Apple scab, frogeye leafspot, fireblight? It would be best if you can provide a photo of the problem. A close up shot of the leaves in question would help. A shot of the branch would help too. If blackened it might be fireblight.

Here's some of the leaves and what they look like on the tree.

Julie:

The first shot with leaves showing black spots on them definitely looks like a fungal issue. Some ornamental crab varieties are very prone to scab, leafspot ect if conditions are right and the pathogen is present. A preventative spray with a fungicide would help but in your case it may only slow down the spreading of the issue. Most fungicides are preventative not curative. Best to spray prior to infection.

The second shot is odd because you have so many dead leaves and some that are brown part way from the tip of the leaf. I have to wonder if something else is also going on.

I also see some of your branches are covered with lichens. Is this tree in part shade? Just asking as usually lichens like moist and shade but they can grow on tree bark even in a sunny site. It will not hurt your tree but if the tree is in shade, it definitely can be more prone to fungal issues as the foliage will not dry off as fast after a rain.