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