Why Early Diagnosis Matters

Trees are slow communicators. By the time a mature oak or ash looks obviously unwell, the problem may have been developing for two or three growing seasons, and the options for treatment have narrowed considerably. Learning to read the early signals — a patch of sunken bark, a thinning crown, leaves turning colour weeks ahead of schedule — gives you time to act while there is still something to save, and to protect the shrubs, hedging and fruit trees planted nearby.

Get into the habit of walking your garden or woodland two or three times a year with a purpose: once in late spring when leaves have fully flushed, once in high summer, and once in autumn, when the difference between normal seasonal change and genuine decline is easier to judge. Take photographs of anything that looks unusual and note the date. Patterns over time tell you far more than a single snapshot.

Cankers, Sores and Bleeding Bark

A canker is a dead, often sunken area of bark and cambium, and it is one of the most reliable early indicators of trouble. Run your hand along the trunks and main limbs of your trees where you can reach safely. You are feeling for depressions, rough raised callus tissue, cracked bark or dark staining.

• Bleeding canker of horse chestnut shows as rusty-red or black sticky fluid oozing from the bark, often with a sour smell. It can affect the trunk or the branches.
• Bacterial canker on cherry and plum causes gumming — amber-coloured resin weeping from sunken patches, frequently accompanied by dieback further out along the branch.
• Apple and pear canker produces target-like rings of dead tissue with a raised edge, usually on younger wood and around old pruning cuts.
• Sooty bark disease on sycamore gives a smoky, dark staining under flaking bark, often with sparse, small leaves in the crown above.

Any canker that completely girdles a stem will kill everything above it. If you find one on a branch, cut back into clean wood well below the affected area, sterilising your tools between cuts.

Dieback and Thinning Crowns

Dieback is simply the progressive death of twigs and branches, usually starting at the tips and moving inward. Stand back and look at the whole tree against the sky. A healthy crown is dense and even; a declining one is sparse, with dead stubs sticking out like broken fingers.

• Ash dieback typically begins with dark lesions on the bark and wilting, blackened leaves on individual shoots, followed by dieback in the upper crown and a characteristic "stag-headed" look.
• Phytophthora ramorum on larch causes rapid decline, with wilted, grey-brown needles and dark cankers with a resinous bleed on the trunk.
• Dutch elm disease often reveals itself as a sudden wilt on one branch in midsummer, with the leaves curling and turning yellow-brown while still attached.

Record which parts of the crown are affected. Dieback confined to one side often points to a localised root or stem injury; dieback spread evenly through the crown suggests a systemic problem.

Unusual Leaf Loss, Colour and Timing

Some leaf change is entirely normal, so timing is everything. Early leaf fall in August, or yellowing on just one branch of an otherwise healthy tree, is not. Look closely at the leaves themselves.

• Honey fungus often shows as wilting, yellowing foliage and premature leaf drop on one or more branches, with white fungal sheets under the bark at the base and black bootlace strands in the soil.
• Silver leaf on plum, cherry and related species gives the leaves a silvery sheen, with a purple-brown stain in the wood when a branch is cut across.
• Horse chestnut leaf miner creates brown blotches that merge until the whole leaf looks scorched by midsummer; unsightly, but rarely fatal.
• Verticillium wilt causes sudden wilting and dieback on one side of maples, ash and other broadleaves, often with greenish streaking in the wood beneath the bark.

Discolouration with no obvious insect feeding, especially if it appears on several trees at once, is worth investigating further.

Checking the Base and Roots

Many serious tree problems start below ground, where you cannot see them. Push a thin cane into the soil around the base of a suspect tree. Soft, waterlogged ground, a spongy trunk base or a distinct mushroom smell may point to Phytophthora root rot. Fungal fruiting bodies — honey-coloured toadstools in clusters, or bracket fungi on the trunk — indicate decay inside the wood, and their presence is a prompt to have the tree assessed for safety.

When to Bring in a Tree Surgeon

Diagnosing from the ground has limits. If you find a canker on a main stem, dieback affecting more than a third of the crown, a hollow or leaning trunk, or any fungal growth at the base of a tree near a path, building or road, call in a qualified tree surgeon for a closer look. They can inspect the crown safely, take samples, and advise on the realistic options: pruning out infection, managing the tree for safety, or felling and replanting with something more resistant.

Where a disease is spreading, remove affected material promptly and do not compost it on site. Clean your boots and tools before moving to the next tree, and think carefully about what you replant. Species diversity is the single best insurance policy a British garden or small woodland can have.