Rhizobium bacteria are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes. Additionally, pathogenic Agrobacterium species have recently been assigned to the genus Rhizobium. These pathogenic bacteria were formerly known as Agrobacterium tumefaciens or Agrobacterium radiobacter, Agrobacterium vitis, Agrobacterium rhizogenes and Agrobacterium rubi, now the first part of the name is changed to Rhizobium.
Rhizobium rhizogenes or Rhizobium radiobacter causes crazy roots, hairy roots and crown gall.
Life cycle and appearance of Crazy roots, hairy roots, crown gall
The primary source of infection by Rhizobium rhizogenes has not yet been defined. The assumption is that the bacteria survive in the soil, but it is possible that first infection is from the propagation material. Infection also takes place via wounds. After infection, galls are formed which break open, releasing many bacteria. These are dispersed by rain and wind and by humans and tools. Until now, no bacteria have been isolated from crazy roots. This disease seems to be caused by a change in the hormonal system of the plant resulting from the Rhizobium infection. Rhizobium rhizogenes puts a Ri-plasmid in the plant tissue and causes a genetic change in the plant.
The four pathogenic Rhizobium species cause different symptoms, like galls, tumours and excessive root growth. All four species cause the growth of galls on the stems, which are first white and later turn brown. Only Rhizobium radiobacter causes galls and tumours on underground plant parts and on the crown. Furthermore, infection is characterized by excessive root growth (crazy roots). In greenhouse vegetables, the roots overgrow the substrate and in severe epidemics the drippers get clogged. In tomato, excessive vegetative growth occurs.