Maple bladder gall is a common example of leaf galls. Small green bumps appear on the tops of silver and red maple leaves, turning bright red. This is due to eriophyid mites feeding on newly developing leaves. While it may look bad, in reality the health of the tree is not threatened. Control is not practical or necessary.
Galls frequently appear on oaks.
They may small bumps or larger, more visible growths. These are caused by a very small wasp. Some may also affect twigs, such as the gouty oak gall, and actually cause some dieback. Most leaf galls on oak are not damaging, however. Leaves of hackberry trees often have the hackberry nipplegall, caused by an insect called a psyllid.