Bio-integration of rhizobacteria and biogenic silver nanoparticles against Pseudomonas syringae pv. syringae causing plum bacterial canker.

BACKGROUND

Plum is a nutritionally and medicinally valuable fruit with high economic value; however, in temperate regions, its yield continuously suffers from bacterial canker caused by Pseudomonas syringae pathovars.

RESULTS

To bridge the research gap, a study was conducted to optimize the use of rhizobacteria and biogenic silver nanoparticles (AgNPs) for controlling bacterial canker in plums. Rhizobacteria were isolated from plum orchards and identified (through 16 S rRNA gene sequencing, phylogenetic analysis using MEGA) and further evaluated for plant growth-promoting characteristics (siderophore production, production of hydrogen cyanide, solubilization of phosphate, and biofilm formation). The results revealed that 7 isolates out of 19 rhizobacterial isolates produced biofilm, of which Rw-1, Rw-6, Rw-7, and Hj-2 were Pseudomonas fluorescens, while Rw-12 and Hj-5 were identified as Bacillus subtilis. Besides, biogenic AgNPs were prepared in different concentrations (0.2%, 0.4%, and 0.6%) by using Cannabis sativa plant extract. X-ray diffraction (indicated the size of crystalline green silver nanoparticles of ~ 20 nm), Fourier-transform infrared spectroscopy (confirmed the presence of alkyne molecules, polyphenols, and carbonyl compounds on the AgNPs surface), and scanning electron microscopy (SEM) were used for characterization of the nanoparticles. Additionally, the antibacterial activity of AgNPs was determined by the disk diffusion assay with 18-hour cultures of P. syringae. In vitro experiments showed that the highest inhibition zone (24.32 mm) was caused by the 0.6% AgNPs concentration, proving the strongest antibacterial activity. The subsequent glasshouse tests confirmed that a consortium of two strains of Bacillus with 0.6% AgNPs allowed the lowest disease incidence (18.13%), proving efficient disease suppression. Combination of Bacillus and Pseudomonas strains with 0.6% AgNPs recorded significantly higher disease incidence, with the probable reason being incompatibility among these bacterial genera.

CONCLUSIONS

The research findings emphasize the potential of applying compatible rhizobacterial consortia and biogenic (C. sativa) silver nanoparticles as a potent substitute for synthetic pesticides in controlling plum bacterial canker.