Why British gardens suit apple diseases so well

An apple tree is a long-term commitment, and in our climate it will spend much of its life in conditions that fungi adore. Mild, damp springs, warm humid summers and plenty of rain-driven spores mean that a tree which shrugs off disease in a drier country can look decidedly sorry here by August. The good news is that most of the problems you will meet in a British garden or allotment are recognisable, and most can be managed by paying attention to airflow, hygiene and timing rather than reaching for a sprayer.

Get to know four culprits in particular: canker, scab, powdery mildew and brown rot. Once you can name what you are looking at, the practical response usually becomes obvious.

Apple canker: the sunken wound that keeps spreading

Canker is the most serious of the four, because it kills wood rather than just spoiling fruit. Look for sunken, dead patches of bark, often centred on a bud, a pruning cut or a snapped branch. The bark over the affected area may look darker and cracked, and in wet weather you can sometimes see small creamy-white or reddish pustules of spores pushing through the surface. Cut into the edge of a canker and you will find dead, brown tissue instead of healthy green-white wood.

Left alone, cankers girdle a branch, which then dies back. The fungus spreads most readily during wet autumn and winter weather, entering through wounds — including the wounds you make yourself with the secateurs.

• Varieties differ markedly: Cox's Orange Pippin and many modern dessert apples are prone, while Egremont Russet, Bramley's Seedling and Ashmead's Kernel cope far better.
• Prune out cankered wood in dry late-summer weather, cutting back into clean wood rather than leaving a stump, and clean your blades between cuts.
• Never compost cankerous prunings; burn them or take them to the tip.

Apple scab: the blotchy one

Scab is the disease behind those dark, corky, cracked patches on apples that make a Cox look as though it has been dropped on gravel. It also shows on the leaves, where you will see velvety olive-green to black blotches, often along the veins. Leaves may yellow and drop early; fruit develops scabby lesions that crack as it swells, giving rots an easy way in.

Scab overwinters mainly in fallen leaves on the ground beneath the tree, releasing spores during wet spring weather to infect new growth. This is why a tree growing on grass that is never cleared is far more affected than one where the leaf litter is tidied up.

• Rake up and remove fallen leaves in autumn — this single habit does more than anything else.
• Avoid overhead watering, which extends the wet periods the fungus needs.
• Look for scab-resistant varieties such as Discovery or Katy if you are planting afresh.

Powdery mildew: the white ghost on new growth

Unlike scab, mildew does not need rain. It thrives in warm, dry spells with humid air and a sheltered, airless position, and it particularly likes trees that are short of water at the roots. You will notice a grey-white powdery coating over the tips of shoots and on young leaves, which then curl, stiffen and become distorted. Affected shoots are stunted and often fail to fruit.

The fungus survives the winter inside infected buds, so those shoots are the source of next year's trouble. Pruning them out in late winter, while the tree is dormant, removes the infection before it can spread.

• Cut out mildewed shoot tips and buds during winter pruning.
• Water trees properly in dry spells and avoid heavy nitrogen feeds, which push soft, susceptible growth.
• Improve airflow by thinning congested branches and keeping the tree well spaced from hedges and fences.

Brown rot: the one that ruins the crop in storage

Brown rot announces itself with a soft, pale brown patch that spreads quickly across the fruit, followed by concentric rings of creamy-grey pustules. The fruit shrivels into a dark, leathery mummy that either clings to the branch or drops. Wasps, birds, codling moth damage and scab lesions all create the small wounds the fungus needs to get started, which is why a tree with other problems often loses its crop to brown rot as well.

The same group of fungi causes blossom wilt in spring, where flowers and the shoot tips just behind them wither and die, staying on the branch instead of dropping. In storage, one infected apple can pass rot to its neighbours, so inspecting the crop regularly really matters.

• Remove rotting and mummified fruit from the tree and the ground as soon as you spot them — do not leave them lying about.
• Handle fruit carefully at harvest; bruises are open doors.
• Store apples in a cool, dry, well-ventilated place and check them weekly, discarding any that show soft brown patches.

Cultural controls that do the heavy lifting

None of these diseases needs harsh chemistry to be kept in check in a domestic setting. What they respond to is consistent garden hygiene and a tree that is not stressed. Fungi overwinter in fallen leaves, mummified fruit, cankered wood and infected buds, so the most valuable thing you can do is break that cycle each winter.

Keep the ground beneath the tree clear of leaf litter and rotting fruit. Prune for an open, airy framework and prune out disease in dry weather, cutting back to healthy wood and cleaning tools as you go. Thin fruitlets in early summer so the crop has room to swell and air can move between the apples. Feed modestly, mulch to hold moisture, and water deeply in dry spells rather than little and often.

Above all, choose varieties that suit your plot. If canker has been a misery on your allotment, plant something with real resistance next time, and the whole business of growing apples becomes far more enjoyable. A well-grown tree in a British garden will still show the odd scabby leaf or mildewed tip — that is normal. The aim is not a flawless tree, but one that carries a decent crop and survives our weather with dignity.