Few things are more disheartening than nursing a young apple tree through its first few years only to watch the leaves mottle, the fruit blemish, or a branch die back. The good news is that most of the problems UK growers meet are recognisable long before they do serious damage — provided you know what you are looking at. A ten-minute walk round your trees every fortnight from bud burst to leaf fall will teach you more than any book. Here is what to watch for, and how to tell the main culprits apart.

Start by Learning What Healthy Looks Like

Before you can spot disease, you need a baseline. On a healthy apple tree in summer, leaves are an even green with a slight sheen, evenly sized across the canopy. New shoots are firm and straight. The bark on young wood is smooth and olive-brown; on older wood it is grey and slightly flaky, but never sunken or cracked. Fruit develops a clean skin unless it has been rubbed by branches or wind.

Walk round each tree at least once a fortnight and check three zones: the leaves (both surfaces), the bark on trunk and main branches, and — once they form — the fruit. Note anything that looks out of place, and check whether it is spreading. Most fungal problems get worse over weeks; nutritional or weather damage tends to appear suddenly and then stop.

Apple Canker: Sunken Bark and Dead Patches

Apple canker is the most serious of the common diseases because it kills wood rather than just spoiling fruit. Look for sunken, dead patches of bark on branches, often centred on an old pruning cut, a bud, or a leaf scar. The bark over the affected area cracks, peels, and may look darker than the surrounding wood. Cut into the edge of a suspect patch with a sharp knife and you will see a wedge of dead, brown tissue, sometimes with fine concentric rings.

In wet weather from late spring onwards, cankers produce small creamy-white pustules that ooze salmon-pink spores — this is the giveaway. Above a canker, shoots and spurs die back, and fruit may rot around the stalk. Cankers are worse on heavy, wet soils and on varieties such as Cox's Orange Pippin and Gala.

• Prune out infected branches in dry weather in July or August, when spores are least active, cutting back at least 15cm into clean wood.
• Disinfect secateurs between cuts and burn or bin the prunings — never compost them.
• Improve drainage and avoid high-nitrogen feeding, which pushes soft growth that cankers exploit.

Apple Scab: Olive Blotches on Leaves and Fruit

Scab is the disease most UK gardeners meet first, and wet springs make it far worse. On leaves you will see olive-green to brown velvety patches, usually on the upper surface, which darken to near-black as they age. Badly affected leaves curl, yellow, and drop in midsummer, weakening the tree.

The fruit symptoms are unmistakable: corky brown or black scabs, often near the calyx end, which crack the skin and let in rots. Fruit may grow misshapen and split. Because scab overwinters in fallen leaves on the ground, hygiene is half the battle.

• Rake up and remove fallen leaves in autumn — bag them for council green waste rather than adding them to your compost heap.
• Prune to open up the centre of the tree so leaves dry quickly after rain.
• If you grow susceptible varieties such as Cox or Bramley, consider a sulphur-based spray at bud burst and again at petal fall, following the label exactly.
• For new plantings, look at more resistant varieties such as Ashmead's Kernel, Discovery, or Egremont Russet.

Powdery Mildew: White Dust and Curled Shoots

Powdery mildew looks quite different from scab. Instead of dark blotches, you get a pale grey-white powdery coating on young leaves and shoot tips, as though someone has dusted the tree with flour. Infected leaves curl upwards, become brittle, and may turn silvery or purple-tinged. Shoot tips are stunted and distorted, and fruit can carry a fine net-like russeting.

Unlike most fungi, mildew does not need free water on the leaf — it thrives in warm, dry days with humid nights, and in still air. It overwinters inside the buds, so spring growth emerges already infected.

• Prune out mildewed shoot tips in winter, cutting back to clean, firm wood.
• Water the roots, not the canopy, and avoid over-feeding with nitrogen.
• Thin congested growth in summer so air moves through the tree.
• Varieties such as Discovery, Lord Lambourne, and James Grieve show good resistance.

Don't Blame Disease for Everything

Several common complaints get mistaken for disease. Bitter pit — small sunken brown spots just under the skin, usually on the blossom end — is a calcium disorder, not an infection; it is worse on light soils and in dry summers. Brown rot shows as firm brown fruit with concentric rings of creamy spores, and usually follows wasp or scab damage. Woolly aphid produces fluffy white colonies on bark and old wounds, but the fluff is waxy and sticky, not powdery, and ants often farm it. And corky russeting round the stalk end is often just spring frost or wind, harmless and edible.

A Simple Seasonal Check Routine

Spring: watch emerging leaves for scab blotches and mildewed tips; check buds and spurs for canker pustules. Early summer: inspect fruit weekly as it swells — this is when scab and canker symptoms show clearly. Late summer: look for cankers with spore pustules after rain; this is also your pruning window. Autumn: clear fallen leaves and rotting fruit, and check the trunk for new cankers. Winter: prune out mildewed tips and dead wood while the tree is dormant, and note which varieties struggle where. Keep a notebook — after two or three seasons you will know your trees' particular weak spots, and spot trouble while it is still small.