ABSTRACT
Twenty five isolates of N2-fixing bacteria and eleven actinomycetes were chosen from 117 microorganisms isolated from desert soils and different plant roots to study their effect on Rhizoctonia solani, the causative of the root-rot disease. The data revealed that Azotobacter Rf and Azospirillium Kc were the most active for nitrogen fixation [210 and 133 ppm], root colonization [61.3% and 65%] and antagonistic effect on the pathogenic fungus R. solani [15 and 17 mm]. The two strains were identified as Azotobacter chroococcum and Azospirillium lipoferum. The actinomycetes isolate, Streptomyces Ncu was the most active strain for root colonization ability [41.4%] and antagonistic effect against R. solani [22 mm]. This strain was completely identified as Streptomyces lydicus