Thousand cankers disease

Geosmithia morbida, vectored by Pityophthorus juglandis

A tiny beetle carrying a fungus that makes one small canker per gallery — thousands of them, until a walnut is girdled from the outside in.

Fungus No cure Spread by insectsSpread by contact Spreads to neighbors

What you'll see

Yellowing and thinning of the upper crown, then branch dieback working downward over two to four years.
Numerous tiny beetle entry holes, less than a millimeter across, in the bark of dying branches.
Small dark brown cankers under the bark, each centerd on a beetle gallery, running together as they multiply.
Epicormic sprouts breaking out on the trunk below the dying crown.
A tree dead within three or four years of the first thinning, with the roots dead too.

Why it happens

The walnut twig beetle tunnels under the bark and carries a fungus with it. Each gallery produces one small canker; a single beetle does no harm, but the beetle breeds in the tree and the cankers accumulate in their thousands until the food-conducting tissue is destroyed all round. Neither organism is dangerous alone, and together they kill a mature black walnut.

Native to Arizona walnut in the southwest and moved east on firewood and logs, where black walnut has no resistance. Hot dry western summers favor the beetle; drought-stressed trees go down fastest.

Treating it

There is no cure. Have a confirmed infested tree removed and the wood chipped or burned on site.
Do not move walnut firewood, logs or chips out of the area — this is how the disease travels.
Water and mulch other walnuts in the garden; vigorous trees resist the beetle better.
Report a suspected case to your state agriculture department, since the distribution is still being mapped.
Do not prune walnuts in the beetle flight season, which runs through the warm months.

Keeping it away

Buy firewood where you burn it and never transport walnut wood.
Keep walnuts well watered through drought, which is the single thing a gardener can control.
Plant a mixture rather than a group of black walnuts.
Inspect the upper crown of walnuts each summer for early thinning.
Debark or chip any walnut wood you must keep, so the beetle has nowhere to breed.

Who gets it

Black walnut above all; butternut, English walnut and Arizona walnut are affected more lightly.