Rhodococcus fascians (known as Corynebacterium fascians until 1984) is a Gram positive bacterial phytopathogen that causes leafy gall disease. R. fascians is the only phytopathogenic member of the genus Rhodococcus; its host range includes both dicotyledonous and monocotyledonous hosts. Because it commonly afflicts tobacco (Nicotiana) plants, it is an agriculturally significant pathogen. Physiology and morphology R. fascians is an aerobic, pleiomorphic, actinomycete that is nonmotile and does not form spores. When grown on the surface of an agar plate, colonies are orange in color and appear both smooth or rough. Virulence R. fascians can be a pathogen of plants, both angiosperm or gymnosperm. Infected plants show typical symptoms, such as leaf deformation, witches broom and leaf gall, which development depends on the plant's cultivar, plant's age, and the bacterial strain. Leaf deformation consists of widening of parenchyma and growth of vascular system, resulting in wrinkling of laminae and widening of veins. Leafy gall is a gall originated from a bud which would not develop under normal conditions. All effects coming from the infection of R. fascians do not depend on plant cells transformation (as for Agrobacterium tumefaciens or Agrobacterium rhizogenes), but on expression of virulence-related genes of bacterium and on the production of compounds that can interfere with normal plant growth and development. During the infection, R. fascians usually stays outside vegetal tissues, near a junction or cavity of a plant's cell walls, maybe to avoid environmental stresses. Presence of R. fascians was also observed in intercellular spaces inside tissues (in leaf or galls) and even inside cell walls. Presence of R. fascians on the infected plant is necessary, not only for the initiation of infection, but also for its maintenance. Virulence genes Virulence in R. fascians is controlled by genes on a plasmid (strains lacking that plasmid are not virulent) and on the chromosome. Using deletion mutations, it was possible to identify three loci on the plasmid: fas, att, and hyp, and one locus on the chromosome, vic. The fas is an operon made of six genes (orf 1-6) and a regulatory gene, fasR. Because deletions of some fas genes give a non-virulent phenotype, for fas a main role in virulence was proposed . Gene fasR is an araC-like transcriptional regulator. Its transcription can be induced in vitro in cultures containing certain carbon sources (such as glucose, sucrose, arabinose, glycerol, pyruvate, mannitol, mannose) or nitrogen sources (such as histidine),