Why early diagnosis matters

Trees rarely collapse without warning. Most problems announce themselves weeks, months or even years in advance, through wilting leaves, thinning crowns, dead twigs, bleeding bark or fungal growths at the base. Learning to read those signs lets you act while a tree is still worth saving, or make a calm decision about removal before a limb ends up on the greenhouse.

It also helps to know what you are looking at. Many symptoms overlap: drought stress, compaction from parked cars, waterlogging, mower damage and herbicide drift can all mimic disease. Before assuming the worst, check the site. Is the soil waterlogged or bone dry? Has the ground been built on, dug over or driven across? Are neighbouring trees of the same species affected? A tree under stress is far more vulnerable to pathogens, so the two often arrive together.

Ash dieback: the most widespread concern

Ash dieback, caused by the fungus Hymenoscyphus fraxineus, is now present across most of the UK and is the disease you are most likely to meet in woodland, hedgerows and gardens alike. Early signs appear in the crown: leaves wilt and blacken in midsummer rather than turning gold in autumn, and they often cling to the twigs instead of dropping.

Look closely at the bark on young stems and branches. Diamond-shaped or lens-shaped lesions, often dark brown to purple, mark the point of infection. Cut into one and you may find a brown stain beneath. As the disease progresses, whole branches die back, the crown thins from the outside in, and ash produces a burst of epicormic growth — tufts of leaves sprouting directly from the trunk and main limbs. Finally, lesions girdle the stem and the tree dies. Watch for the blackened, withered shoots that persist through winter, which is often the clearest confirmation.

Some ash show tolerance, so do not assume every affected tree must be felled. Where a tree overhangs a road, footpath or building, though, the brittle deadwood ash dieback creates is a genuine hazard and should be assessed.

Honey fungus and other root rots

Honey fungus (Armillaria species) attacks and kills the roots and lower trunk of a huge range of broadleaved and coniferous trees. Above ground, the first clue is often a crown that thins, wilts or dies suddenly in summer, sometimes with unusually heavy flowering or fruiting as a stressed tree makes a last effort. Below ground, the real evidence is hidden.

Peel back the bark at the base of a suspect tree and look for white, fan-shaped sheets of mycelium between bark and wood, smelling distinctly of mushrooms. Search the soil for black, bootlace-like strands (rhizomorphs) running through the ground. In autumn, clusters of honey-coloured toadstools may appear around the base, though their absence proves nothing.

Phytophthora species cause similar root and collar rots, often showing as dark, tarry bleeding from bark on oak, beech, alder and others, sometimes with a rusty or vinegary smell. Because these pathogens persist in soil for many years, replanting the same species in the same spot usually fails.

Cankers, bleeding and wilt diseases

Bark that bleeds, cracks or sinks is a warning sign. Horse chestnut bleeding canker produces sticky, dark ooze on trunks and branches, often followed by sparse, yellowing foliage and crown dieback. Canker fungi on apple, willow and poplar create sunken, dead patches that slowly girdle a limb.

Dutch elm disease remains active in the UK. Look for leaves that wilt and turn yellow-brown on individual branches, often in midsummer, while the rest of the crown stays green. Strip the bark from an affected twig and you will see brown streaking in the sapwood. Because the elm bark beetle carries the fungus, prompt removal and destruction of affected trees protects nearby elms.

Not every blemish is fatal. Wounds, old pruning cuts, frost damage and mechanical injury all invite secondary infections, and many trees compartmentalise decay successfully for decades.

When to call an arborist

Book a professional assessment if you notice any of the following:

• A sudden thinning or dieback of the crown, particularly on one side of the tree
• Deadwood, cracked limbs or a hollow trunk over a path, driveway, play area or building
• Fungal brackets on the trunk or buttresses, or honey fungus strands in the soil nearby
• Bleeding, sunken or splitting bark on the main stem
• Root disturbance from building work, drainage or new paving
• Any tree that has begun to lean, lift the ground or show a new soil crack at the base

A qualified arborist can confirm the diagnosis, assess the risk and advise on options — pruning, crown reduction, bracing, monitoring or removal. They will also know whether a Tree Preservation Order, conservation area designation or felling licence applies before any work begins.

Keep a simple record of what you see and when: photographs taken from the same spot each season are invaluable. Trees are long-lived, and small changes are far easier to judge against a history. If in doubt, ask early. A conversation costs far less than a fallen oak.