Fusarium wilt suppressive soils: an example of disease-suppressive soils

The existence of soils that are naturally suppressive to diseases induced by soilborne plant pathogens provides good opportunities to study situations where biological control is effectively working. In most cases, suppressiveness is fundamentally based on microbial interactions between the pathogen and some populations of the saprophytic microflora. However, these biotic interactions are dependent on the abiotic characteristics of the soil. In the case of soils suppressive to fusarium wilts, it is obvious that pH and the nature of the clays are important factors interacting with the microbial populations responsible for suppressiveness. Competition for nutrients, mainly carbon and iron, has been demonstrated to be one of the mechanisms by which suppressive soils control fusarium wilts. Populations of non-pathogenic Fusarium oxysporum and fluorescent Pseudomonas spp. are, at least partly, responsible for competition for carbon and iron, respectively. Moreover, these antagonistic populations have other modes of action which can contribute to their biocontrol activity. Strains of both non-pathogenic F. oxysporum and P. fluorescens are being developed as biocontrol agents. Studies of suppressive soils suggest biological control could also be achieved by enhancing the natural level of suppressiveness that exists in every soil.