Is the ability of biocontrol fluorescent pseudomonads to produce the antifungal metabolite 2,4- diacetylphloroglucinol really synonymous with higher plant protection?

• The antifungal compound 2,4-diacetylphloroglucinol (Phl) contributes to biocontrol in pseudomonads, but whether or not Phl + biocontrol pseudomonads display higher plant-protecting activity than Phl – biocontrol pseudomonads remains to be demonstrated. This issue was addressed by assessing 230 biocontrol fluorescent pseudomonads selected from a collection of 3132 bacterial isolates obtained from 63 soils worldwide. • One-third of the biocontrol pseudomonads were Phl + and almost all Phl + isolates also produced hydrogen cyanide (HCN). The only Phl + HCN – strain did harbor hcn genes, but with the deletion of a 134 bp hcnC fragment corresponding to an ADPbinding motif. • Statistical analysis of biocontrol isolate distributions indicated that Phl production ability was associated with superior disease suppression activity in the Pythium– cucumber and Fusarium– tomato pathosystems, but this was also the case with HCN production ability. However, HCN significance was not as strong, as indicated both by the comparison of Phl – HCN + and Phl – HCN – strains and by correlation analyses. • This is the first population-level demonstration of the higher plant-protecting activity of Phl + biocontrol pseudomonads in comparison with Phl – biocontrol pseudomonads.