Inhibitory Effect of Paenibacillus polymyxa GBR‐462 on Phytophthora capsici Causing Phytophthora Blight in Chili Pepper

Abstract In the present work 25 strains of Paenibacillus polymyxa isolated from rotted ginseng roots were screened for their antimicrobial activity against Phytophthora capsici in vitro. Based on antimicrobial activity, 15 strains categorized as strongly antimicrobial, among them GBR‐462 was found as the most active, and five strains each as weekly antimicrobial and no antimicrobial. Antimicrobial activity was influenced by the initial inoculum density, as strains of P. polymyxa with a strong antimicrobial activity (including P. polymyxa GBR‐462) showed the antimicrobial activity against P. capsici and could form biofilm only when they were applied at the higher initial inoculums, 108 cfu/ml. No inhibitory effect was noted on the mycelial growth and zoospore germination of the pathogen when applied at the lower inoculum density of 106 cfu/ml of P. polymyxa GBR‐462. However, sporangium formation and zoospore release was significantly inhibited at the lower inoculum density. Also light and electron microscopy revealed the structures of sporangia aberrant with no or few healthy nuclei, indicating sporangium and zoospore formation inhibited at the lower inoculum density. Application of P. polymyxa GBR‐462 into potted soil suppressed disease progression as well as disease severity; disease severity was reduced by 30% as compared to untreated pots, suggesting P. polymyxa GBR‐462 could be a potential biocontrol agent against Phytopthora capsici.