Studies of soils suppressive to fusarium wilts have indicated that the phenomenon is fundamentally microbiological in nature, resulting from complex microbial interactions between the pathogen and all, or a part of, the saprophytic microflora. Many groups of micro-organisms have been proposed as having a role in this process. The most consistent results show that non-pathogenic Fusarium oxysporum and fluorescent pseudomonads are the main agents for biological control. Each group has been effective in reducing the severity of fusarium diseases in several crops under experimental conditions. All the problems associated with application of antagonists have not yet been solved, but special situations exist where biological control could be applied soon. In Europe, the present development of soil-less systems to grow tomatoes, cucumbers, carnations and a few other species of flowers and vegetables in greenhouses, offers a unique opportunity to use antagonists. A strain of nonpathogenic Fusarium isolated from soil from Chateaurenard can be established in the soil-less substrates, or in the potting mixtures, before transplanting the plants and in these conditions can prevent colonization of the substrates by the pathogens. It gave good control of fusarium wilts of tomatoes, carnations and cyclamens, and of crown and root rot of tomatoes due to F. oxysporum f.sp. radicis lycopersici. This non-pathogenic strain, produced by liquid fermentation and mixed with talc, is easy to introduce into soil-less substrates after having been suspended in water or in liquid fertilizer. Experiments have been under way in commercial greenhouses for several years, but it will be necessary to wait at least two more years to complete all the tests required for the registration of the product as a biopesticide. Similar experiments have been conducted with a strain of fluorescent pseudomonad also isolated from the soil of Chateaurenard, which shows poor antagonistic effect when applied alone but consistently improves disease control when applied in combination with the non-pathogenic Fusarium. In fact, application of both antagonists together always gave total control of the diseases, the yield in the treated plots being equal to that in the healthy control. These experiments show that combining antagonists can improve the consistency of biological control.
