Fuente: WIPO "tobacco agriculture" The invention discloses a bacillus strain for preventing and treating two diseases and a microbial agent and a preparation method and application of the microbial agent. The bacillus strain for preventing and treating the two diseases is Bacillus amyloliquefaciens (CJ20), CJ20 was collected in the China General Microbiological Culture Collection Center (CGMCC) which is the collection unit, on October 11th, 2012, and the collection number is CGMCC NO.6666. The microbial agent of CJ20 is prepared by carrying out slant culture, seed culture and fermentation on CJ20. The preparation method comprises the steps of slant culture, seed culture, fermentation and preparation of the microbial agent; the microbial agent of CJ20 is used for preparing a pesticide for controlling tobacco root knot nematode diseases and tobacco black shank. CJ20 is a preparation containing active components and has obvious prevention and treatment effects both on the tobacco black shank and root knot nematode.