Apple Scab & Pear Scab

Apple Scab & Pear Scab Disease 

These diseases can both orchard apples and pears and their respective ornamental equivalents such as crab apples and weeping pears.

Scab, which is caused by the fungus Venturia inaequalis, is most common in warmer, more humid parts of the UK, mostly in the South and West. It should not put you off growing apples and pears in those areas, as choosing the right varieties and good hygiene make a huge difference, before you even reach for a fungicide.

Scab Symptoms

Scab attacks almost all parts of the tree: new stems, foliage, flowers and fruit. It stunts and distorts leaf growth, and can clearly be seen as brown patches of dead tissue on apple leaves, or black patches on pear foliage.
Leaves with scab turn yellow and fall earlier than usual. If the attack is severe, it can defoliate the tree.
Fruit tends to be undersized and scabby all over (see picture); as it grows, these scabs harden and often crack. The disease is only skin deep, so the fruit is perfectly edible when peeled, which is some consolation. 

Scab Causes

Relatively warm autumns and winters cause leaves to fall later, but emerge earlier than usual. This means that the fallen ones have less time to decompose before the vulnerable new leaves appear, thus providing a better home for over wintering scab spores (among other pests & diseases). In the spring, scab spores settle on the tree, regenerate and release more spores in late March to early April, and then again and again at intervals of a few weeks. The disease therefore appears to be progressive, when it is in fact caused by waves of infection.

Scab Prevention

As is so often the case in gardening, hygiene is crucially important.

• Raking up and burning all Autumn leaves in the orchard alone will bring a significant reduction in infection the following spring, for scab and most other fungal and insect pests.
• Pruning out and burning infected young shoots, which will have cracked, coarse bark in winter also removes overwintering spores.
• Well planted and watered trees, that are mulched with woodchips or other organic matter in the spring, are stronger and more resistant than stressed one.
• Before bud burst, mow the orchard, which helps to break down remaining leaves from the previous year, and disturbs the fungus before it is ready to spread.
• Grow resistant varieties. Resistant eating apples include Arthur Turner, Ashmeads Kernel, Beauty of Bath, Charles Ross, Discovery, Egremont Russet, Ellison's Orange, Golden Delicious, Greensleeves, Kidds Orange Red, Lord Lambourne, Sunset, and Tydeman's Late Orange. Resistant cookers include Grenadier, Lanes Prince Albert, Lord Derby and Reverend W. Wilks. Of the pears, Beurre Hardy is the most resistant.
• Plant your orchard in a poly-culture where, at the very least, no two trees of the same cultivar are next to each other. Better yet, plant a sequence like apple - cherry - pear - plum, thus keeping the species separated. This prevents any disease or pest from having a wall-to-wall banquet. For bonus points, mingle a few nitrogen fixing trees like Red Alder, Alnus rubra (which produces fewer suckers than other alders, making it easier to manage) in there and coppice them for firewood - credit to Stefan Sobkowiak in Canada for promoting this practice.

Scab Chemical Treatment

At the time of writing, there are no seriously effective, legal chemical controls for the home grower. The nearest things to an organic fungicide against scab are copper based - sulphur is less efficient, and some "sulphur shy" varieties really don't like it.

Follow the manufacturer's instructions - generally, the first spray should take place when the buds are closed, and the second when they are barely beginning to show the first signs of colour as they open. Spray when the air is still, in the evening when there are fewer pollinating insects around.