Answer: Based on your description, this may be the disease called apple scab. Here are some photos of it.
For a home apple tree, the best prevention of this (and many other problems) is to do a thorough clean up every fall, removing all fallen leaves and any left over fruit/leaves on the tree or on the ground, and disposing of it in the trash. You would also put any prunings in the trash. This should limit sources of reinfection for the next year.
Too, some varieties have much better resistance to scab than others. If your tree is especially susceptible, you may also need to use a fungicide.
Please contact your University of Maryland state extension service for information about growing apples in the home orchard and their recommendations on spraying as well as more definite diagnosis of the problem. They will be able to tell you what to use and how/when is best to use it for maximum results. Their web site has contact information including an 800 number for Maryland residents to call.
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