Contribution of studies on suppressive soils to the identification of bacterial biocontrol agents and to the knowledge of their modes of action

In some soils, disease severity of soilborne diseases remains low despite the presence of pathogenic populations and conditions favourable to disease expression. Natural suppressiveness has been reported for a wide range of soilborne diseases and is specific, for a given soil, to a class of disease. Different types of soil suppressiveness have been described according to their origin and stability: (i) acquired with time after a monoculture of a susceptible host-plant in the presence of the pathogen and eliminated by breaking the monoculture by a non-host crop, for take-all, and (ii) native and stable, for fusarium wilts and balck root rot. Natural suppressiveness of soils to diseases has been related to their indigenous microflora. More precisely, suppressiveness has been ascribed to the total soil microflora (general suppressiveness) and to specific antagonistic microbial populations (specific suppressiveness). Among these populations, a major role has been given to fluorescent pseudomonads. Their implication in soil suppressiveness was shown to be related to siderophore-mediated iron competition (suppressive soils to fusarium wilts) and antibiosis (suppressive soils to take-all and tobacco root rot). Progresses in the knowledge of the microbial populations involved in the natural soil suppressiveness have allowed the isolation of biocontrol agents such as P. fluorescens CHA0, 2-79, Q8r1-96, C7 from soils naturally suppressive to black root rot, take-all and fusarium wilts, respectively. Modes of action of these biocontrol agents are related to antibiosis, competition and/or induced systemic resistance. Reciprocal feedbacks between studies on mechanisms of natural soil suppressiveness and modes of action of biocontrol agents are described. Prospects are finally given on the application of molecular approaches to identify new bacteria, genes and metabolites involved in soil suppressiveness.