Temporal dynamics of components of the microflora, including Trichoderma sp.in relation to patch development of the disease incited by Rhizoctonia solani AG 2-2 in a sugar-beet field

English. The soil-borne phytopathogenic fungus, Rhizoctonia solani AG 2-2 incites damping off and root rot in sugar beet (Beta vulgaris L.). The disease occurs in the form of patches of dead or necrotic plants which show temporal and spatial dynamics within and between seasons. The patches never occur at the same place where they were observed in the previous season. The soil from inside the patches was found less conducive towards the disease caused by R. solani AG 2-2 as compared to the soil from healthy areas. The objective of the present study was to characterize the mechanisms involved in the build up of disease patches caused by R. solani AG 2-2 in a sugar beet field, to evaluate the temporal dynamics of the microflora in relation to the pathogenic activity, and to identify the putative biocontrol agents playing a role in the reduced conduciveness of soil.Different components of microflora were related to the reduced conduciveness of soil inside the patches. Soil inoculum density was higher inside the disease patches with soil inoculum potential reduced and there seemed to be a progressive decrease in the inoculum density of R. solani AG 2-2 measured by real time PCR with the development of patches. The fungal and bacterial community structures were significantly modified inside the disease patches which also showed a progressive deviation with the development of disease. Modification in the community structures of Trichoderma spp. and increase in their densities inside the patches were evident in the early and the end of the season respectively. Apparently, the equilibrium established between microflora and the infectious activity of R. solani as described above was broken by an exceptional attack of an aerial disease caused by Cercospora beticola. This reveals the role of the plant exudates in response to the necrotic attack as to an aerial attack in the structure of the soil microflora.The present study marked the role of Trichoderma spp. in reduced soil conduciveness along with some unidentified microorganisms. The Trichoderma isolates picked from inside the patches were more antagonistic towards the pathogen as compared to the ones from outside. Most of the isolates identified by morphological and molecular tools belonged to T. velutinum and T. gamsii. The isolates showed different mechanisms to antagonize R. solani AG 2-2 including production of volatile and water soluble metabolites, mycoparasitism as well as induced systemic resistance in the host. T. gamsii T30 proved to be the best antagonistic strain based on the different tests used ranging from Petri dishes to microcosms and bioassays and may present a potential biocontrol of R. solani AG 2-2.In conclusion, although mechanisms contributing to the development of patches caused by R. solani in a sugar beet field have been established, some others arised and need to be investigated such the vulnerability of the soil suppressiveness acquired during the patch built up, the high soil inoculum potential remaining as a risk of disease occurrence in the healthy areas, the role of the intra specific diversity based on the antagonistic activity of some populations of Trichoderma and, the mechanism of interactions between these populations and R. solani.

French. Afin d'identifier des mécanismes impliqués dans le développement des foyers causés par Rhizoctonia solani AG2-2 sur betteraves, des échantillons de sol ont été prélevés le long de transects traversant des zones infectieuses et des zones saines dans un champ cultivé en betteraves sucrières. Le potentiel infectieux et la résistance du sol aux maladies dues à R. solani ont été mesurés et comparés. Le sol devient résistant au sein des foyers.L'analyse des structures de communautés bactériennes et fongiques par des méthodes génétiques et biochimiques ont mis en évidence l'émergence de Trichoderma spp. au sein des foyers. Des isolements et une caractérisation génotypique, phénotypique et fonctionnelle de ces Trichoderma spp. ont révélé qu'au sein d'une même espèce (T. gamsii) des populations antagonistes de R. solani s'étaient développé dans les foyers. En milieu contrôlé, la population de T. gamsii testée inhibe fortement le développement et la croissance de celle de R. solani AG 2-2 dès que celle dernière atteint une densité seuil. Cela expliquerait que le sol devienne résistant au cœur du foyer mais que l'apparition et le développement initial du foyer ne soient pas contrôlés. Des prélèvements réalisés au cours du temps montrent l'évolution de la structure des communautés microbiennes aboutissant au développement du foyer et révèlent que l'incidence d'une maladie aérienne causée par Cercospora beticola sur le feuillage de la betterave n'est pas neutre vis-à-vis de la microflore tellurique.