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

Aims: Four well-described strains of Pseudomonas fluorescens were assessed for their effect on pea growth and their antagonistic activity against larg e Pythium ultimum inocula. Methods and Results: The effect of Pseudomonas strains on the indigenous so il microflora, soil enzyme activities and plant growth in the presence and absence of Pythium was assessed. Pythium inoculation reduced the shoot and root weights, root length, and the number of lateral roots. The effect of P ythium was reduced by the Pseudomonas strains. Strains F113, SBW25 and CHAO increased shoot weights (by 20%, 22% and 35%, respectively); strains Q2-87 , SBW25 and CHAO increased root weights (14%, 14% and 52%). Strains SBW25 a nd CHAO increased root lengths (19% and 69%) and increased the number of la teral roots (14% and 29%). All the Pseudomonas strains reduced the number o f lesions and the root and soil Pythium populations, while SBW25 and CHAO i ncreased the number of lateral roots. Pythium inoculation increased root an d soil microbial populations but the magnitude of this effect was Pseudomon as strain-specific. Pythium increased the activity of C, N and P cycle enzy mes, while the Pseudomonas strains reduced this effect, indicating reduced plant damage. Conclusions: Strains SBW25 and CHAO had the greatest beneficial characteris tics, as these strains produced the greatest reductions in the side effects of Pythium infection (microbial populations and enzyme activities) and res ulted in significantly improved plant growth. Strain SBW25 does not produce antifungal metabolites, and its biocontrol activity was related to a great er colonization ability in the rhizosphere. Significance and Impact of the Study: This is the first critical comparison of such important strains of Ps. fluorescens showing disease biocontrol po tential.