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.
Elms often get galls such as the cockscombgall, caused by an aphid.
This irregular gall looks like rooster's combs on the leaves. Other shade tree, shrub, fruit crop, and even perennial flower foliage may have galls appearing. Treatment is rarely suggested, and would have been needed prior to the gall forming. This usually is not practical. Once the gall had formed, even if the pest is killed, the gall remains since it is actually plant tissue.