Ecological basis for biocontrol of damping-off disease by Pseudomonas fluorescens 54/96

Pseudomonas fluorescens 54/96, originally isolated from the rhizosphere of sugar beet, has been shown to be commercially effective in field trials for the suppression of a number of fungal diseases of seedlings. In vitro and microcosm-based assays revealed that both the timing and method of application of bacteria were important for effective control of Pythium ultimum, the causative agent of damping-off disease. Following transposon mutagenesis (Tn5lac), mutants deficient for the suppression of Pythium ultimum infections of peas were isolated. Three major classes of insertional mutants of Ps. fluorescens 5/96 were identified which either inhibited sporulation, reduced mycelial growth or affected the regulation of bacterial metabolic activity. Evaluation of the metabolic capability of pathogen and antagonist revealed evidence for direct competition, as both the fungus and bacterium had similar sole carbon source nutrient utilization profiles. Further comparisons of the activity of the transposon mutants indicated that although the mechanisms of disease control were multifactorial, the most significant factor was the prevention of rapid spore germination in the presence of pea seeds.