Occurrence of Leaf Spot on Solidago canadensis caused by Colletotrichum aenigma in China

Solidago canadensis is a rhizomatous perennial plant native to North America that has invaded eastern China and continues to spread northward and westward (Uzelac et al. 2025). In June 2025, leaf spot on S. canadensis was obsered in Chibi (29.811199° N, 113.577812° E), Xianning City, Hubei Province, China. The disease incidence reached 10% across surveyed fields covering 100 square meters. Initially, small necrotic spots appeared, which rapidly expanded into irregular grayish-brown lesions with a reddish-brown outer ring. Collected leaf samples were used to isolate the pathogen by single-spore isolation method (Choi et al. 1999). The isolates grown on potato dextrose agar (PDA) plates at 25°C in the dark for 5 to 7 days of incubation with similar morphology, and two representative isolates, QM-1 and QM-2 were used for further study. The colony surface presents a circular, white, cottony morphology and undergoes a brownish-gary discoloration. Conidia were unicellular, oval, and hyaline, usually in size from 10.47 to 21.16 × 3.14 to 6.98 μm in size (n = 50). The morphological characteristics were consistent with published descriptions of the Colletotrichum sp. (Liu et al. 2022). For accurate classification, genomic DNA of two representative isolates was extracted from mycelia using a DNA extraction kit (TSINGKE Biotech Co., Ltd.). For molecular identification, the regions of actin ( ACT ), glyceraldehyde-3-phosphate dehydrogenase ( GAPDH ), and ribosomal internal transcribed spacer (ITS) (Weir et al. 2012) were amplified and sequenced. The obtained sequences were deposited in GenBank (ACT: PZ213906, PZ213907; GAPDH: PZ213908, PZ213909; ITS: PZ658286, PZ658287. BLASTn results revealed that the sequences of ACT, GAPDH , and ITS were identical to C. aenigma ICMP 18608 (JX009443, JX010044, JX010244). A phylogenetic tree was constructed using the neighbor-joining method in MEGA 7 (Kumar et al. 2016). The two isolates fell into the clade of C. aenigma , supported by bootstrap value of 96%. Pathogenicity tests were conducted using three healthy S. canadensis plants. Conidia were collected and re-suspended in sterilized distilled water (1×106 conidia/mL). Five leaves per seedling were selected for inoculation, each of conidial suspension was inoculated on two sites near midrib and sterile water was used as controls. All the treated plants were covered with plastic bags to maintain high humidity and placed in a greenhouse at 25°C with a photoperiod of 12 h. Grayish-brown lesions similar to those in the field appeared on leaves 5 days after inoculation. While no symptoms were observed on the control leaves. The same fungus was reisolated from the diseased leaves based on morphological characterization and sequence analyses but not from the healthy controls, which fulfilled Koch’s postulates. The same treatment was repeated for three times. Previous studies have reported that C. aenigma can cause anthracnose on the leaves of various economic crops in China, such as apple, tree peonies, mulberry, and walnut (Wang et al. 2021; Wang et al. 2023; Zhang et al. 2021). To the best of our knowledge, this is the first report of C. aenigma causing anthracnose on S. canadensis in China, which can lay a theoretical foundation for disease diagnosis, epidemic dynamics, and the development of effective management strategies.