Biocontrol of Rhizoctonia solani damping-off of tomato with Bacillus subtilis RB14

Abstract

Bacillus subtilis RB14, which showed antibiotic activities against several phytopathogens in vitro by producing the antibiotics iturin A and surfactin, was subjected to a put test to investigate its ability to suppress damping-off of tomato seedlings caused by Rhizoctonia solani. To facilitate recovery from soil, B. subtilis RB14-C, a spontaneous streptomycin-resistant mutant of RB14, was used. Damping-off was suppressed when the culture broth, cell suspension, or cell-free culture broth of RB14-C was inoculated into soil. Iturin A and surfactin were recovered from the soils inoculated with the cell suspension of RB14-C, confirming that RB14-C produced them in soil. The gene lpa-14, which was cloned from RB14 and required for the production of both antibiotics, was mutated in RB14-C, and a mutant, RΔ1, was constructed. The level of disease suppressibility of RΔ1 was low, but RΔ1(pC115), a transformant of RΔ1 with the plasmid pC115 carrying lpa-14, was restored in suppressibility. These results show that the antibiotics iturin A and surfactin produced by RB14 play a major role in the suppression of damping-off caused by R. solani. RB14-C, RΔ1, and RΔ1(pC115) persisted in soil during the experimental period and were recovered from the soil, mostly as spores.