Optimization and evaluation of a compound bacterial agent degrading autotoxins of tobacco

Abstract:

[Background] Long-term continuous cropping of tobacco (Nicotiana tabacum) leads to the accumulation of autotoxins, which aggravates the incidence of tobacco bacterial wilt (TBW) caused by Ralstonia solanacearum and causing great economic losses of tobacco production. [Objective] To develop a compound bacterial agent capable of controlling TBW in the field with long-term continuous cropping of tobacco. [Methods] We formulated compound bacterial agents with screened strains capable of degrading autotoxins. Orthogonal design and single factor experiments were employed to optimize the strain ratio and the dosages of additives. The inhibition performance of the compound bacterial agent on TBW was evaluated in a greenhouse and in a field with continuous cropping of tobacco for 15 years. [Results] Orthogonal experiments showed that the optimal ratio of Bacillus sp. NO1, Brucella sp. NO8, Bacillus sp. NO9, and Bacillus sp. NO10 in the compound bacterial agent was 1:3:4:2. Single factor experiments showed that the best vector was silica. The best wetting agent and dispersant were sodium hexametaphosphate (SHMP) and sodium butylnaphthalene sulfonate (SBNS), respectively, which were optimized to be added at the dosages of 2%. The optimal concentration of the stabilizer glycerin was 1.0%. Pot experiments showed that both the degradation rate of six autotoxins and the inhibition rate of TBW by the compound bacterial agent reached over 78%. The results of the field experiment showed that the compound bacterial agent diluted by 100 folds had significant degradation effects on six autotoxins in the tobacco field with continuous cropping for 15 years. Moreover, the agent significantly increased the height, stem circumference, and waist leaf length and width of tobacco plants, thus promoting the growth and development of tobacco. In addition, the agent regulated the rhizosphere microbiota of tobacco, as manifested by the decreased relative abundance of Clostridium_sensu_stricto_1, Ralstonia, and Cellulomonas and the increased relative abundance of Devosia, Flavobacterium, and Sphingomonas. The agent decreased the incidence rate and disease index of TBW from 92.22% and 48.19% to 18.15% and 9.52%, respectively, with a control effect of 80%. [Conclusion] The optimized compound bacterial agent significantly reduces the incidence rate and disease index of TBW in the field with long-term continuous cropping of tobacco, which provides a solution for the prevention and control of TBW.