Optimization of the Fermentation Conditions of Compound Microbial Fertilizer and its Potential for Controlling Apple Replant Disease

Apple replant disease (ARD) negatively affects the growth and yield of apple plants worldwide. Here, seven strains of Bacillus QSB-6, HSB-2, XNRB-3, XC1, 2RS-9, 4R-5, and 9R-5 with strong antagonistic effects on the main pathogens that cause ARD were screened through the plate confrontation test. These strains were compatible with each other and could be co-cultured; thus, the antagonistic bacteria were compounded according to a volume ratio of 1:1 to make a compound bacterial solution. Subsequently, single factor and orthogonal experiments were used to select and optimize the fermentation medium and fermentation conditions. The optimal fermentation conditions were peptone 10.00 g·L-1, ammonium chloride 5.00 g·L-1, maltose 20.00 g·L-1, rotation speed 180 r/min, temperature 30 ℃, inoculation amount 5%, and pH 7. The fermentation medium was mixed with the microbial fertilizer carrier at an inoculation amount of 5% to make a compound microbial fertilizer. Finally, the control effect of compound microbial fertilizer was verified under potted conditions and field experiment. The application of compound microbial fertilizer in replanted soil could promote the growth of Malus hupehensis Rehd. Seedlings and apple seedlings. It also increased the plant height, ground diameter, aboveground fresh weight, underground fresh weight, aboveground dry weight, underground dry weight, the total length of roots, root surface area, and the number of root tips and forks. The activity of superoxide dismutase, peroxidase, catalase, sucrase, urease, and phosphatase also increased significantly, and the malondialdehyde activity decreased significantly. The number of bacteria in soil was significantly increased, and the number of fungi in soil was decreased under compound microbial fertilizer application. The relative abundance of fungal communities in the soil has changed. Among them, the relative abundance of Fusarium is significantly reduced. Compared with CK1, the relative abundance of T3 is reduced by 68.87%. Therefore, the application of compound fertilizer with seven Bacillus can optimize the microbial structure of replanted soil, promote the growth of M.hupehensis Rehd.Seedlings, and alleviate ARD. Compound fertilizer could thus be used as a biological control agent for ARD.