Characterization of Colletotrichum siamense Causing Leaf Anthracnose on Cornus officinalis and Its In Vitro Sensitivity to Fungicides in China

Cornus officinalis is a valuable traditional Chinese medicinal (TCM) plant species with both therapeutic and ornamental attributes. It is widely used in TCM prescriptions to nourish the liver and kidneys and constitutes a critical component of numerous classical formulas. In recent years, the large-scale cultivation of this medicinal plant has been expanded in Xixia County, Henan Province, China. Field investigations have revealed widespread brown leaf spot, accompanied by reductions in yield and quality. In this study, symptomatic leaves were collected for pathogen isolation. Tissue isolations consistently yielded a Colletotrichum fungus, and morphology combined with multi-locus phylogenetic analyses (the internal transcribed spacer, glyceraldehyde-3-phosphate dehydrogenase, chitin synthase, actin, and β-tubulin) identified the pathogen as Colletotrichum siamense. Pathogenicity assays (conducted by either wounding and inoculating detached leaves with a mycelium plug or spraying a conidium suspension on healthy potted plants) reproduced field symptoms, and the pathogen was re-isolated, thereby fulfilling Koch’s postulates. In vitro fungicide assays showed that carbendazim, tebuconazole, and prochloraz were highly effective against the pathogen, providing preliminary information for chemical management. This is the first documentation of C. siamense causing leaf anthracnose on C. officinalis and provides a basis for developing targeted control strategies to mitigate disease impacts and preserve yield and quality.