Aims Root rot caused by Fusarium solani is an important disease seriously affecting the yield and quality of Astragalus membranaceus. Therefore, this study was performed to elucidate the antifungal activities and mechanisms of cinnamaldehyde treatment against F. solani and its control effect for A. membranaceus root rot. Methods and Results Cinnamaldehyde significantly inhibited mycelial growth and spore germination of F. solani in dose-dependent, and the median effective concentration was 178.68 μl l^-1. Furthermore, scanning electron microscopy (SEM), propidium iodide (PI) staining, cell leakage experiments and ergosterol quantitation illuminated that cinnamaldehyde could alter the mycelial morphology, damage the plasma membrane and hinder the biosynthesis of ergosterol. Besides, cinnamaldehyde induced the generation of reactive oxygen species (ROS) by synergistically upregulating the genes encoded subunits for NADPH oxidase. The disease suppression efficacy of 600 μl l^-1 cinnamaldehyde against A. membranaceus root rot was 92.98 ± 6.08% (P < 0.05) under greenhouse conditions. Conclusions This study proved that cinnamaldehyde could markedly inhibit the growth of F. solani in vitro and effectively suppress the occurrence of A. membranaceus root rot, perhaps by inducing oxidative damage, which results in the distortion of F. solani, and the destruction of cell membrane integrity and permeability. Significance and Impact of the Study This study first explores the antifungal mechanisms of cinnamaldehyde against F. solani in vivo and vitro, thereby providing a promising candidate for disease bio-control.

Pepper (Capsicum annuum L.) is a lucrative crop, but problems arise when it is cultivated intensively, such as soil-borne disease outbreaks, excessive chemical fertilization, and soil degradation, reducing pepper’s yield and quality. Here, we experimentally investigated how the biocontrol agent Bacillus subtilis affects pepper growth, disease incidence, fruit quality, soil physicochemical properties, and its rhizosphere microbial community. In plate confrontation assays, this strain has a strong antagonistic effect against Phytophthora capsici, a major plant pathogenic oomycete causing pepper blight, inhibiting its mycelial growth by 56%. Field experiments show that, compared with the fertilizer control group (CF+OF), adding a biocontrol agent (CF+OF+KC) significantly reduces disease incidence, to 3% (a 85.7% reduction). Notably, the activity of systematic host defense enzymes (PAL, PPO) was negatively associated with disease incidence. In response, pepper yield increased by 17%, while its vitamin C content—a key fruit quality indicator—was greatly enhanced, by 55.1%. In terms of microbial community composition, the biocontrol agent-treated group had increased relative abundances of specific bacterial taxa, such as Lactobacillus, Sphingomonas, and Kosakonia, with less-abundant fungal genera, namely Pseudeurotium, Wardomyces, and Fusarium. The abundance of specific bacterial taxa, such as Lactobacillus, was positively associated with pepper fruit’s vitamin C content and soil available nitrogen. Inferred functional profiling suggested that biocontrol agent treatment was associated with an enrichment of microbial growth- and metabolism-related pathways while reducing the predicted abundance of pathogen-associated virulence pathways. Collectively, these findings provide a theoretical framework and technical basis for leveraging beneficial microbial inoculants in eco-friendly pepper cultivation, contributing to its future sustainability.