Isolation, characterization, and optimization of cultivation parameters of two Streptomyces species for potential biocontrol of Rhizoctonia solani causing root rot disease

Abstract Background Numerous significant crops are infected by the devastating soil-borne fungal pathogen Rhizoctonia solani . This study set out to isolate and screen actinobacteria with potential biocontrol capability against this pathogen. Results After screening, two isolates AC 82 and AC 143 were selected for further study according to their capacity to reduce the growth of R. solani (54.31 and 50%, respectively). These isolates were identified as Streptomyces lividans and S. rochei depending on their biochemical and morphological properties plus 16S rRNA gene sequenceing. The antifungal mechanisms of both strains were seen through the light and SEM images. The selected strains could also produce hydrolytic enzymes. Single-factor and response surface techniques were used to optimize the fermentation and culture conditions for both strains. Three variables (molasses, corn steep liquor (CSL) and inoculum age) for S. lividans and four variables (molasses, CSL, inoculum age and incubation time) for S. rochei were identified as the most significant variables. Using low-cost novel medium ingredients (molasses and CSL) for Streptomyces greatly enhanced the antifungal activity of both strains. The overall suppression of R. solani growth reached 92 and 91% with an improvement of 53 and 68% for S. lividans and S. rochei , respectively, when compared to the basal medium. The greenhouse experiments revealed that the fermentation broth of both strains increased the percentages of surviving green bean plants (75–80%) in comparison to control (25–30%). Conclusions The findings of this work highlight the possibilities for using both Streptomyces strains as promising biocontrol agents in R. solani management.