The bumps are small growths called galls. They do not hurt the plant, and once they have formed there is nothing you can do to get them off the leaves. Although galls can be caused by wounds or diseases, many are caused by small insects and mites. These animals lay eggs or feed on a leaf or small branch, an action that stimulates the plant to grow and cover the injured area. The immature insect lives and grows inside the gall.

Once the gall is formed, it will not take over the leaf or spread to other parts of the plant. Galls are harmless unless they girdle a branch, and then only that branch will be injured. Galls can be prevented only at the time the adult insect is laying eggs. Once the gall begins to form, no insecticide can penetrate it. It is almost impossible to time an insecticide application to kill the adult, and, in fact, insecticides may do more harm than good by accidentally killing the natural predators that attack the galls. Because of this, and because the galls do not hurt the plant, chemical control is not recommended. There are many different types of galls formed by insects and mites. Different galls are found in different parts of the country.