Crown gall

Crown gall (Agrobacterium tumefaciens) is a tumour-forming disease of over 140 species of dicot.

Symptoms
Crown gall manifests itself initially as small swellings on the root or stem near the soil line, and occasionally on aerial portions of the plant. Young tumors, which often resemble the callus tissue that results from wounding, are soft, somewhat spherical and white to cream colored. As tumors become older, their shape becomes quite irregular, and they turn brown or black. Tumors may be connected to the host surface by only a narrow bit of tissue, or may appear as a swelling of the stem, not distinctly separate. The tissue can be spongy and crumbling throughout the gall or can be woody and knot-like. Several tumors may occur on the same plant and may rot from the surface of the plant completely or partially, possibly developing repeatedly in the same area season after season. Additional symptoms include stunting, chlorotic leaves, and plants may be more susceptible to adverse environmental conditions and secondary infection.

Control
Introduction of pathogenic A. tumefaciens strains can be avoided by thorough inspection of nursery stock for crown gall symptoms. Susceptible varieties should not be planted in soils known to be infested with the pathogen. These soils should be planted in a monocotyledonous crop like corn or wheat for several years. Nursery stock should be certified crown gall-free and should be budded rather than grafted. If the threat of crown gall exists, all practices that wound tissue should be avoided and chewing insects should be controlled.