Yikes - that looks like apple scab disease brought on by damp weather and it is very hard to control once the leaves have turned yellow. It will infect any fruit that does appear as well. You can try using a fungicide made for apple scab disease like Ortho Tree and Shrub Fruit Spray and also clean up fallen leaves especially in the fall so the disease spores do not reinfect again. Encourage better air circulation by pruning the tree into an open vase shape and keep the tree healthy by mulching the soil and giving enough water. If apple scab is common in your area consider replacing your tree with a resistant variety such as "Freedom" apple trees. Keep growing, Marianne Binetti