Isolation, characterization and evaluation of the biocontrol potential of Pseudomonas protegens RS-9 against Ralstonia solanacearum in Tomato

Tomato (Solanum lycopersicum) is a common vegetable crop with worldwide consumption of its edible fruit. Bacterial wilt in tomato causes substantial yield loss. Biological control using microbial agents such as rhizobacteria has gained considerable attention over the last decade. In this study, we isolated a bacterial strain, RS-9 from the rhizosphere of a healthy tomato plant in a bacterial wilt diseased field that strongly inhibited the growth of bacterial wilt pathogen Ralstonia solanacearum. Further, we characterized the antagonistic bacterium RS-9 and tried to understand the mechanism behind. Phylogenetic tree analysis based on similarity percentage identified RS-9 as Pseudomonas protegens. This strain showed potential inhibition of different phytopathogenic fungi in vitro and could produce 2,4-diacetylphloroglucinol (2,4-DAPG), hydrogen cyanide (HCN), pyrrolnitrin and pyoluteorin. To identify the key bioactive compound in RS-9, an antagonism defective mutant was generated by transposon mutagenesis. The HPLC analysis of mutant and parent strains identified 2,4-DAPG as the main compound responsible for suppression of R. solanacearum. Evaluation of RS-9 in greenhouse studies revealed inoculation of field soil in pots with RS-9 led to significant increase in plant height and dry weight of tomato plant and the incidence of bacterial wilt reduced by 65.6%. The bacterial strain RS-9 outcompeted the pathogen by reducing its population by 63% in soil as compared to the ‘pathogen only control’ as well as with the antagonism defective mutant, while maintaining its own population at high density. In conclusion, this study revealed Psuedomonas protegens RS-9 to be a promising strain for biocontrol of bacterial wilt of tomato.