Dutch elm disease

Ophiostoma novo-ulmi

also called DED

A fatal fungal wilt of elms carried by bark beetles: leaves on whole branches yellow, wilt and hang brown, and the wood shows brown streaks under the bark.

Fungus No cure Spread by insectsSpread by contact Spreads to neighbors

What you'll see

Leaves on a branch suddenly yellow, wilt and turn brown in summer, yet stay hanging on the tree.
Peel the bark from a wilting shoot and the outer wood shows brown or purplish streaks and spots along the grain.
Cut a twig across and a dark ring or scattered dark dots mark the blocked water vessels.
Dieback spreads from branch to branch, and shoot tips often curl into a shepherd’s-crook.
Small D-shaped beetle exit holes and winding galleries appear under the dead bark.

Why it happens

An aggressive fungus that blocks the tree’s water-conducting vessels. It is carried from tree to tree by elm bark beetles as they feed, and passes directly between neighboring trees through natural root grafts.

It spreads through the growing season as the beetles are active; mature elms in warm summers are worst affected, and stands linked underground by grafted roots go down together.

Treating it

There is no cure for an infected mature tree; fell it promptly to protect others nearby.
Strip and burn or bury the bark, as the beetles breed under it and spread the fungus.
On a prized tree, cut out an isolated infected limb well below the streaking as soon as it shows — this only sometimes works.
Sever root grafts between an infected tree and its neighbors by trenching to slow underground spread.

Keeping it away

Plant a modern disease-resistant elm cultivar where you want elm.
Do not store or stack elm logs or brash with bark on, which harbor the breeding beetles.
Report and remove dying elms quickly to break the beetles’ life cycle in the area.
Watch mature elms through summer so an early infected branch can be caught and cut out.

Who gets it

Elms of all kinds — English, wych and smooth-leaved elm most of all, and to a lesser degree Zelkova.