Biocontrol activities of rhizobacteria associated with apple, apricot and kiwi rhizosphere against bacterial canker caused by Clavibacter michiganensis

Bacterial canker, a destructive disease caused by Clavibacter michiganensis subsp. michiganensis causes significant economic losses to tomato production worldwide. Biological control has been proposed as an alternative to current chemical protectants, however, a little headway has so far been made in developing biocontrol methods against this destructive bacterial pathogen. To narrow this knowledge gap, we investigated the antagonistic capacity of different rhizobacterial isolates from three horticultural crops viz., apple, apricot and strawberry against C. michiganensis under in vitro conditions. The potential antagonistic strains showing in vitro inhibition against C. michiganensis were further screened for multifarious plant growth promoting and biocontrol activities (P-solubilization, IAA production, siderophore production, lytic enzyme activity, and ability to fix atmospheric nitrogen). Increase in concentration of cell free supernatant from 0.25 to 1.00% (v/v) revealed significant increase in antagonistic activity against C. michiganensis with most prominent increase being observed in isolate S1 with 9.90 mm zone of inhibition at 0.25% concentration, which increased to 15.40 mm with increase in concentration to 1.0%. Five most efficient antagonists identified under in vitro conditions were tested for biocontrol potential against C. michiganensis under net house conditions. Isolate S1 showed maximum reduction in disease incidence (70.00%) with minimum disease severity recorded (28.55%), besides significant increase in various plant growth parameters. The isolate was identified as Bacillus amyloliquefaciens by 16S rDNA sequencing. Therefore, strain S1 holds considerable biocontrol as well as growth promoting potential and could be used as a potential biocontrol agent against bacterial canker of tomato.