Bacterial diseases can be grouped into four broad categories based on the extent of damage to plant tissue and the symptoms that they cause, which may include vascular wilt, necrosis, soft rot, and tumours. Vascular wilt results from the bacterial invasion of the plant’s vascular system. The subsequent multiplication and blockage prevents movement (translocation) of water and nutrients through the xylem of the host plant. Drooping, wilting, or death of the aerial plant structure may occur; examples include bacterial wilt of sweet corn, alfalfa, tobacco, tomato, and cucurbits (e.g., squash, pumpkin, and cucumber) and black rot of crucifers. Pathogens can cause necrosis by secreting a toxin (poison). Symptoms include formation of leaf spots, stem blights, or cankers. Soft rot diseases are caused by pathogens that secrete enzymes capable of decomposing cell wall structures, thereby destroying the texture of plant tissue—i.e., the plant tissue becomes macerated (soft and watery). Soft rots commonly occur on fleshy vegetables such as potato, carrot, eggplant, squash, and tomato. Tumour diseases are caused by bacteria that stimulate uncontrolled multiplication of plant cells, resulting in the formation of abnormally large structures.
