Evaluation of rhizospheric Pseudomonas and Bacillus as biocontrol tool for Xanthomonas campestris pv campestris

Xanthomonas campestris pv campestris (Xcc), causing black rot, is one of the most yield-limiting and destructive pathogens of cruciferous crops. The intention of this study was to evaluate the potential of rhizobacteria in black rot management. Fifty-four isolates from rhizosphere soil of Brassica campestris were screened against Xcc. Two isolates namely, KA19 and SE, with inhibition radius >11 mm were selected. The combined use of them produced an average inhibition zone of 18.1 ± 1.4 mm radius (P < 0.05). 16S rRNA gene sequencing and phylogenetic analysis identified KA19 and SE as the nearest homologs (>99.4%) of Pseudomonas aeruginosa and Bacillus thuringiensis, respectively. In greenhouse study, both isolates were effective (P < 0.05) in reducing black rot lesions compared to untreated control involving either a foliar spray or the combined seed soak and soil drench. However, the combined strains (KA19 + SE) were significantly more effective (P < 0.05) when the mode of application was combined seed and soil drench. The lipid content of seeds increased significantly with the application of these strains, especially with SE alone and in combination. After 9 weeks, the Xcc population was significantly lower in soil treated with combined strains (P < 0.05). KA19 produced extracellular siderophores, influenced by various carbon sources and identified as 4-hydroxy-2-nonyl-quinoline by NMR. In Bacillus SE, two antibacterial factors corresponding to autolysins (β-N-acetylglucosaminidase) and AHL-lactonases were established. This study would strengthen our understanding for application of different rhizobacteria with various active principles like Pseudomonas and Bacillus as ingredients of a biocontrol mixture.