The ability of Pseudomonas fluorescens F113 to produce the antibiotic 2,4-diacetylphloroglucinol (DAPG) is a key factor in the biocontrol of the phytopathogenic fungus Pythium ultimum by this strain. In this study, a DAPG-producing strain (rifampin-resistant mutant F113Rif) was compared with a nearly isogenic DAPG-negative biosynthesis mutant (Tn5::lacZY derivative F113G22) in terms of the ability to colonize and persist in the rhizosphere of sugarbeets in soil microcosms during 10 plant growth-harvest cycles totaling 270 days. Both strains persisted similarly in the rhizosphere for 27 days, regardless of whether they had been inoculated singly onto seeds or coinoculated in a 1:1 ratio. In order to simulate harvest and resowing, the roots were removed from the soil and the pots were resown with uninoculated sugarbeet seeds for nine successive 27-day growth-harvest cycles. Strains F113Rif and F113G22 performed similarly with respect to colonizing the rhizosphere of sugarbeet, even after nine cycles without reinoculation. The introduced strains had a transient effect on the size of the total culturable aerobic bacterial population. The results indicate that under these experimental conditions, the inability to produce DAPG did not reduce the ecological fitness of strain F113 in the rhizosphere of sugarbeets.

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