An investigation of biocontrol activity Pseudomonas and Bacillus strains against Panax ginseng root rot fungal phytopathogens

Ginseng (Panax ginseng) is one of the most common medicinal herbs in Korea and the production of ginseng is affected by root rot diseases caused by fungal phytopathogens, which remain a threat for profitable agricultural productivity. In this study, seven fungal genera (Ilyonectria sp., Neurospora sp., Cladosporium sp., Eutypella sp., Aschersonia sp., and Fusarium sp.) were isolated from infected ginseng root rot samples and examined for pathogenicity. Fungal pathogens were isolated by placing a small piece of infected ginseng root on potato dextrose agar, screened based on colony morphology, and identified based on 18s rRNA sequencing and phylogenetic analysis. Pathogenicity of the isolated fungal strains was evaluated based on pathogen penetration on the host surface that was documented by scanning electron microscopy (SEM). To isolate antagonistic strains, a total of 300 bacterial strains that were isolated from South Korean mine soil were screened using a dual culture assay. On screening, we found two potent antagonistic strains, Pseudomonas aeruginosa (D4) and Bacillus stratosphericus (FW3), that were identified based on molecular characterizations and shown to control isolated root rot fungal pathogens. Both strains were optimized for mycelial growth inhibition tests with different growth media and it was determined that nutrient broth is highly suitable for the antagonistic activity assay. In addition, both antagonistic strains were found to produce various bioactive metabolites. From our findings, the isolated P. aeruginosa and B. stratosphericus strains are excellent candidates to control ginseng root rot caused by pathogenic fungi.