galls

Galls are abnormal growths that form on plants in response to various organisms, most commonly insects and mites but also fungi, bacteria, nematodes, and some parasitic plants. They occur on leaves, stems, roots, or fruits and are often highly specific to the plant genus and the inducing organism.

Germination and development begin when a gall-inducing organism, such as a gall wasp or mite, lays eggs or otherwise interacts with plant tissue. The organism releases chemicals or mechanical stimuli that alter the plant’s growth hormones, triggering localized cell division and differentiation. The resulting gall provides a protected habitat and a nutrient source for the developing larva or spore, while the host plant’s tissue may be distorted but not immediately killed.

Galls vary widely in size, shape, and texture—ranging from smooth, spherical structures to woody or fleshy masses. Some are conspicuous, while others are subtle. Notable examples include oak galls (often called oak apples) formed by cynipid wasps on oaks, mossy rose galls on wild roses caused by Diplolepis rosae, and the galls produced by cedar-apple rust fungi on junipers that release spores onto apples or related plants. Crown galls caused by the bacterium Agrobacterium tumefaciens form tumor-like outgrowths at the crown of various woody plants, and some root-knot nematodes create galls on roots.

Ecologically, galls can influence plant vigor and aesthetics. They are of interest in biology and ecology as examples of plant–insect–fungus interactions, and historically some galls have been harvested for tannins and other compounds. Control, when needed, focuses on managing the inducing organism or protecting susceptible hosts.