Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.
Most plant pathogenic bacteria are rod-shaped (bacilli). In order to be able to colonize the plant they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are known: uses of Cell wall-degrading enzymes, Toxins, Effector proteins, Phytohormones and Exopolysaccharides
Pathogens such as Erwinia, use Cell wall-degrading enzymes to cause soft rot. Agrobacterium changes the level of auxins to cause tumours with phytohormones. Exopolysaccharides are produced by bacteria and block xylem vessels, often leading to the death of the plant.
Bacteria control the production of pathogenicity factors via quorum sensing.
Significant bacterial plant pathogens:

• Burkholderia
• Proteobacteria
• Xanthomonas spp.
• Pseudomonas spp.
• Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, and it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system."