It's a fungal disease called black spot. Hard to get rid of completely, but you can help reduce the effect.
First, always make sure that all the fallen leaves are taken away in autumn, as the spores over-winter on them. This also applies to any fruit left hanging, or that has dropped onto the ground. You can make sure that they are destroyed by burning them, removing them from the site or getting them completely rotted down in an active compost heap (difficult over winter).
In spring, when the blossom comes out is around the time that the fungus becomes active again. You could spray the entire tree with a fungicide then. It's now thought best not to spray when you see signs of the black spot as this could result in it becoming resistant to fungicides. You could keep a close eye on the leaves, and remove any that start to show signs of the disease.
Good hygiene, i.e. removing leaves etc, is probably the most effective way of dealing with black spot in a small scale area.
