Abstract: 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 108 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.