First Report of Stem Rot Caused by <i>Colletotrichum panzhouensis</i> on Huangjing ( <i>Polygonatum kingianum</i> )

Polygonatum kingianum (dian huang jing), a perennial herb valued in traditional Chinese medicine for its bioactive polysaccharides and steroidal saponins, is increasingly threatened by stem rot disease across major cultivation regions in China (Tan et al., 2021). In August 2024, severe stem rot symptoms were observed in commercial fields in Panzhou County, Liupanshui, Guizhou Province, China (25°82′N, 104°53′E; elevation 2,113 meters). Initial symptoms included water-soaked, orange-red lesions on rhizomes, progressing to extensive soft rot, vascular browning, and tissue maceration. Aboveground symptoms featured progressive wilting, chlorosis, stunted growth, and eventual plant death. Five diseased underground stem tissues were surface-sterilized with 2% NaClO for 60 second, followed by three sterile-water rinses, then plated on potato dextrose agar (PDA) and incubated at 25°C in darkness. Three morphologically similar isolates (GF-1 to GF-3) were purified via mono-spore culturing. Colonies developed dense, cottony white aerial mycelia transitioning to pale gray after 7 days, with profuse salmon-orange conidial masses. Conidia were hyaline, aseptate, cylindrical with obtuse ends, measuring 13.2-16.8 × 4.5-5.7 µm (15.1 ± 0.9 × 5.0 ± 0.3 µm). Appressoria were dark brown, clavate to irregular, measuring 6.5-10.2 × 4.8-7.1 µm. For molecular identification, genomic DNA was extracted from the isolates (Zhao et al., 2020). Five loci were amplified: act, chs1, gapdh, his3, ITS, and tub2. PCR conditions followed standard protocols (Liu et al., 2022). A maximum-likelihood phylogeny (1,000 bootstraps) of concatenated sequences (act-chs1-gapdh-his3-ITS-tub2) constructed using MEGA-X version 10.1.6 (Kumar et al., 2018) showed that the isolates (GF-1 to GF-3) form a monophyletic clade, which was denoted as a new species (Colletotrichum panzhouensis). Pathogenicity tests used 2-year-old healthy P. kingianum plants grown in autoclaved soil in a pot (8.7 cm height, 9.7 cm upper diameter, and 6.7 cm bottom diameter). Each pot was irrigated with 5 ml of conidial suspension (10⁶ conidia/ml). Control plants received sterile water. Plants were maintained in a greenhouse (25°C; 85% RH; 12-h photoperiod). All inoculated plants underground stem developed water-soaked lesions within 9 days, progressing to severe necrotic rot and wilting at 17 days post-inoculation, matching field symptoms. Controls remained asymptomatic. The pathogens were reisolated from lesions and identified by morphological and molecular data, fulfilling Koch’s postulates. To our knowledge, this is the first global report of C. panzhouensis causing stem rot on P. kingianum. This finding underscores that C. panzhouensis may pose a significant threat to the sustainable cultivation of Polygonatum kingianum. Urgent research on host resistance, fungicide efficacy, and cultural controls is warranted.