Biocontrol of tomato bacterial spot by novel Bacillus and Pseudomonas strains

Bacterial spot is a disease that affects tomato worldwide reducing its yield and quality. It is caused by different Xanthomonas spp., among which is Xanthomonas vesicatoria . Copper-based bactericides are generally used to control this disease, although nowadays sustainable strategies are being searched to efficiently replace their use. Our aim was to select native bacteria from tomato rhizosphere with biocontrol properties against X. vesicatoria . We selected, characterized, and identified three novel strains, two closely related to Bacillus velezensis (VMA11p and VM05) and one closely related to Pseudomonas soli (VMAP1), that in vitro antagonized X. vesicatoria. We evaluated the efficacy of the three rhizobacteria and their cell-free supernatants to control bacterial spot using the model tomato- X. vesicatoria in plants grown in pots, in greenhouse conditions. Bacterial suspensions of VMA11p and VMAP1, applied to the soil by irrigation, significantly ( P < 0.05) reduced bacterial spot severity by 53.9% and 44.2%, respectively. Nevertheless, the most effective strategy to control bacterial spot was achieved using the cell-free supernatant produced by VMA11p, VM05 or VMAP1 applied as foliar spray, which significantly ( P < 0.05) reduced the severity of the disease by 98.5%, 94.2% and 75.2%, respectively. None of the treatments reduced the growth of tomato plants. Our results suggest that the use of these novel strains of Bacillus and Pseudomonas and/or their metabolic products could be used for the development of biocontrol strategies for the management of bacterial spot in tomato.