The Fusarium root rot-controlling effect and growth-promoting effect of the bacteria in the rhizosphere of Capsicum annuum

Abstract:

[Objective] To screen out the bacterial strains that can control the Fusarium root rot and promote the growth of Capsicum annuum L. and clarify their disease-controlling and growth-promoting effects. [Methods] The soil samples were collected from the rhizosphere of healthy C.annuum plants, and then plate confrontation method was employed to screen the biocontrol bacteria with Fusarium solani and F.oxysporum as the indicator fungi. After that, selective media were used to screen the growth-promoting bacteria with inorganic phosphorus-solubilizing, organic phosphorus-solubilizing, nitrogen-fixing, or potassium-solubilizing activities. Further, the disease-controlling and growth-promoting effects of the selected strains were determined qualitatively and quantitatively. The amount of phosphorus solubilized, nitrogen fixed, and potassium solubilized were respectively determined by molybdenum-antimony anti-colorimetric method, Kjeldahl method, and flame atomic absorption spectrometry. We then determined the 16S rDNA sequences of the strains with excellent characteristics and prepared the bacterial inoculants with different combination formula. Finally, pot experiments were carried out to measure the disease-controlling and growth-promoting effects of the inoculants. [Results] We screened out 323 strains with excellent functions, including 78 antagonistic strains, 87 organic phosphorus-solubilizing strains, 107 inorganic phosphorus-solubilizing strains, 128 nitrogen-fixing strains, and 123 potassium-solubilizing strains. Some strains had multiple functions, and 6 combinations with excellent characteristics were obtained, which involved 8 strains. Strains XP271 and XP181 were identified as Bacillus subtilis, XP125 as B.tequilensis, XP236 as B.halotolerans, XP79 as B.megaterium, XP171 as B.circulans, XP248 as Cellulosimicrobium funkei, and XP167 as Pseudomonas synxantha. The inoculants were prepared according to two combination formula, which demonstrated the best controlling effect of 88.52% on Fusarium root rot and increased the plant height, branch number, and biomass of C.annuum by about 10 cm, 2 branches, and 5-21 g. Moreover, after the inoculation, the content of available nitrogen, available phosphorus and available potassium in the rhizosphere soil, as well as the activities of soil enzymes such as urease, sucrase, and alkaline phosphatase increased, while the activity of soil catalase decreased. Additionally, the microbial biomass carbon, nitrogen-fixing genes, and nitrogen-fixing microorganisms in the soil increased significantly. [Conclusion] The rhizosphere soil of C.annuum harbors rich bacteria with excellent disease-controlling and growth-promoting effects, which can be prepared into inoculants for the control of Fusarium root rot and the growth-promoting on C.annuum.