The effect of mode of application of Bacillus subtilis RB14-C on its efficacy as a biocontrol agent against Rhizoctonia solani

Bacillus subtilis RB14-C, which produces the antibiotic iturin A, was investigated for its effectiveness as a biocontrol agent against Rhizoctonia solani infecting tomato using seed coating and/or direct introduction of the bacteria to the soil. The ability of RB14-C to colonize plant roots and produce iturin A in soil, depending on the method of bacterial application, was also determined. Seed coating and the combined treatment (soil and seed bacterization) did not protect seedlings against damping-off caused by R. solani. By contrast, RB14 introduced only to the soil controlled the disease. The total number of RB14-C bacteria on the roots of plants grown from coated seeds was significantly lower than on the roots of plants grown in soil mixed with the bacteria. In the combined treatment, application of B. subtilis with seeds to soil preinoculated with this bacterium, at first suppressed the population of RB14-C in the soil. Then the colonization was generally uniform. The concentration of iturin A in non-planted soil was highest at the beginning of the experiment (i.e. after application of the bacterial suspension) but then decreased, and was undetectable 3 days after incubation. However, after seed planting the antibiotic was produced again around young roots. Bacteria introduced to the soil as a seed coating also released the antibiotic around the seeds.