bacteria within the PHYLUMPURPLE BACTERIA. They are found in the SOIL, mainly in the RHIZOSPHERE. Most can infect dicotyledonous plants (see DICOTYLEDON and induce GALLS or ADVENTITIOUS roots. The species are generally defined on the basis of their plant PATHOGENIC properties. For example, Agrobacterium rhizogenes causes hairy root disease and Agrobacterium tumefaciens causes crown gall disease.