Leaf closeup photos, upper and lower side, will help. Do you have an apple? A photo of that may help, also.
Provide a history of the tree and your current practices. For example, have you been doing winter maintenance?
EC631 is Extension’s publication about disease management. With additional photos I hope to tell you which disease(s) your tree has. Note that good pruning and careful removal of fallen fruit and leaves is recommended control for most diseases and pests.
Your tree looks too tall to reach, but it might just be the photo angle. EC1005 about restoring an old neglected apple tree may be helpful to you, even though your tree doesn’t look old. 

Hello!

Wow, this sure looks like a rust disease. Can you take another photo showing the underside of those affected leaves? Do you see any bumps on the underside of the leaves? That will give us a big clue to help solve this mystery.

The cool wet spring made for great conditions for foliar issues! We're seeing all sorts of problems this year. 

I attached a picture of the underside of some leaves. The brown spots are raised bumps. 

Thanks for that photo-very helpful. This does look like a disease caused by a rust fungus. This spring was an exceptionally good year for leaf diseases-including this one! 
In general you'll want to practice good sanitation by raking up and disposing of the leaves that fall. Cross your fingers that next spring is warmer and drier so the rust doesn't get the upper hand.

Thank you!