Leaf spot disease has recently emerged on Euonymus fortunei in Shanxi Province, China, causing noticeable foliar lesions and reducing ornamental value. The causal agent was identified as Alternaria alternata based on morphological characteristics and multilocus phylogenetic analysis using five loci (Alt a1, gapdh, RPB2, ITS, and OPA10-2). Biological characterization showed that mycelial growth was optimal at 27 ℃ on oatmeal agar, whereas conidial germination was favored under alkaline conditions (pH 9.0). The pathogen was sensitive to NaCl stress but showed no significant response to different light regimes. In vitro fungicide sensitivity assays revealed marked differences in inhibitory efficacy between mycelial growth and conidial germination, indicating strong developmental stage-dependent responses. Certain fungicide mixtures exhibited enhanced inhibitory effects compared with single compounds, suggesting potential synergistic interactions. Microscopic observations further revealed fungicide-induced morphological abnormalities, including hyphae swelling, roughened surfaces, reduced sporulation, and deformation of conidial germ tubes. This study represents the first report of A. alternata causing leaf spot disease on E. fortunei in China and provides a basis for understanding pathogen biology and developing effective chemical management strategies for this emerging ornamental disease.