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Mechanisms involved in spatial and temporal mobility of disease patches caused by Rhizoctonia solani in sugar beet field: Induction of antagonists within disease patch Muhammad Anees (1), Arne Tronsmo (2), Véronique Edel-Hermann (1), Nadine Gautheron (1) Vincent Faloya (3) and Christian Steinberg (1) (1) INRA, Université de Bourgogne, UMR 1229 Microbiologie du Sol et de l’Environment, CMSE, 17 rue Sully, BP86510, F-21065 Dijon, France ; (Phone: +33-3-80-69-30-50; E-mail: [email protected]) (2) Department of chemistry, biotechnology and food science, PO box 5003, NO-1432 Aas, Norway (3) Unité Expérimentale de Bretenière, INRA, BP86510, F-21065 Dijon, France Rhizoctonia solani AG 2-2 causes damping off and root rot in sugar beet in patches which are highly mobile both on spatial and temporal scales. They never occur in the same place where they were in the last season. The present study was to uncover the mechanisms underlying the dynamics of disease patches. Soil was sampled from within diseased patch, healthy area and in between the two regions in sugar beet field, previously inoculated with R. solani AG 2-2 and tested for inoculum potential and soil suppressiveness towards the pathogen as well as genetic and physiological structure of bacteria and fungi. Potential antagonists, Trichoderma were isolated from within and outside the disease patch and studied for their antagonistic ability against the pathogen both in vitro and in vivo in two different soils. It was observed that soil inoculum potential was higher within diseased patch than in healthy area. However, the dormant pathogen in healthy area was stimulated by addition of buckwheat meal more than that in diseased patch. In addition soil from diseased area was more suppressive towards the pathogen. We did not observe significant differences in bacterial genetic structure between different treatments. However, diseased area could be easily distinguished from healthy area by their bacterial physiological fingerprints. Diseased patch and healthy area were distinguishable both on the fungal genetic and physiological structures. In addition to this, Trichoderma isolates from diseased patch were more antagonistic than the ones from outside the patch both in vitro and in vivo in both types of soils. These results suggest that R. solani when active may stimulate the development of antagonistic organisms. Their presence may inhibit the formation of patches in the subsequent season in that part of the field.