Behavior of a virulent strain derived from Agrobacterium radiobacter strain K84 after spontaneous Ti plasmid acquisition

The behavior of the virulent transconjugant K84N6 derived from Agrobacteriu m radiobacter strain K84 after spontaneous Ti plasmid transfer in crown gal l tissue in a biocontrol experiment was studied and compared with the behav ior of the wild-type A. tumefaciens donor of the Ti plasmid. The main diffe rence between the strains was a greatly reduced ability of the transconjuga nt to catabolize nopaline. Host range, ability to induce tumors in several fruit trees, and stability of the pathogenic determinants in isolates from tumors did not differ between the strains. Nevertheless, in a biocontrol ex periment, the transconjugant was not controlled by strain K84 or K1026 in p each x almond hybrids and survived in the plant rhizosphere for 9 months wi th larger population densities than the wild strain. The appearance and per sistence in soil of strains harboring a Ti plasmid in the K84 chromosomal b ackground could represent a risk in the medium term, if they show good comp etitive ability.