The fungal spores over winter on fallen leaves or in severe infestations in twig lesions. Wet or dull weather around blossom time will encourage the spread of this disease. The fungal fruiting bodies are produced in spring and infect young leaves which develop white spots and then become blackened, then grey and some leaf  blistering may occur. The spores are blown or splashed onto the leaves and fruit. Dark spots appear on the fruit and turn into a blackish brown corky area which may nearly cover the whole fruit. 

To combat this infection rake up fallen leaves and burn them. Prune out and destroy any scabby shoots. This will reduce over wintering places for the fungal spores. 

Spray with a suitable fungicide such as Systhane Fungus Fighter from bud burst. As with all chemicals follow the manufacturer's recommendations.