Antagonistic bacteria are common soil inhabitants with potential to be developed into biofungicides for the management of seedling damping-off, root rot, and other soil-borne diseases of various crops. In this study, antagonistic bacteria were isolated from a commercial potato field and screened for their growth inhibition of fungal and oomycete pathogens in laboratory tests. The biocontrol potential of the 3 most effective antagonistic bacteria from the in vitro tests was evaluated against seedling damping-off and root rot of cucumber caused by Pythium ultimum. Based on phenotypic characteristics, biochemical tests, and sequence analysis of 16S-23S rDNA gene, the 3 antagonistic bacteria were identified as Pseudomonas fluorescens (isolate 9A-14), Pseudomonas sp. (isolate 8D-45), and Bacillus subtilis (isolate 8B-1). All 3 bacteria promoted plant growth and suppressed Pythium damping-off and root rot of cucumber seedlings in growth-room assays. Both pre- and post-planting application of these bacteria to an infested peat mix significantly increased plant fresh masses by 113%-184% and percentage of healthy seedlings by 100%-290%, and decreased damping-off and root rot severity by 27%-50%. The peat and talc formulations of these antagonistic bacteria applied as seed or amendment treatments to the infested peat mix effectively controlled Pythium damping-off and root rot of cucumber seedlings and enhanced plant growth. The survival of all 3 antagonistic bacteria in peat and talc formulations decreased over time at room temperature, but the populations remained above 10(8) CFU/g during the 180-day storage period. The peat formulation of a mixture of 3 bacteria was the best seed treatment, significantly increasing the plant fresh masses by 245% as compared with the Pythium control, and by 61.4% as compared with the noninfested control. This study suggests that the indigenous bacteria from agricultural soils can be developed and formulated as biofungicides for minimizing the early crop losses caused by seedling damping-off and root rot diseases.

Novel strains of rhizobacteria, Pseudomonas fluorescens Pf 9A-14, Pseudomonas sp. Psp. 8D-45 and Bacillus subtilis Bs 8B-1, showed broad-spectrum antagonistic activity and provided suppression of Pythium damping-off and root rot of cucumber. Their biocontrol potential was further investigated for suppression of additional seedling diseases of cucumber (Phytophthora capsici) and radish (Rhizoctonia solani). Bacterial strains were also characterised for production of antibiotics, metabolites, volatiles, phytohormones and lytic enzymes. Seed and pre-plant applications of all three antagonistic bacteria as cell suspension and talc or irradiated peat formulations to the infested potting mix provided overall high level of suppression of Phytophthora damping-off and root rot of cucumber (66-85% healthy seedlings) and relatively low level of suppression of Rhizoctonia damping-off of radish (18-38% healthy seedlings). Bacterial treatments also resulted in higher plant fresh masses. Seed coating with irradiated peat formulation of a mixture of three bacteria resulted in superior control of Phytophthora damping-off and root rot of cucumber and much higher plant fresh masses. The presence of genes for biosynthesis of phenazine-1-carboxylic acid, 2,4-diacetylphloroglucinol, pyrrolnitrin and pyoluteorin was confirmed in Pseudomonas strains, and that of fengycin, bacillomycin, bacilysin, surfactin and iturin A in B. subtilis Bs 8B-1. All three strains produced HCN, salicylic acid, indole-3-acetic acid, protease and -1,3-glucanase. Both Pseudomonas strains produced siderophores and only P. fluorescens Pf 9A-14 showed phosphate solubilisation and chitinase activity. All three strains inhibited pathogen growth by producing volatiles, and gas chromatography-mass spectrometry analysis revealed eight compounds in Pf 9A-14, 10 in Bs 8B-1 and 4 in Psp 8D-45, some with known antifungal activity. The antagonistic and plant-growth promotion activities of these strains might be due to production of antibiotics, metabolites, lytic enzymes or phytohormones.

Indigenous antagonistic bacterial isolates have the potential to be developed as biofungicides for minimizing early crop losses due to soilborne diseases caused by Fusarium and other soilborne pathogens.