Rhizobium radiobacter

Rhizobium radiobacter, formerly classified as Agrobacterium tumefaciens, is a Gram-negative soil bacterium within the family Rhizobiaceae. It is a notorious plant pathogen known for its ability to induce tumor-like growths, commonly referred to as crown gall, on various plant species. The bacterium is scientifically significant due to its natural ability to transfer a portion of its genetic material into the plant genome.

The disease caused by this pathogen, crown gall, is a major issue in nurseries and orchards. It affects a vast range of dicotyledonous plants, including rosaceous fruit trees (apples, peaches, roses), grapes, and several vegetable crops. The symptoms are characterized by the formation of fleshy, undifferentiated galls that eventually become woody, necrotic, and impair the plant's vascular system, leading to stunted growth.

The biology of R. radiobacter is centered on the Ti (tumor-inducing) plasmid. When the bacteria encounter chemical signals from wounded plant tissues, they initiate the transfer of T-DNA into the host plant's cells. This genetic material integrates into the plant's chromosomes and induces the production of plant hormones, forcing the host to produce galls and specialized nutrients called opines, which the bacteria feed upon.

Development and spread are facilitated by moist soil conditions, optimal temperatures between 20°C and 28°C, and physical trauma to roots or stems. The pathogen can survive as a saprophyte in the soil for extended periods. It is frequently introduced into new areas via infected nursery stock or contaminated tools used during agricultural operations, making sanitation a critical factor in management.

The economic impact of the disease is severe, as infected plants become unproductive and susceptible to secondary infections. Control strategies are largely preventative:

• Sourcing certified disease-free propagation material.
• Practicing strict hygiene, including disinfecting cutting tools between plants.
• Careful handling of root systems during transplanting to minimize injury.
• Soil solarization or crop rotation in heavily infested fields.