INRA-CMSE, Laboratoire de Recherches sur la Flore Pathogene le Sol, 17 Rue Sully, BV 1540-21034 Dijon, Franc

Diseases caused by soil-borne plant pathogens are among the most difficult to control. Among the reasons accounting for this difficulty, one is related to the complexity of the soil environment where many factors of both abiotic and biotic origin interact with the pathogens. The best example of the role of the soil interfering with the pathogens is provided by soils that are naturally suppressive to diseases; i.e., soils in which disease incidence remains low despite the presence of the pathogen, a susceptible plant species and climatic conditions favourable to disease expression. Suppressive soils show that the soil is not a neutral milieu where the pathogens interact freely with the plant roots. Conversely, the soil interferes in many ways with the relationships between and among microorganisms, plants and pathogens. These observations led to the concept of (1) soil inoculum potential describing the capacity of a soil to cause disease, and (2) soil suppressiveness describing the capacity of a soil to control disease. Although very useful for an epidemiology approach to the diseases, these concepts are too global to provide explanations of' the mechanisms by which the soil interfaces with the microbial populations. To propose cosy strategies of disease control it is therefore important to analyse the different types of interactions taking place in soil. It its first needed to describe the physicochemical environment in the micro-organisms are embedded and how these physicochemical characteristics influence the population dynamics and the activity of microbial populations. Influence of some factors such as pH on disease development has been clearly established the role of other soil components such as clays, organic matter or cations such as Cu, Fe, Mn, Zn on microbial activities is still largely ignored.

Plant pathologists have been interested by the antagonistic interactions between the saprophytic, microflora and the pathogens. Many studies have been devoted to competition for nutrients, mycoparasitism and antibiosis as important mechanisms controlling the population dynamics of plant pathogens in the soil. However, these mechanisms also play a role in the interactions between populations of saprophytic microorganisms and contribute to the success or the failure of the establishment of a given population in a specific ecological niche. In contrast to this great interest focused on the microbial interactions, relationships between microflora and fauna in the soil have seldom been considered. However, there is a great diversity of species constituting the microfauna and mesofauna, interacting with the microflora. The prey-predator relationship has been extensively studied especially in the case of amoebae feeding on bacteria. The population dynamics of prey and predator have been accurately described but the consequences of predation, especially the possible stimulation of the metabolic activities of the prey have been ignored. Finally, the rhizosphere. i.e., the zone where plant roots modify the soil environment is considered as the place where most of the significant interactions between plant microorganisms occur and a considerable amount of work has been dedicated to it. Only recently has the use of modern technology enabled us to discover the fundamental bases of the dialogue between the plant and some symbiotic microorganisms. Much more needs to be elucidated on the influence of root exudates on the modification of the microbial balance in the, rhizosphere, and the determination of rhizosphere competence.

This symposium will not review all of these important phenomena taking place in soil but it will focus on a few of them that deserve the interest of plant pathologists willing to achieve integrated control of soil-borne plant pathogens.