Scab (which is caused by a fungus - Venturia inaequalis) can infect both apples and pears and it attacks almost all parts of the tree - new growth, foliage, flowers and fruit. Scab stunts and sometimes distorts leaf growth. It is clearly visible, and can be seen as brown patches of dead tissue on apples (black patches on pear foliage). Leaves with scab, yellow and fall earlier than usual. If the attack is severe, the tree can be defoliated. Fruit tends to be undersized and exhibits the same blotchiness and as it grows these harden and often crack. Put simply, the fruit looks "scabby" (see picture). Actually, as far as the fruit is concerned, the disease is only skin deep, and it is perfectly edible if peeled.
We have had a spate of relatively warm autumns and winters which have meant leaves have fallen later, and emerged earlier than usual. The consequence of this is that they have less time to disintegrate/decompose and so provide a better home for over wintering pests and diseases - especially scab. In the spring, apple scab spores settle on the tree, regenerate and release more spores in late March- early April and 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.
As is so often the case in gardening, hygiene is crucially important. Rake up and burn all leaves in the orchard. This alone will make a significant difference to the level of infection the following spring. Pruning out and burning infected young shoots (the disease causes them to have cracked, rather coarse bark) in the winter also removes overwintering spores. Feeding is important too - well planted trees, that are mulched with good, well rotted organic matter in the spring are stronger and more resistant that weedy, underfed specimens. Before bud burst, it pays to mow the orchard - this helps break down any remaining leaves from the previous year and disturbs the fungus before it is ready to spread. Growing resistant varieties of apple and pear is also effective. 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, Tydeman's Late Orange and cookers that are less likely to succumb include Grenadier, Lanes Prince Albert, Lord Derby and Reverend W. Wilks. Of the pears, Beurre Hardy is the most resistant. Ultimately however, scab can only be eradicated with chemical use. Systhane is an excellent protectant and is widely available. Once an outbreak is spotted, it should be dealt with as soon as possible using a suitable eradicant. There are many on the market, but those containing mancozeb are very effective. If the terrors of scab have not completely put you off (seriously, good hygiene alone severely limits its ability to affect apple trees and pears) why not have a look at our large (and increasing) range of fruit trees.