The endophytic strain Bacillus velezensis OEE1: An efficient biocontrol agent against Verticillium wilt of olive and a potential plant growth promoting bacteria

The use of antagonists in the biocontrol of soilborne pathogens is a promising approach to manage many diseases. In this context, Bacillus velezensis strain OEE1, isolated from endogenous root tissue of olive trees, showed the most effective antifungal activity under in vitro conditions against V. dahliae with inhibition rate exceeding 92% using the dual culture method to investigate effects of diffusible compounds. This antagonism was also demonstrated with volatile and extracellular compounds where fungus growth was reduced by about 40% and 60% respectively. Gnotobiotic study of axenic in vitro olive plants, artificially inoculated with V. dahliae, using B. velezensis OEE1 cells led to a considerable reduction in Verticillium wilt severity and the disease index effectively decreased for preventive and curative treatments. Vegetative growth showed equally an interesting improvement. These potentialities are provided due to the production of numerous secondary metabolites detected by antibiotic biosynthesis genes amplification. The efficacy of B. velezensis OEE1 to suppress Verticillium wilt of olive tree was evaluated also under field conditions. This experiment revealed that B. velezensis OEE1 significantly decreased final mean disease severity index (FMS), percentage of dead plants (PDP) and area under disease progress curve (AUDPC) and reduced similarly microsclerotia density in the naturally infested soil.