oak galls

Oak galls.

A gall is an abnormal growth that occurs in plants. It may be caused by insects, mite, nematodes, fungi, or bacteria. By definition, galls are self-limiting growths requiring the continued presence of the inciting organism for full development. They thereby differ from tumors, where abnormal growth continues in the absence of the inciting organism. However, in crown gall disease of crucifers (e.g., cabbage), the bacterium Agrobacterium tumefaciens is required for the initiation of the gall, but subsequent growth becomes tumorous and does not require the presence of the bacterium. The commonly seen spongy "apple" gall on oak trees is caused by the gall wasp larva.