Plant Growth Promoting Rhizobacteria (PGPR) Induce Cucumber Root-Rot Protection under Plastic Tunnels Conditions

Eighteen isolates of plant growth promoting rhizobacteria (PGPR) were isolated from samples of four crops grown in reclaimed fields at Nobariya district (Behera Governorate). The isolates were in vitro tested for their antagonistic effect against the three fungal pathogens causing root-rot of cucumber, i.e. Fusarium solani, Pythium ultimum and Rhizoctonia solani, as well as its effect on plant growth. Nine isolates showed moderate to strong inhibition at least to one of these pathogens and increased vigour index of cucumber. Selected isolates were tested for production of indole acetic acid (IAA) and phosphorous solubilisation capability. The biocontrol ability of these antagonistic isolates was evaluated using an in vivo assay. Accordingly, three isolates were selected and used for soil treatment to control root-rot of cucumber in pot experiments. Bacterial isolates mixtures induced significant protection were the same as the individual isolates that significantly reduced symptoms of root-rot pathogens individual or its mixture in pot experiment. Pseudomonas fluorescens (P-11), P. fluorescens (Cu-2) and Bacillus subtilis (Cu-4) were effective for controlling root-rot disease, under plastic tunnels condition. Mixture of the three isolates was most effective to decrease percentage of root-rot and disease severity compared with other treatment and untreated treatment. Seeds treated with P. fluorescens (P-11), P. fluorescens (Cu-2) and B. subtilis (Cu-4) isolates and challenge inoculated with mixture of P. ultimum, F. solani and R. solani significantly reduced disease compared with check treatment and induced the plants to synthesize peroxidase and chitinase enzymes. The maximum phenolic content was observed in P. fluorescens (P-11) and P. fluorescens (Cu-2) pretreated plants and challenge inoculated with the pathogens, and the higher amounts of phenolics were noticed even on 10th day after the pathogens challenge inoculation.