Ash dieback

What you are looking at

From a distance, an ash with dieback has a thin crown and bare branch tips standing above the foliage, so the outline against the sky looks like antlers. The thinning works from the top and the outside inwards, and it gets worse each year.

Closer to, the leaves carry dark brown or orange blotches that run down into the leaf stalk. Affected leaves wilt, blacken and hang on the tree instead of falling. On the stems, look for lesions shaped like a diamond or a flattened oval, usually centred on the point where a side shoot joins.

A tree that has lost a lot of its crown often produces bushy clumps of shoots straight out of the trunk and the main limbs. That is the tree trying to replace the canopy it has lost, and it is a sign of how far things have gone rather than of recovery.

Underneath, in the litter from last year, look for the fungus itself. Small white or cream cups a few millimetres across stand in rows along blackened ash leaf stalks from July onwards. That is where the spores come from.

Where it came from and where it is going

The fungus is native to east Asia, where it lives harmlessly on a related ash that evolved alongside it. It reached Europe in the 1990s and was found in Britain in 2012, first on imported nursery stock and then in established woodland.

It is now effectively everywhere in Britain. Current expectations are that it will kill most of the ash in the country, with estimates generally in the range of seventy to eighty-five per cent.

Ash is one of the commonest trees in Britain, so this is a landscape-scale event rather than a garden problem. That said, a great many gardens have an ash in them or next to them, and what to do about that one tree is a fair question with a clear answer.

Do not fell a healthy ash

Somewhere between one and five trees in a hundred carry enough genetic tolerance to survive infection. Those trees are the entire future of ash in Britain, and they look exactly like every other healthy ash right up until the moment their neighbours die and they do not.

Felling ash pre-emptively, or clearing a stand because the disease is in the area, removes the tolerant individuals along with the rest. Several conservation programmes are now working from precisely these survivors.

The exception is safety, and it is a real one. A tree that has lost much of its crown becomes brittle and its root plate weakens, partly because honey fungus moves in on a weakened ash. Affected trees drop limbs and blow over. Anything overhanging a path, a road, a building or a boundary needs looking at by an arborist, and that assessment needs repeating as the disease progresses.

There is no treatment. Nothing available to a gardener, a forester or a commercial grower cures this or slows it.

What to do about it

Watch the tree rather than felling it, unless it is a safety problem.

Get an arborist to assess anything overhanging a path, a road or a building, and have it looked at again each year.

Take out dead and dangerous wood while leaving the living tree standing.

Rake up and bury or cover the fallen leaf stalks under a garden ash, which lowers the spore load immediately around it.

Water the tree in a drought and keep machinery off the roots.

Do not plant a new ash, and do not move ash logs, brash or leaves off the site.

Report a new find through Forest Research’s TreeAlert if you are somewhere the disease has not been recorded.

What it gets mistaken for

Honey fungus kills from the roots up, with white fungal sheets under the bark at ground level and black bootlaces in the soil. It commonly follows dieback into the same tree, so finding it does not rule the other out.

Verticillium wilt takes one branch at a time and stains the wood brown inside, with nothing showing on the outside of the stem.

Drought browns the leaf margins across a whole crown during one hot spell and then stops. Dieback keeps taking branches year after year.

Ash bud moth and various leaf-mining insects damage foliage without producing stem lesions or crown dieback.

Squirrel bark stripping leaves obvious gnaw marks with the wood exposed, usually in a band around a limb.