They may be simple lumps or complicated structures, plain brown or brightly colored. There are 1500 species of gall producers the majority of which are insects and mites. Some galls form where insects or mites feed or lay eggs. They may also develop as a response to infections by several kinds of fungi, bacteria and viruses. Galls affecting leaves are seldom if ever a serious problem.
Galls affecting twigs such as the gouty oak gall and horned oak gall can be more serious.
In most cases, galls are unsightly but not damaging to the tree. Small plants may be stunted because the water and nutrient circulatory system of the plant may be damaged. Infested leaves, which can be twisted and curled, are usually able to carry out photosyntheses at near normal levels. Less striking and non-apparent galls that can occur on twigs, small branches and roots can, over time, kill and weaken portions of a tree or cause a general decline in plant vigor. Galls can be confused with normal parts of the tree, for example seed bearing structures or insects such as scale.