Isolation of Antagonistic Rhizosphere Bacteria Toward Phytophthora capsici Induce Phytophthora Blight in Pepper (Piper nigrum)

Vietnam has been the largest exporter of pepper globally in recent years. However, the quick death disease caused by Phytophthora capsici is spearing rapidly, causing notable damage to many concentrated cultivation areas. The regular application of chemical pesticides to combat the diseases at pepper farms has increased the certain environmental problems. To reduce pesticide usage, biological methods for controlling P. capsici have been implied. In this study, rhizosphere bacterial strains were isolated from pepper plantations, and their chitinolytic and phytophthora antagonistic activities were evaluated. The chitinolytic activity was conducted on an agar medium supplemented with chitin (CM), and the antagonistic activity was done using a dual culture inhibition assay. As the result, 46 strains (signed NH1 - NH46) were isolated based on morphological distinctions in the CM medium. Out of the 46 isolated strains, 8 strains including NH7, NH10, NH11, NH27, NH31, NH32, NH33, and NH46, which accounted for 17 % of the isolates, showed high chitinolytic activity. In the dual culture assay, the strain NH7 showed the highest effectiveness that inhibiting the P. capsici mycelial growth with an antagonistic distance of 20.33 mm, followed by three strains NH27, NH32, and NH46 antagonistic distances of 13.67 – 15.33 mm. These strains were further identified as E. cloacae, S. flaveus, K. pneumoniae, and B. amyloliquefaciens through 16S rRNA sequencing. A phylogenetic tree showed a closed connection between the antagonistic isolated strains and antagonistic bacteria reported previously.