Why Early Spring Is Your Best Chance to Look

By the middle of March most British trees are waking up. Buds are swelling, sap is moving, and within a few short weeks the canopy will be dense enough to hide almost anything. That window between bud burst and full leaf is the single most useful period in the arboricultural calendar for spotting trouble. The tree shows you last season's damage on its newest growth, and there is no foliage to obscure the trunk, the branch junctions or the root flare.

It is also the moment when many pathogens are gearing up. Fungal spores that overwintered in fallen leaves and cankers are released in spring rain, and insects such as the elm bark beetle begin moving as temperatures climb. Catching a problem now — even if that just means getting a proper diagnosis — leaves you far more options than discovering it in August.

Discoloured, Mottled and Distorted Leaves

Start at eye level and work upwards. Healthy new growth should be an even colour for the species; anything blotchy, silvery or unnaturally dark deserves a closer look.

• Silvery sheen on plums and cherries can indicate silver leaf, where the leaf surface separates from the tissue beneath. Cut a branch and look for a dark stain in the wood.
• Olive-brown velvety patches on apple and pear foliage are characteristic of scab, which thrives in damp spring weather.
• Dark brown or black lesions spreading from the leaf margins on ash, often with wilting shoots, is a classic early sign of ash dieback.
• Pale mottling and tunnels between the veins on horse chestnut usually points to leaf miner, which becomes obvious from early summer onwards.

Note whether the discolouration is confined to one branch, one side of the tree, or the whole crown. A single affected limb often means localised damage; a whole crown suggests something systemic at the roots or in the vascular system.

Cankers, Lesions and Bleeding Patches

A canker is a dead, sunken area of bark, and in early spring they are at their most visible because surrounding tissue is swelling and healing while the canker is not. Run your eyes along the trunk and main limbs, particularly around old pruning wounds and branch forks.

• Rusty or dark sticky fluid oozing from horse chestnut bark is the classic signature of bleeding canker.
• Amber-coloured gum on cherry and plum, often with sunken dead patches and shoot dieback, suggests bacterial canker.
• Sunken, cracked bark with concentric rings on apple and pear is likely fungal canker, which spreads in wet weather.
• Diamond-shaped dark lesions at branch junctions or the base of ash shoots are one of the most reliable indicators of ash dieback.

Photograph anything suspicious with something for scale, and avoid cutting into the bark "to see" — open wounds are an invitation to further infection.

Unusual Growth: Brooms, Galls and Fungal Bodies

Not every oddity is a disease. Oak galls, for instance, are caused by tiny wasps and do very little harm. But dense, twiggy clusters known as witches' brooms — often seen on ash, birch and cherry — frequently signal a chronic underlying problem and should be assessed properly.

Look also for:

• Bracket fungi at the base of the trunk, which can indicate significant internal decay.
• Black bootlace-like strands or white sheets of mycelium under the bark, the hallmarks of honey fungus.
• Sticky honeydew and black sooty mould on leaves and surfaces below, often the first clue to an aphid or scale insect population.
• Sudden prolific epicormic growth along the trunk, which can be a tree's response to stress or decline.

Bark, Buds and the Root Flare

Dead buds that fail to break, or a whole section of the crown that stays bare while the rest leafs out, tell you as much as any lesion. Check for vertical frost cracks, storm damage and old strimmer or mower wounds at the base — these are common entry points for decay.

Scrape back a little mulch and check that the root flare is visible. Trees planted too deeply, or buried under years of soil and grass, often decline slowly and inexplicably. Waterlogging and compaction around the roots cause symptoms above ground that can easily be mistaken for disease.

Getting the Diagnosis Right

Before you reach for a saw or a spray, gather information. Note the species, roughly how much of the crown is affected, when you first noticed it, and whether neighbouring trees of the same species look similar. Several affected trees in a row suggests something spreading; a single struggling specimen often points to site conditions.

Some problems are notifiable, and moving infected timber or foliage around the country can make matters much worse. Keep pruned material on site, clean your tools between trees, and never transport wood from a suspect tree.

If you are unsure, ask a qualified arboriculturist to take a look — most will give you an honest view on whether you are dealing with a cosmetic blemish or something that needs action. Many of our native trees live perfectly well with a degree of disease; the skill lies in knowing which warning signs to take seriously, and acting while you still have choices.