Identification and characterization of beneficial bactetria to control pea Ascochyta blight disease

Abstract

Ascochyta blight is one of the major diseases threating pea production worldwide. It is caused by a complex of fungal pathogens including Ascochyta pisi, Ascochyta pinodes, Ascochyta pinodella and Phoma koolunga. Among these pathogens, A. pinodes alone can cause yield loses of up to 50% and was the focus of this study. Our goal was to identify and characterize beneficial bacteria that can reduce the disease severity under greenhouse conditions. By screening 350 bacterial strains isolated from pea roots and soils, we found ten isolates, including Pseudomonas spp., Bacillus spp., Peribacillus spp., and Penibacillus spp., showed effective control of Ascochyta blight in greenhouse assays. All the 10 isolates inhibited A. pinodes on culture plates, indicating bacteria-derived antibiotics likely play a role in the disease control. To identify the antibiotics, we used a soil bacterium Pseudomonas protegens Pf-5 as a model because of its known production of a broad spectrum of antibiotics that are toxic to many fungal pathogens. Antibiotic production of Pf-5 requires a global regulator GacA/GacS. We found that wild type Pf-5, but not its Delta gacA mutant, inhibited A. pinodes on culture plates and reduced disease severity in greenhouse, supporting that antibiotic production contributes to disease control. Using a set of eight Pf-5 mutants, each lack one specific antibiotic, we found that a mutant lacking rhizoxin production showed decreased pathogen inhibition and reduced disease control. These results suggest that rhizoxin of Pf-5 contributes to the growth inhibition of A. pinodes and the disease control of Ascochyta blight in greenhouse conditions.