Mixtures of suppressive bacteria enhance biological control of tomato bacterial wilt

Tomato bacterial wilt is one of the most destructive soil-borne plant diseases in the world, causing considerable economic losses. Biological control is an effective and environmentally friendly strategy to reduce disease incidence and yield loss. In this study, six Streptomyces-like strains, NEAU-Z2, NEAU-D18, NEAU-88, NEAU-174, NEAU-C12 and NEAU-66 exhibiting high antagonistic activities against Ralstonia solanacearum were isolated from tomato rhizosphere soil and used for further study. Of which strains NEAU-D18 and NEAU-88 were compatible with each other and the mixed strain (NEAU-D18 + NEAU-88) showing the strongest antibacterial activity against R. solanacearum were prepared. Hydroponic experiment at seedling stage showed that strains NEAU-D18 and NEAU-88 had the best control efficacy at the spore concentration of 108 cfu ml-1, which were 66.6% and 80%, respectively. Therefore, strains NEAU-D18 and NEAU-88 were inoculated into tomato seedling roots with different proportions (i.e., 1:1, 1:2, and 2:1) at the spore concentration of 10(8) cfu ml(-1), and the ratio of 1:1 showed the best control efficacy (100%) on tomato bacterial wilt. Pot experiment also indicated that the mixed strain (NEAU-D18 + NEAU-88) had better biocontrol efficacy than that of single strains. Moreover, the mixed strain and R. solanacearum could utilize 31 of 51 tested compounds of tomato root exudates, indicating the mixed strain was more competitive than the single strains (27, 29) in theory. Furthermore, in vitro nutritional competition experiment showed that the incidence of tomato bacterial wilt with the addition of compounds that could not be utilized by R. solanacearum and mixed strain was equal to that of the control group (CK4-1), but the biocontrol efficacy was great different after adding six compounds that could be utilized by R. solanacearum and mixed strain, which confirmed that the nutritional competition occurred between the mixed strain and R. solanacearum. In addition, solid-state fermentation was used to prepare bioorganic fertilizer of mixed strain. The application of bioorganic fertilizer could effectively suppress the occurrence of tomato bacterial wilt, and the incidence was only 21.67 at 14 days post-challenge inoculation, while 85% of tomato plants developed bacterial wilt with organic fertilizer treatment. In conclusion, this study demonstrated that the mixed strain (NEAU-D18 + NEAU-88) showed better control efficacy than that of single strains and was a potential biocontrol agent for controlling tomato bacterial wilt.