The Impact of Biofilm Forming Rhizobacteria on Rhizoctonia solani Damping-off of Tomato Running title: The Impact of biofilm forming rhizobacteria on Rhizoctonia solani

In this study, 180 rhizobacteria were isolated from different fields of East and West Azarbaijan and provinces of Iran. These bacterial isolates were screened based on their antagonistic potential against Rhizoctonia solani and their ability to form robust biofilm. Out of these isolates, 49 isolates exhibited more that 30% antagonistic activity against R. solani. Different methods were used to screen the isolates with high ability of biofilm formation. Out of 180 isolates, 51 isolates were able to form considerable amount of biofilm as determined by crystal violet staining (CVS) method. Pellicle formation (PF) bioassay was performed in three different media. Most of the robust biofilm forming isolates from CVS method could also form robust biofilms in PF method, only if MSNGP (medium with glycerol and pectin) medium was used. In root colonization bioassay, isolates N168, N95 and N94 that were robust or mediocre biofilm forming isolates in MSNGP, could efficiently colonize plant roots but the population of isolates N100 and N87 (none-biofilm forming isolates) on tomato roots were lower than 6 log10 CFU g-1 root. However, results of the pathogenicity system with R. solani damping off of tomato showed that, biofilm formation and colonization ability of these isolates doesn't have significant effect on their biocontrol activity