Ash dieback

Hymenoscyphus fraxineus

also called chalara, chalara dieback of ash

A fungal disease of ash spread on the wind: blackened wilting leaves, diamond-shaped dark lesions on the stems, and progressive crown dieback.

Fungus No cure Airborne spores Spreads to neighbors

What you'll see

Dark brown-black blotches form where the leaf meets the stalk, and the leaves wilt and hang blackened.
Elongated diamond-shaped lesions, often centerd on a leaf scar, sink into the bark of young stems.
The crown thins and dies back from the top and outside inwards, leaf by leaf and branch by branch.
The tree responds with bushy tufts of new shoots lower down the trunk and main limbs.
Peel a stem lesion and the wood beneath is stained brownish-gray.

Why it happens

A fungus that infects ash leaves in summer, then grows down into the twigs and branches, killing the wood. It fruits on the previous year’s fallen leaf stalks on the ground and releases spores that blow long distances on the wind.

Spores are shed in warm, humid weather from midsummer into autumn; young trees and dense woodland with a damp leaf litter succumb fastest.

Treating it

There is no cure; the aim is to manage the tree, not save it.
Leave a lightly affected tree standing where it is safe to, as a minority show useful tolerance.
Fell or reduce a tree only on safety grounds when it overhangs a path, road or building and is failing.
Take care with felling, as diseased ash becomes brittle and dangerous to work.

Keeping it away

Do not plant new ash where the disease is established.
Retain and protect any healthy, tolerant ash that may carry resistance for the future.
Clear and destroy fallen ash leaves under a valued young tree to reduce the local spore load.
Buy in nothing but certified ash stock, and avoid moving ash plants or leaves between sites.

Who gets it

Native ash (Fraxinus excelsior) above all, along with other Fraxinus species grown as ornamentals.