Sounds like apple scab. My neighbor's crab apple is also shedding leaves. Scab overwinters on fallen leaves and in the spring when the green tissue is exposed the scab lesion on the old leaves discharge spores into the air. The spores sprout on wet apple leaves and if the infection period is long enough they will infect the leaves. In a week or two you will see gray green spots on the leaves and these spots will enlarge. Infected leaves often fall off the tree. The spores from the old leaves are almost all gone now but the fungi growing on the leaves are now releasing spores and these spores can infect new leaves now. You can save the new leaves by spraying them with a fungicide to control scab. I recommend materials containing captan. Scab will not kill the tree but losing its leaves will weaken the tree. Most home owners do not think controlling the disease is worth having to apply fungicide sprays every 7 to 10 days in the spring from green tip to about a month after bloom.
We had a very wet spring and apple scab is very common. Here is a link to information about the disease.http://www.ipm.msu.edu/diseases/apple_scab .