Gall

Galls are abnormal growths produced by plants in response to chemical stimuli from insects, mites, nematodes, fungi, or bacteria. The causal organism essentially hijacks the plant's growth processes to create a customized shelter and food source. Galls come in a remarkable variety of shapes, sizes, and colors depending on the causal agent and the host plant.

Common gall-forming organisms include cynipid wasps on oaks (producing the familiar round oak apple galls), eriophyid mites on maple leaves (causing felt-like patches or finger-like projections), and the bacterium Agrobacterium tumefaciens (causing crown gall on many woody plants). Each gall-former typically produces a distinctive gall type on specific plant tissues.

Most leaf and twig galls are cosmetically concerning but do not seriously harm established trees. The plant tissue in the gall is essentially sacrificed, but the overall impact on tree health is minimal unless populations are extremely high. Exceptions include root crown galls and galls that girdle young branches. Chemical control of galls is rarely practical or necessary. The most effective approach is to maintain overall tree health and tolerate the presence of these fascinating biological structures.