Why apple trees in Britain struggle

Few garden trees repay attention like an apple. Yet our damp, mild climate is almost tailor-made for fungal trouble. Scab, canker and powdery mildew all thrive in the sort of weather we get most springs and summers, and a tree left unpruned for a few years soon develops the still, humid interior where these problems take hold.

The good news is that all three announce themselves clearly. Learn the signs, act early, and most garden trees will carry on cropping for decades.

Apple scab: the most familiar blemish

Scab is caused by a fungus that overwinters in fallen leaves and in infected wood, releasing spores in spring. It is worst in wet seasons, especially on susceptible varieties such as Cox's Orange Pippin.

Look for velvety, olive-green to sooty black patches on the upper surface of leaves, sometimes with a paler edge. Badly affected leaves turn yellow and drop in midsummer, which weakens the tree and reduces next year's crop. On the fruit you will see dark, corky, scabby lesions, often cracked and misshapen around the calyx end.

• Rake up and bin fallen leaves rather than composting them — this breaks the fungus's life cycle.
• Thin congested shoots in summer so leaves dry quickly after rain.
• Prune out badly marked spur systems in winter, cutting back to clean wood.
• Feed with a balanced fertiliser rather than a nitrogen-heavy one, which pushes soft, vulnerable growth.

Apple canker: the bark disease that girdles branches

Canker is the most serious of the three. It kills bark and wood, and if it rings a branch completely, everything beyond it dies. It usually enters through a pruning wound, a leaf scar or a frost-damaged bud.

The first sign is often a small, sunken, dead-looking patch on the bark. As it grows you will see the surface crack and flake, revealing concentric rings of reddish-brown tissue beneath. In damp weather, especially autumn, tiny creamy-white pustules appear in a ring around the edge. Whole spurs and young shoots wither; fruit can develop small circular rots around the eye. On older trees, cankers may be several centimetres across and clearly sunken.

Cut out affected wood with a clean saw, going back well into healthy tissue until you see unblemished white wood and green bark. Because the fungus is most active in wet winter weather, the best time to prune canker is a dry spell in late summer or early autumn, when spores are less likely to land on the fresh cut. On a bad canker, cut back to a side shoot or remove the branch entirely rather than leaving a stub. Clean your tools with methylated spirits between cuts and afterwards.

Powdery mildew: the white coating on young growth

This fungus does not need wet leaves at all — high humidity is enough. It shows as a grey-white, powdery film on young leaves and shoot tips, which then curl, pucker and distort. Affected shoots are stunted, and the tips may die back and turn silvery-brown.

Mildew overwinters inside dormant buds. Those buds open later than the rest and look pale, silvery and slightly swollen — a useful clue in early spring. Cut them out as soon as you spot them, along with any distorted shoot tips, cutting back to healthy wood.

• Prune affected growth in early summer when the fungus is visible and dry weather follows.
• Water around the base rather than over the foliage, and water thoroughly but less often.
• Avoid high-nitrogen feeds, which produce the soft leafy growth mildew loves.

Other problems worth knowing

Not everything odd on an apple tree is one of the big three, and it helps to rule out other causes before you reach for the saw.

• Brown rot: a soft brown rot of fruit with concentric rings of buff-coloured spores on the surface. Fruit mummifies and hangs on the tree. Remove and bin every affected fruit, on and off the tree.
• Woolly aphid: white cotton-wool-like clumps in cracks and around old pruning wounds, causing knobbly galls that can later become canker. Brush them off with a stiff brush in winter.
• Bitter pit: small sunken brown spots on the fruit, a calcium deficiency rather than a disease. Water steadily through dry spells and avoid heavy winter pruning.
• Honey fungus: white fungal sheets beneath the bark at the base of a dying tree, with black bootlace-like strands in the soil. There is no cure; remove the tree and do not replant an apple in the same spot.

Pruning, airflow and a simple yearly routine

Most of the work is prevention. An open-centred tree — a goblet shape with a clear middle and well-spaced main branches — dries fast and is far harder for these fungi to colonise. Aim for branches roughly 45 to 60 centimetres apart, removing anything crossing, rubbing or growing inwards.

Prune in winter to shape the tree, but keep canker cuts for a dry spell in late summer if your tree is prone to it. Always cut to a bud or a side shoot, never leaving a snag, and make sure your blade is sharp so the bark is not torn.

Through the year, keep to a simple rhythm. In spring, cut out mildewed buds and tips. Through summer, thin heavy crops and take out crowded shoots so light and air reach the fruit. In autumn, rake up leaves and clear fallen fruit. In winter, prune for shape and mulch with well-rotted compost, keeping it clear of the trunk. Do that, and your tree will shrug off most of what the British weather throws at it.