Abstract:To explore the antagonistic mechanism of rhizospheric bacteria against Phytophthora nicotianae van Breda de Haan, five antagonistic Pseudomonas fluorescens strains were isolated from rhizospheric soils of tobacco fields in Chongqing. Efficient antagonistic strain against P.nicotianae was screened using dual culture technique and metabolites inhibitory test. Among these strains, P-72-10 showed the highest suppressive effect; it produced an inhibition zone of 13.0 mm (radius) and an inhibitory rate at 68.57%. Furthermore, the growth of pathogen’s hypha was also suppressed by the extracellular metabolites of strain P-72-10 from 25.39% to 46.03%. P.nicotianae was observed microscopically excessive branching of mycelia and malformation of mycelia tips; thick-walled cells with concentrated plasmas and chlamydospore-like cells. When tested on the tobacco plants grown in greenhouse, strain P-72-10 got a good biocontrol effect on tobacco black shank infection and effects on the resistant and susceptible varieties of tobacco were 53.57% and 66.37% respectively.