Ash is one of the most common trees in the British countryside, and ash dieback has changed the way we manage it. The disease, caused by the fungus Hymenoscyphus fraxineus, has spread across most of the UK. If you own or manage woodland, you need to spot it early — not to save every tree, but to make safe, sensible decisions about which ones to keep, which to monitor, and which to remove before they become a hazard.
This checklist walks you through the symptoms in the order you are most likely to see them, then covers what to do next. It is not a substitute for a professional survey, especially where trees are close to roads, footpaths or buildings.
What ash dieback does to a tree
The fungus attacks the tree's vascular system. It blocks the movement of water and nutrients, causing shoots and branches to die back from the tips. In the early stages, a tree may look thin and sparse, with fewer leaves than its neighbours. Over several years, the crown dies back, the trunk becomes brittle, and the tree becomes unsafe long before it actually falls.
Think of it as a slow blockage rather than a sudden kill. A tree can carry the fungus for years, producing fewer leaves each season and sending out stress growth from the trunk. By the time large branches are dying, the wood inside may already be weaker than it looks.
Some ash trees tolerate the disease far better than others. A small number survive with only mild symptoms, and these trees are valuable — they may carry genetic resistance. So identification is not just about finding trees to remove. It is also about spotting the ones worth keeping.
Check the crown first
Start with the leaves and shoots, ideally between June and September.
- Wilting, blackened leaves. Leaves may turn brown or black and stay attached to the tree, rather than falling cleanly in autumn.
- Premature leaf fall. A tree may drop its leaves in late summer while others are still green.
- Sparse, thin crowns. Look for gaps in the canopy where branches have died.
- Shoot dieback. The tips of branches die back, and small twigs snap easily.
Leaf symptoms alone are not conclusive. Ash trees can lose leaves for many reasons, including drought and frost. Look for a combination of symptoms before you make a judgement. A hedgerow ash that drops leaves in August after a dry spell may simply be stressed; a woodland ash with blackened leaves, sparse twigs and diamond-shaped bark lesions is a much stronger suspect.
Move to the trunk and branches
This is where the most reliable signs often appear. Look closely at the bark on smaller branches and the main trunk.
- Diamond-shaped lesions. Dark, sunken patches on the bark, often with a paler centre, are a classic sign. They may be several centimetres long.
- Epicormic growth. Bushes of new shoots growing from the trunk or lower branches are a sign of a tree under stress. The tree is trying to replace the leaves it has lost.
- Dark staining under the bark. If you peel back a small piece of bark from a dead or dying branch (only where it is safe to do so), you may see brown or black discolouration in the wood beneath.
On young ash, lesions can girdle a stem quickly and kill the tree within a season. On mature trees, the decline is usually slower, but the damage to the timber makes the tree unpredictable. A 15-year-old planted ash can die within one or two seasons, while a 100-year-old roadside tree may decline for a decade — yet become brittle much sooner than its appearance suggests.
Check the base and root flare
Collar rot — where the fungus attacks the base of the trunk — is the most dangerous stage for tree safety. Look for:
- Dark, sunken lesions at the base of the trunk, sometimes extending into the roots.
- Loose or cracked bark around the root flare.
- Fungal brackets or a white, fan-like growth under the bark, which can also indicate honey fungus rather than ash dieback.
A tree with a significant lesion at the base can fall with little warning. If you see this, keep your distance and get a professional opinion before doing anything else. Treat any basal lesion as a red flag, especially if the tree leans, has dead branches overhead, or stands within falling distance of a road, path, building or power line.
Symptom lookalikes
Not every sick ash has ash dieback. Before you act, rule out other common problems:
- Squirrel damage. Stripped bark with clear tooth marks, often on the main stem.
- Ash rust. Bright orange or yellow pustules on leaves in summer.
- Frost damage. Young shoots killed in late spring, usually with no bark lesions.
- Drought stress. Wilting leaves across the whole crown, often on dry, shallow soils.
- Honey fungus. White mycelium under the bark and black, bootlace-like strands in the soil.
If you are unsure, take clear photographs of the leaves, bark and base of the tree, and ask a qualified arboriculturist or your local woodland officer. Include something for scale — a coin, hand or notebook — and note the date and location. A few good photos can save a site visit and help you compare symptoms over time.
Timing your survey and keeping records
You will get the clearest picture between June and September, when the crown is in leaf. A mid-summer walk can reveal dieback before autumn leaf fall masks it. In winter, you can still look for diamond lesions, dead twigs and epicormic growth, but you will miss leaf symptoms.
Record each tree or group with its location, estimated age or size, percentage crown dieback, presence of basal lesions, proximity to targets, and date. A simple notebook or smartphone map is enough. Use a scoring system to prioritise:
- Low risk: mild crown symptoms, no basal lesion, away from paths, roads and buildings.
- Medium risk: 25–50% crown dieback, no basal lesion, but within falling distance of a footpath, fence or vehicle track.
- High risk: more than 50% crown dieback, a basal lesion, dead branches, or within falling distance of a road, building or power line.
This is not a substitute for professional risk assessment, but it helps prioritise. Revisit high-risk trees after storms, and keep notes so you can see whether a tree is declining slowly or collapsing quickly.
Wildlife, safety and legal checks
Before felling, check for nesting birds, usually between March and August, and for bats and other protected species. Mature ash often has cavities, cracks and loose bark used by bats. If bats are likely, get advice from a licensed ecologist. Never work alone under a diseased tree, and never cut a tree that is already unstable unless you are trained and equipped.
You may need a felling licence for larger volumes of timber. Safety felling and trees that are dead, dying or dangerous often have exemptions, but rules differ across the UK. Check with the Forestry Commission, Natural Resources Wales, Scottish Forestry or DAERA before you start. Also check for tree preservation orders and conservation area designations, which may require consent.
Reporting and removal decisions
Reporting rules differ across the UK and have changed as the disease has spread. In much of England and Wales, ash dieback is no longer subject to the same statutory reporting requirements as when it first arrived, but you can still record suspected new sightings through TreeAlert. In Scotland, use TreeAlert or contact Scottish Forestry; in Northern Ireland, use TreeCheck. Check current guidance before reporting, as advice changes.
Removal decisions should be based on risk, not on the presence of the disease alone. A tree with mild symptoms and no targets nearby may be left to see if it tolerates the disease. A tree with a basal lesion next to a road should be assessed immediately. Where you remove ash, consider retaining resistant survivors and planting a diverse mix of native species — oak, field maple, lime, hornbeam, birch and wild cherry — rather than replacing ash with a single species.
If a tree has a basal lesion and a target within falling distance, treat it as a priority — not a waiting game.
Finally, remember that ash dieback is a long-term change, not a one-off problem. A woodland owner's job is to keep people safe, keep records, and keep the best ash. The trees that survive may be the foundation of the next generation of ash woodland.
Photo: Erik Mclean / Pexels
