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RESEARCH PRODUCT

The soil as a reservoir for antagonists to plant diseases

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The soil is often considered the milieu providing support for plant roots, water and nutrients for plant growth. But it is also considered a hostile environment harbouring plant pathogenic nematodes, bacteria and fungi. The most common attitude is to try to eliminate the plant pathogenic organisms by biocidal treatments such as methyl bromide fumigation, which are dangerous for man and the environment. Beside this pathogen eradication strategy, another approach to control soil-borne plant diseases consists in studying the plant-pathogen interactions at the cellular and molecular level to create new resistant cultivars or to develop new plant protection products based on elicitation of plant defence reactions. This field of research only focuses on plant pathogen interactions, not taking into account the environment in which they take place. Although a plant disease results from the intimate interaction between a plant and a pathogen, the importance of these direct interactions should not hide the role of environmental factors which influence disease severity. These indirect interactions are particularly important in the case of diseases induced by soil borne pathogens. Indeed, the pathogens are not freely interacting with the plant; they are included in the soil matrix and thus can not escape to the soil environment. Both their inoculum density and infectious capacities are controlled by the soil. Evidence of these interactions is given by the existence of soils that suppress diseases. In suppressive soils disease incidence or severity remains low in spite of the presence of the pathogen, a susceptible host plant and climatic conditions favourable for disease development. These suppressive soils provide examples of biotic and abiotic factors affecting the pathogen, the plant or the interaction between plant and pathogen. In other words, suppressive soils provide examples where biological control, similar to conservation biological control, is active in nature. Therefore many studies have been devoted to the understanding of soil suppressiveness in order to use suppressive mechanisms in biological control strategies. Since in many cases, antagonistic micro-organisms play a role in soil suppressiveness, the soil has been seen as a reservoir of potential biological control agents. For the two or three last decades the main approach was to identify effective antagonists in soil and try developing them as biological control agents. Most of the biological control agents on the market, even when aerial diseases are targeted, have been isolated from soil. But in order to control soil-borne diseases, one must admit that this strategy has not been as successful as expected. Indeed, even if the soil harbours effective antagonists, soil suppressiveness is due to an association of mechanisms and micro-