Cocolonization of the Rhizosphere by Pathogenic Agrobacterium Strains and Nonpathogenic Strains K84 and K1026, Used for Crown Gall Biocontrol

The crown gall biocontrol agent strain K84 and three mutants derived from it, K1026 (Tra⁻ deletion mutant of pAgK84), K84 Agr⁻ (lacking pAgK84), and K1143 (lacking pAgK84 and pNoc), significantly reduced gall formation caused by two pathogenic strains resistant to agrocin 84 in peach × almond seedlings planted in infested soil. Cocolonization of roots by pathogenic and nonpathogenic strains was observed in these biocontrol experiments under field conditions. In spite of the efficient biocontrol observed, average populations consisting of 10² and 10⁶ pathogenic agrobacteria per g of root were found 8 months after planting. The total numbers of pathogenic bacteria on roots were similar for plants treated with the biocontrol strains and for the untreated plants. Strain K84 and the genetically engineered organism K1026 survived at a level of 10⁶ agrocin 84-producing bacteria per g of root. The population size of genetically engineered strain K1026 was not significantly different than the population size of wild-type strain K84 8 months after root inoculation. Strains K84 and K1026 controlled two pathogens resistant to agrocin 84 without reducing the total number of pathogenic bacteria in the root system. In addition, this study shows that some biological control activity of strain K84 against agrocin 84-resistant pathogens is independent of plasmids pAgK84 and pNoc.