The potential of endophytic bacteria from Prosopis cineraria for the control of Pythium aphanidermatum-induced damping-off in cucumber under saline water irrigation

The biocontrol efficacy of endophytic and rhizospheric bacteria from Prosopis cineraria, a halophyte, was evaluated against cucumber damping-off incited by Pythium aphanidermatum under different salt-water irrigation (50, 100, 150 and 200 mM NaCl) conditions. A total of 6 endophytes and 8 rhizobacteria differing in their morphology were isolated from the roots and rhizosphere soils of P. cineraria, respectively on nutrient agar medium containing 2.5% NaCl. Among them, 4 endophytic bacterial isolates, designated as PC1, PC3, PC5 and PC6 inhibited P. aphanidermatum growth in dual-culture assay. The isolate PC5 showed the highest mycelial growth inhibition (42.9%), followed by PC6 (34.8%), PC1 (24.3%) and PC3 (23.8%). These isolates were identified as Pseudomonas putida (PC1), Acinetobacter johnsonii (PC3), Pseudomonas aeruginosa (PC5) and Pseudomonas mosselii (PC6) based on maximum-likelihood analyses of 16S rDNA sequence data. These bacterial strains were compatible among them and capable of forming biofilm. P. aeruginosa PC5 produced volatile compounds that inhibited P. aphanidermatum growth by 57.3% in an in vitro assay. Analysis of the volatile compounds of P. aeruginosa PC5 revealed the presence of dimethyl disulfide as a major component (46.2%) followed by lactic acid (21.3%). Greenhouse test results revealed that soil application of these bacterial strains had no significant effect on damping-off incidence under non-saline irrigation. However, under saline water (50 mM NaCl) irrigation A. johnsonii PC3 showed the highest biocontrol efficacy with 82% reduction in the disease incidence, with no significant effects at concentrations of 100 mM NaCl and above.