Hello
Your leaves have a few types of gall on them, most look like saucer galls. After our mild winter and this warm, wet spring, we are seeing high numbers of a variety of insects. So, it isn’t surprising to see this, since galls are made by tiny insects. 
https://bygl.osu.edu/index.php/node/2185
While unsightly, they do no permanent harm to the trees. Gall forming insect populations often cycle from high to low, and next year you may see very few.
 If we enter a droughty period this summer or fall (more than 2 weeks without substantial rain) keep the trees most affected by brown or dropped leaves well watered, without drowning the tree. 

Thanks very much