Identification of rice blast disease-suppressing bacterial strains from the rhizosphere of rice grown in Pakistan

Sixteen bacterial strains isolated from the roots and rhizosphere of rice plants growing in saline and non-saline soils from the Shorkot area of Pakistan were tested for their ability to promote plant growth and reduce the incidence of rice blast disease. When applied to the soil, many of the isolated rhizobacterial strains increased seedling growth and/or suppressed rice blast disease in greenhouse-grown plants of the cultivars Super Basmati and Azucena, but each cultivar responded to different subsets of the bacteria. In the cv Super Basmati, increased blast resistance was correlated with the production of siderophores by the rhizobacteria. Several strains inhibited the growth of the causative agent of rice blast disease, the fungal pathogen Magnaporthe grisea, in an in vitro dual culture assay. Direct bioantagonism was correlated with disease resistance in Super Basmati, but not in Azucena, and direct antagonism as a cause for the reduced disease incidence is also unlikely since no epiphytic colonisation of leaves was detected. Rhizosphere colonisation by the bacteria in plants grown in sterile sand was correlated with disease resistance in Super Basmati, but not in Azucena. As well as the differences in strains that protected each cv against blast disease, we also found that there were differences in the ability of some strains to protect plants against blast depending on soil type. Hence, there are complex interactions between rhizobacteria and rice plants with respect to biocontrol of rice blast disease, dependent upon both rice cv and soil type. The identity of strains that promoted high levels of disease protection, including three that performed well across all plant cultivars and growth conditions, was determined by 16S rRNA gene sequencing.