Biocontrol of soil-borne fungal plant diseases by 2,4-diacetylphloroglucinol-producing fluorescent pseudomonads with different restriction profiles of amplified 16S rDNA

Fluorescent pseudomonads producing the antimicrobial compound 2,4-diacetylphloroglucinol (Phl) are being studied extensively for use as biocontrol agents of soil-borne fungal diseases. Some of them can produce pyoluteorin (Plt) in addition to Phl, whereas others synthesise only Phl. Here, a collection of seven Phl^sup +^ Plt^sup -^ pseudomonads, seven Phl^sup +^ Plt^sup +^ pseudomonads and seven Phl^sup -^ biocontrol pseudomonads were compared for protection of plant roots against fungal pathogens. The seven Phl^sup +^ Plt^sup +^ pseudomonads were identical by restriction analysis of amplified spacer ribosomal DNA (spacer ARDRA), whereas the Phl^sup +^ Plt^sup -^ pseudomonads and especially the Phl^sup -^ biocontrol pseudomonads were quite diverse by spacer ARDRA. Collectively, the Phl^sup +^ Plt^sup -^ pseudomonads proved superior to the Phl^sup +^ Plt^sup +^ pseudomonads and the Phl^sup -^ biocontrol pseudomonads for protection of tomato against Fusarium crown and root rot (in rockwool microcosms) or cucumber against Pythium damping-off (in non-sterile soil microcosms). There was no correlation between protection in vivo and inhibition of the corresponding fungal pathogen on plates. However, there was a significant correlation between the amount of Phl produced on plates and protection of tomato against Fusarium crown and root rot, but not with protection of cucumber against Pythium damping-off. Interestingly, the minority of strains unable to produce HCN, an extracellular protease, or both, were among those unable to protect plants in both pathosystems. A seedling assay was developed to compare pseudomonads for suppression of Fusarium crown and root rot in vitro, and a significant correlation was found between disease severity in vitro and in vivo. Overall, results suggest that promising biocontrol pseudomonads may be identified based on the ability to produce Phl and/or specific ARDRA-based fingerprints.[PUBLICATION ABSTRACT]