Apple Scab

Crab apple trees commonly get apple scab and over the years can cause the tree to weaken and die.  It presents usually in mid summer when the leaves turn yellow or gold and prematurely fall from the tree.  

Clean up all the leaves and fruit this fall. Next spring, spray the trees with lime-sulfur while the trees are still dormant and again after leafing. Use a fungicide known as Captan. Repeat again in 10 to 14 days.

I also got this response from a gardening question and answer board:

It may have cedar apple rust, a fungus that effects crab apple and apple trees, the leaves start out with a small yellow spot that gets bigger and has some red around it normally. Then the leaves fall off. There's not much you can do about it this year, but next year you will need to spray the tree after it blooms. Spray it with copper sulphate every seven days for a month. that should keep it from getting it. Source(s): State Certified Ornamental & Turf Specialist since '85