Biocontrol potential and mode of action of entomopathogenic bacteria Xenorhabdus budapestensis C72 against Bipolaris maydis

Southern corn leaf blight (SCLB) caused by Bipolaris maydis is an important foliar disease of maize. In this study, a nematode-symbiotic bacterium Xenorhabdus budapestensis strain C72 was identified with remarkable inhibiting effect on mycelial growth and spore germination of B. maydis. The in vitro assay revealed that C72 cell-free culture media (CFCM) with thermostability exhibited broad-spectrum antifungal activities against other several important plant pathogenic fungi. The early colonization of B. maydis were significantly impaired by CFCM treatment under phytotron condition. This antagonism is likely to be the main contributor to the highly efficient plant protection of 40% (v/v) CFCM treatment against B. maydis, and the relative control effect reached to 59.15% and 77.96% in greenhouse and field experiments, which was comparable to the effect of fungicides. Moreover, we found that extracellular enzymes secreted by symbiotic bacterium may be one of the reasons for the antifungal potential of C72. Beside direct antagonistic effects provided by the bacterium, defense related genes were induced in maize after CFCM treatment. In summary, this study reported the first systematic evaluation of the effect of X. budapestensis C72 in controlling SCLB and exploration of its mode of action, then indicating that entomopathogenic bacteria have the potential to become a new and efficient biological control resource for plant fungal disease management.