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 i t, K1026 (Tra(-) deletion mutant of pAgK84), K84 Agr(-) (lacking pAgK84), a nd K1143 (lacking pAgK84 and pNoc), significantly reduced gall formation ca used by two pathogenic strains resistant to agrocin 84 in peach x almond se edlings planted in infested soil. Cocolonization of roots by pathogenic and nonpathogenic strains was observed in these biocontrol experiments under f ield conditions. In spite of the efficient biocontrol observed, average pop ulations consisting of 10(2) and 10(6) pathogenic agrobacteria per g of roo t were found 8 months after planting. The total numbers of pathogenic bacte ria on roots were similar for plants treated,vith the biocontrol strains an d for the untreated plants. Strain K84 and the genetically engineered organ ism K1026 survived at a level of 10(6) 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 pathog ens resistant to agrocin 84 without reducing the total number of pathogenic bacteria in the root system. In addition, this study shows that some biolo gical control activity of strain K84 against agrocin M-resistant pathogens is independent of plasmids pAgK84 and pNoc.