Bacillus velezensis T149-19 and Bacillus safensis T052-76 as Potential Biocontrol Agents against Foot Rot Disease in Sweet Potato

Sweet potato (Ipomoea batatas) tuberous roots are used for human consumption, animal feed, and many industrial products. However, the crop is susceptible to various pests and diseases, including foot rot disease caused by the phytopathogenic fungus Plenodomus destruens. Biological control of plant pathogens by Bacillus species is widely disseminated in agrosystems, but specific biological control agents against the foot rot disease-causing fungus are not yet available. Our previous studies showed that two Bacillus strains isolated from sweet potato roots-B. safensis T052-76 and B. velezensis T149-19-were able to inhibit P. destruens in vitro, but data from in vivo experiments using simultaneously the fungus and the bacteria were missing. In this study, both strains were shown to protect the plant from the disease and to mitigate the symptoms of foot rot disease in pot experiments. Total fungal community quantification using real-time PCR showed a significant decrease in the number of copies of the ITS gene when the bacteria were inoculated, compared to the control (with the fungus only). To determine the genes encoding antimicrobial substances likely to inhibit the fungus, their genomes were sequenced and annotated. Genes coding for mycosubtilin, bacillaene, macrolactin, bacillibactin, bacilysin, plantazolicin, plipastatin, dificidine, fengycin and surfactin were found in B. velezensis T149-19, while those coding for bacylisin, lichenysin, bacillibactin, fengycin and surfactin were found in B. safensis T052-76. Altogether, the data presented here contribute to advancing the knowledge for the use of these Bacillus strains as biocontrol products in sweet potato.