Biocontrol of Pythium in the pea rhizosphere by antifungal metabolite producing and non-producing Pseudomonas strains

Abstract

Four well-described strains of Pseudomonas fluorescens were assessed for their effect upon pea growth and their antagonistic activity against large Pythium ultimum inocula. The effect of Pseudomonas strains upon the indigenous soil microflora, soil enzyme activities and plant growth in the presence and absence of Pythium is assessed. Pythium inoculation reduced the shoot and root weights, root length, and the number of lateral roots. The effect of Pythium was reduced by the Pseudomonas strains as follows: F113, SBW25 and CHAO increased the shoot weights (by 20%, 22% and 35% respectively); strains Q2-87, SBW25 and CHAO increased root weights (14%, 14% and 52%); Strains SBW25 and CHAO increased the root lengths (19% and 69%), and increased the number of lateral roots (14% and 29%). All the Pseudomonas strains reduced the number of lesions and the root and soil Pythium populations, whilst SBW25 and CHAO increased the number of lateral roots. Pythium inoculation increased root and soil microbial populations but the magnitude of this effect was Pseudomonas strain specific. Pythium increased the activity of C, N and P cycle enzymes, whilst the Pseudomonas strains reduced this effect, indicating reduced plant damage. Overall, strains SBW25 and CHAO had the greatest beneficial characteristics as these strains produced the greatest reductions in the side effects of Pythium infection (microbial populations and enzyme activities) and resulted in significantly improved plant growth. Surprisingly strain SBW25 does not produce antifungal metabolites, and its biocontrol activity was related to a greater colonisation ability in the rhizosphere.