Apple scab is the fungal disease venturia inaequalis, it only attacks trees belonging to the genus malus.
It occurs annually in all districts, but varies in intensity, and is generally most severe after a wet May.
The fungus spends the winter on fallen leaves and infected shoots, and appears as small blister like pimples on fallen leaves and young shoots.
In spring airborne spores are released from these infected leaves, which causes infections on the newly developing foliage.
As these infections develop, they release large quantities of another airborne spore which spreads further infection throughout the growing season.
The scab marks appear on leaves from mid-spring until leaf fall in autumn.
Young shoots can blister and split making the ideal spot for new spores to over winter.
Black scabby blotches develop on the fruit as it matures causing the skin to distort and or crack.
The eating quality is not always affected by this, but the storing qualities usually are due to the cracked skin leading to premature rotting.
Typical sign of Scab
Some control can be achieved by gathering up and burning fallen leaves, and by cutting out diseased shoots in winter.
Complete control can be obtained by spraying with a fungicide, at green cluster, flower bud, petal fall, and fruitlet stages.
If necessary, make repeat applications at fortnightly intervals until mid-July.
Complete control may not be possible with in large trees due to accessabilty, but with the more modern varieties grown on dwarfing root stock the task should be more practicle.
Some apple varieties are resistant to this disease so it is worth considering this prior to purchase.
Reputable suppliers should be able to advise you which varieties least affected.