ISOLATION AND CHARACTERIZATION OF TWO NEW BACILLUS SUBTILIS STRAINS FROM THE RHIZOSPHERE OF EGGPLANT AS POTENTIAL BIOCONTROL AGENTS

Fusarium wilt is one of the most important diseases of eggplant and the cause of major crop losses. The present work was undertaken to evaluate the possibility of using antagonistic bacteria isolated from eggplant rhizosphere as biocontrol agents to reduce the incidence of wilt caused by Fusarium solani. Out of 141 isolates, two strains AI01 and AI03 which showed maximum antagonistic activity during initial screening against F. solani were selected. The strains were identified as Bacillus subtilis based on physiological and biochemical characters and analysis of the 16S rRNA gene sequence. Antagonistic activity was further tested against ten important fungal pathogens in vitro, which revealed that both strains inhibited these phytopathogens to a desirable extent. Scanning electron microscope observations disclosed the presence of deformity and lysis of fungal mycelia at the zone of pathogen/antagonistic bacteria interaction. Robust swarming motility was exhibited by strain AI01 in dual culture plates, which limited fungal growth. Studies on antifungal metabolites secreted by the bacteria confirmed the presence of siderophore and several hydrolytic enzymes like chitinase, protease, lipase and amylase. AI01 and AI03 were also capable of producing indole acetic acid (IAA), a plant growth-promoting trait, but exhibited no deleterious effects like production of hydrogen cyanide, cellulase and pectinase. AI01 possesses higher ability for biofilm formation on the wells of microtitre plate. Both bacterial isolates showed significant reduction of wilt incidence in eggplant seedlings pre-treated with them under greenhouse conditions. Disease incidence was reduced upto 72% when the soil was sterilized before treatment. Treatment in unsterilized soil was less efficacious. Since both B. subtilis strains exhibited several traits beneficial to the host and showed promising results when applied as bioinoculants, they may be used to develop new, safer and effective formulations as an alternative to chemical fungicides.