You have a couple different things happening.  Both are basically cosmetic.  The bumps are galls caused by a microscopic insect which lays eggs on the leaves.  The leaf responds with that growth (think of it a a scab).  There is no way you could have prevented the galls.  Secondly, you have a sucking insect attacking the leaves.  It may be fourlined plant bugs.  They probably will not be able to do much damage to a tree.  https://extension.umn.edu/yard-and-garden-insects/four-lined-plant-bugs