Mechanisms underlying the biocontrol activity of Serratia marcescens YNAU-SM-1 against Phytophthora infestans in potato

The use of beneficial microorganisms offers a sustainable alternative to chemical fungicides for managing plant diseases. This study aimed to investigate the biocontrol potential and underlying mechanisms of Serratia marcescens (YNAU-SM-1) for controlling potato late blight caused by Phytophthora infestans. Dual culture assay, fluorescent dye staining, spore germination inhibition assays, in vitro bioassays, greenhouse experiments, and biochemical analyses of defense-related enzymes and osmotic regulators were conducted to determine the effects of YNAU-SM-1 using P. infestans HJG02 as a target pathogen. Dual culture assays revealed that YNAU-SM-1 significantly inhibited the mycelial growth of P. infestans HJG02 by 70.26%. Abnormal hyphal expansion and excessive branching were also observed under microscopic observations. Fluorescent dye staining revealed that YNAU-SM-1 induced intracellular accumulation of reactive oxygen species (ROS) in P. infestans HJG02, leading to sporangial inactivation. Spore germination inhibition assays suggested the bacterial strain strongly inhibited sporangial direct germination (98.86%), zoospore release (70.13%), and cystospore germination (100%). Detached leaf, tuber slice, and greenhouse assays confirmed the efficacy of YNAU-SM-1, with disease control efficiencies of 67.62%, 65.48%, and 71.04% under preventive, simultaneous, and curative treatments, respectively. Biochemical analyses indicated that YNAU-SM-1 increased soluble protein and reduced malondialdehyde (MDA) content, and altered the general trend of peroxidase (POD), and soluble sugar content in potato leaves. These findings suggest that YNAU-SM-1 suppresses P. infestans HJG02 through both direct antagonism and the induction of host defenses, highlighting its potential as an effective biocontrol agent for managing potato late blight.