Identification of conserved traits in fluorescent pseudomonads with antifungal activity

A collection of 29 fluorescent pseudomonads, some with known biological con trol activity against a range of phytopathogenic fungi, were characterized phenotypically and genotypically by comparing carbon source utilization pat terns, suppression of Pythium ultimum both in planta and in vitro and the p otential to produce known secondary metabolites. Fatty acid profiling and r estriction fragment length polymorphism (RFLP) analysis of the ribosomal DN A operon (ribotyping) were used to determine the diversity of isolates. A s mall group of genetically related Pseudomonas spp. with similar properties was identified; each isolate produced a diffusible bioactive product in vit ro and was active against Pythium ultimum in planta. However, other isolate s that were able to suppress damping off disease but did not inhibit hyphal extension in vitro clustered outside this group. Phenotypic analyses revea led that the accumulation of C17:0 cyclopropane fatty acid (17CFA) and the production of hydrogen cyanide correlated significantly with biological con trol activity and with the antagonism of fungal development. The potential of 17CFA as a marker for the selection of fluorescent pseudomonads with bio control agent (BCA) potential was demonstrated by the isolation of a novel active strain. This was selected after the screening of 13 clonal groups of fluorescent pseudomonads identified from 500 isolates from the phytosphere of sugar beet. Levels of 17CFA synthesis possibly reflect the efficacy of the rpoS allele in particular strains.