Clavibacter michiganensis subsp. michiganensis is the causal agent of bacterial canker of tomato. The disease was first described in 1910 in Michigan, USA. C. michiganensis subsp. michiganensis (from now on called clavibacter) was initially thought to be a phloem parasite, but was later found to be a xylem-invading bacterium. The host range comprises mainly solanaceous crops such as tomato, pepper, and eggplant. Strains show great variability in virulence and are usually described as being hypervirulent, hypovirulent, or nonvirulent. Clavibacter lacks a type III secretion system, and only a few virulence factors have been experimentally determined from the many putative virulence factors. As the molecular mode of infection by clavibacter is unknown, researchers have avoided intensive work on this organism. Genetic plant mechanisms conferring resistance to clavibacter are apparently complex, and breeders have yet to develop disease-resistant cultivars.