How does crown gall form on stems or roots?

Crown gall is a plant disease caused by the bacterium Agrobacterium tumefaciens. The disease manifests as woody, tumorlike growths known as galls, which typically form on the roots, lower stems, graft sites, or bud unions of susceptible plants.12

The formation of these galls is driven by a specific genetic mechanism: the pathogenic bacterium carries a plasmid, which is a small, circular piece of DNA. When the bacterium infects a plant—often through wounds or natural openings—this plasmid facilitates the conversion of normal plant cells into cells that undergo excessive multiplication.2

Initially, these growths appear as small, cream-colored or greenish enlargements. As the disease progresses, the galls increase in size, become rough-surfaced and wrinkled, and eventually turn brown or black. Because these tumors disrupt the plant's normal development, infected plants often lose vigor and may eventually die. Management strategies include using pathogen-free nursery stock, practicing crop rotation, and protecting plants from physical injury that could provide entry points for the bacteria.12