it has a bright network drawn on it. Leaves drop while still partly green.

On red oaks it moves faster: the whole crown can go from green to bronze over a few weeks in early summer, and the tree can be dead in a season.

The pattern is the strongest clue. Oak wilt spreads through root grafts, so it tends to move outward through a stand of live oaks as an expanding front rather than hitting one tree in isolation. If your oak is declining and the two next to it are fine, oak wilt is less likely than drought, root damage or soil movement.

Confirming it takes a lab. Don't take my word or anyone else's for a diagnosis that ends in removing a mature tree.

What you can do that actually helps

Prune in the right season. Paint wounds during the window. Don't move unseasoned red oak firewood, because fungal mats can form on cut wood and that's how the disease gets a lift to a new neighborhood — if you have red oak firewood of unknown origin, keep it covered with clear plastic and sealed to the ground.

If there's a confirmed infection center nearby, trenching to sever root grafts between infected and healthy live oaks is the established way to stop the underground spread. That's specialist work and it needs doing in the right place, which means a diagnosis first.