Selection of antagonistic bacteria of Clavibacter michiganensis subsp michiganensis and evaluation of their efficiency against bacterial canker of tomato

A 178 bacterial strains, antagonistic towards Clavibacter michiganensis subsp. michiganensis, the causal agent of bacterial canker of tomato, were isolated from bulk soil, the rhizosphere and rhizoplane of tomato, originating from different sites in the Souss-Massa Valley, Agadir, Morocco. The strains were characterized on the basis of the Gram stain, sporulation, fluorescence on King's B medium and physiological tests. All of the strains inhibited C. m. subsp. michiganensis on nutrient-broth yeast extract agar (NBYA), with zones of inhibition ranging from 2-30 mm. The most efficient strains were Gram-negative and most of these were fluorescent pseudomonads. Twenty-four strains (ampicillin and nalidixic acid resistant mutants) were selected for their ability to colonize tomato roots. Less than a third (7/24) completely colonized the roots of all the seedlings. Only six strains colonized the main and lateral roots of 50% of the seedlings, and eleven colonized <50% of the seedlings. The best root-colonizing strains were fluorescent pseudomonads. Eighteen strains were screened in greenhouse pot experiments for control of bacterial canker. Only three of the 18 strains reduced infection when applied as a seed treatment compared to the untreated control. The majority of the strains reduced the infection totally when applied as a seed treatment followed by a root treatment before transplanting compared to the control. This combined treatment was significantly more efficient than the seed treatment alone.