Aims: Four well-described strains of Pseudomonas¯uorescens were assessed for their effect on pea growth and their antagonistic activity against large Pythium ultimum inocula. Methods and Results: The effect of Pseudomonas strains on the indigenous soil micro¯ora, 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 Pythium 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 and CHAO increased 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, while 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-speci®c. Pythium increased the activity of C, N and P cycle enzymes, while the Pseudomonas strains reduced this effect, indicating reduced plant damage. Conclusions: Strains SBW25 and CHAO had the greatest bene®cial characteristics, as these strains produced the greatest reductions in the side effects of Pythium infection (microbial populations and enzyme activities) and resulted in signi®cantly improved plant growth. Strain SBW25 does not produce antifungal metabolites, and its biocontrol activity was related to a greater colonization ability in the rhizosphere. Signi®cance and Impact of the Study: This is the ®rst critical comparison of such important strains of Ps.¯uorescens showing disease biocontrol potential.

The antibiotic, 2, 4-diacetylphloroglucinol (Phl), is produced by a range of naturally occurring fluorescent pseudomonads. One isolate, P.fluorescens F113, protects pea plants from the pathogenic fungus, Pythium ultimum, by reducing the number of pathogenic lesions on plant roots but with a concurrent reduction in the emergence of plants such as pea. The genes responsible for Phl production have been shown to be functionally conserved between the wild type (wt) P. fluorescens strains F113 and Q2-87. In this study the genes from F113 were isolated using an optimised long PCR method and a 6.7-kb gene cluster inserted into the chromosome of the non-Phl producing P. fluorescens strain SBW25 EeZY6KX. This strain is a lacZY, km R marked derivative of the wild type SBW25 which effects biological control against the plant pathogen, Pythium ultimum by competitive exclusion as a result of its strong rhizosphere colonising ability. We describe here the integration of the Phl antifungal and competitive exclusion mechanisms into a single strain, and the impact this has on survival and plant emergence in microcosms. The insertion of the Phl biosynthetic genes from the F113, into the SBW25 chromosome gave a Phl-producing transformant (strain Pa21) able to suppress P. ultimum through antibiotic production. The growth of Pa21 was not reduced in flask culture at 20 o C, compared with its parent strain. When inoculated on pea seedlings, the strain containing the Phl operon behaved similarly to the SBW25 EeZY6KX parent but did not show the tendency of the wt Phl producer F113 to cause lower pea seed emergence. SBW25 EeZY6KX has significantly lower indigenous populations in the root than the F113 and control. This is indicative of this strain's strong colonising presence. Pa21, the Phl-modified strain is able to exclude the resident population from roots to the same degree as the SBW25 EeZY6KX from which it is derived. This suggests that it has maintained its competitiveness around the root systems of plants even with the introduction of the Phl locus. Thus, strain Pa21 possesses the qualities necessary to provide effective