Stem rot with the causal agent Sclerotium rolfsii is a major disease of peanut in Guilan province, Iran. The aim of this investigation was to determine the inhibitory effect of native isolates of peanut root nodulating symbiotic bacteria on this fungus based on in vitro and in vivo studies. Several bacterial strains were isolated and purified from peanut roots collected from different fields. Eight of them were detected as the main symbiotic nodulating strains. These eight isolates were

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... ed as Bradyrhizobium based on 16S rDNA gene analysis and different biochemical tests. The inhibitory effect of these strains on the radial growth of S. rolfsii was studied in vitro using sealed plate and dual culture methods. Strains significantly inhibited radial growth of the fungus on the PDA medium. Br9, Br18 and Br16 were recognized as strong inhibitors and Br14 as weak strain in dual culture method and used in greenhouse experiments. Ability of the selected strains in controlling the stem rot disease, reducing the disease parameters and enhancing the peanut growth parameters was investigated in greenhouse conditions. The strains significantly decreased the white rot index and increased peanut dry matter (P ≤ 0.01) in greenhouse.