Rye (Secale cereale) growth promotion by Pseudomonas fluorescens strains and their interactions with Fusarium culmorum under various soil conditions

The aim of this study was to determine whether the form of N-fertilization (NO3−, urea, mixture of NO3−, and NH4+ [1:1]) and enrichment of the soil with clay could affect the efficiency of Pseudomonas fluorescens strains as plant growth promoting rhizobacteria (PGPR) and as antagonists to Fusarium culmorum (W.G. Smith Sacc) in the rhizosphere of rye [Secale cereale (L.) cv Da image kowskie Z image ote]. We found that P. fluorescens strains 23 and 45 (resistant to streptomycin and kanamycin) produced Fe(III)-chelating compounds (including siderophores) and could inhibit the in vitro growth of F. culmorum (cycoheximide resistant) strain by competition for Fe(III). Plants were grown in pots filled with nonsterile soil for 21 days at constant temperature (20 °C) and soil moisture (60% WHC), without additional illumination. A mixture of NO3− and NH4+ was the best form of fertilization for enhancing the capacity of the P. fluorescens strains to promote plant growth and inhibit Fusarium growth. Enrichment of the soil with clay by the addition of 2% (w/w) bentonite favored growth promotion and antagonism to Fusarium. Growth promotion and pathogen-inhibiting efficiencies were not directly dependent on the sole ratio of Pseudomonas strains to pathogen number in the rhizosphere but were likely dependent on environmental conditions.