Phedimus aizoon is native to east Asian countries that including China, Siberia, Korea, Mongolia, and Japan. In China, the plant is highly valued for use in folk medicine, for detoxification and analgesia, blood pressure, hemostasis, and used as an ornamental. In August 2021, a leaf spot and blight disease were observed on P. aizoon in a 120-ha field in Pizhou, Jiangsu Province, China where disease incidence reached 90%, and almost every leaf was withered. Early symptoms appeared as dark brown lesions on leaf margins that enlarged and coalesced to form large necrotic areas. In efforts to determine the cause of the disease, ten symptomatic leaves were randomly collected from ten different plants at the site. Diseased leaf pieces that measured 5 mm2 were disinfected in 75% ethyl alcohol for 30 s and 7% NaOCl for 60 s, rinsed three times in sterile distilled water, and placed on potato dextrose agar (PDA). Ten fungal isolates obtained by single-spore isolations were selected for further study. These isolates produced colonies that measured 70 to 82 mm in diameter after 7 days growth on PDA. Colonies were black to brown in color with gray-white aerial hyphae on their surfaces. The isolates produced conidia that were ovate to pear-shaped, brown to black in color, with 1 to 4 transverse septa and 0 to 1 oblique septa, smooth surfaced, parietal cells extending into the beak, and measured 10 to 35.5 × 5.0 to 12.5 μm. Conidiophores were brown, erect or curved, branched, with pronounced spore marks, and measured 7.5 to 37.5 × 2.5 to 5.0 μm. All ten fungal isolates were morphologically similar to Alternaria alternata (Simmons 2007). Two representative isolates FC01 and FC02 were used for molecular identification. The internal transcribed spacer (ITS) region, RNA polymerase second largest subunit (RPB2), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), translation elongation factor 1-alpha (TEF1), and Alternaria major allergen (Alt a 1) were amplified with the primers ITS4/ITS5, RPB2-5F2/RPB2-7CR (Khodaei and Arzanlou 2013), gpd1/gpd2, EF1-728F/EF1-986R (Nishikawa and Nakashima 2020) and Alt-for/Alt-rev (Woudenberg et al. 2015). The resulting sequences were deposited in GenBank (ITS, ON584560, ON564492; RPB2, ON729984, ON703241; GAPDH, ON652866, ON652867; TEF1, ON652868, ON652869; Alta1, ON652870, ON652871). Phylogenetic analyses showed 100% identity between FC01 and FC02 and the type strain CBS 916.96. Thus, the fungus was identified as A. alternata based on morphology and molecular analysis. Pathogenicity tests were done by spraying conidial suspensions containing 106 conidia per ml of A. alternata isolates FC01 and FC02 on leaves of five healthy P. aizoon plants, separately. Five control plants were sprayed with distilled water and both sets of plants covered with plastic bags and placed in a greenhouse maintained at 25⁰ C. Plastic bags were removed from plants after 48 h. Dark brown lesions developed on inoculated plants after 16 days and control plants remained symptomless. The pathogenicity tests were conducted three times. A. alternata was reisolated and identified based on morphological and molecular traits, thus fulfilling Koch's postulates. To our knowledge, this is the first report of A. alternata causing leaf blight on P. aizoon in China and worldwide. Based on the plant's medicinal value, further studies should be directed toward control of this disease.