It sounds like these trees may have apple scab, which is a common fungal infection on apples and pear trees. It affects both leaves and fruit so it is well worth tackling immediately. Firstly clear up all leaves that have fallen from the trees and either burn them or dispose of them with household waste as this will prevent the fungus from over-wintering so easily. Spray the tree with a systemic fungicide in the spring, which will be available at all good garden centres. Look out for the active ingredients Difenoconazole or Myclobutanil on the bottle and follow the manufacturers instructions. Also check it is suitable for use on fruit trees. Also consider pruning out any congested branches to allow good air circulation (the fungus thrives in warm damp conditions).