Identifying Key Pathogens and Effective Control Agents for <i>Astragalus membranaceus</i> var. <i>mongholicus</i> Root Rot

Root rot is one of the most serious diseases affecting <i>Astragalus membranaceus</i>, significantly reducing its yield and quality. This study focused on root rot in <i>Astragalus membranaceus</i> var. <i>mongholicus</i>. Pathogenic fungi were isolated and identified. The pathogenicity of seven strains of pathogenic fungi was verified according to Koch’s postulates. The inhibitory effects of eight classic fungicides and nine strains of biocontrol agents on the pathogenic fungi were determined using the mycelial growth rate method. Through morphological and ITS phylogenetic analyses, strains CDF5, CDF6, and CDF7 were identified as <i>Fusarium oxysporum</i>, while strains CDF1, CDF2, CDF3, and CDF4 were identified as <i>Fusarium solani</i>. Indoor virulence tests showed that, among the eight tested fungicides, carbendazim exhibited the strongest inhibitory effect on the mycelial growth of both <i>F. oxysporum</i> and <i>F. solani</i>, with a half-maximal effective concentration (EC₅₀) value of (0.44 ± 0.24) mg/mL, making it a highly promising chemical agent for the control of <i>A. membranaceus</i> var. <i>mongholicus</i> root rot. Among the nine biocontrol agents, KRS006 showed the best inhibitory effect against the seven pathogenic strains, with an inhibition rate ranging from 42.57% to 55.51%, and it can be considered a candidate strain for biological control. This study identified the biocontrol strain KRS006 and the chemical fungicide carbendazim as promising core agents for the biological and chemical control of <i>A. membranaceus</i> var. <i>mongholicus</i> root rot, respectively, providing a theoretical foundation for establishing a dual biocontrol–chemical control strategy. Based on the excellent performance of the biocontrol bacteria and fungicides in the pathogen control tests, future research should focus on field trials to verify the synergistic effect of this integrated control strategy and clarify the interaction mechanism between the antibacterial metabolites produced by the biocontrol bacteria KRS006 and carbendazim. Additionally, continuous monitoring of the evolution of <i>Fusarium</i> spp. resistance to carbendazim is critical to ensure the long-term sustainability of the integrated control system.