A novel pathogen Colletotrichum karstii causing leaf anthracnose of golden camellia ( Camellia sect. Chrysantha ) in China

Anthracnose represents the most severe disease threatening the development of the golden camellia industry (Liang, 1990; Liang et al., 2012). In January 2023, field surveys of golden camellia leaf anthracnose were conducted across major cultivation regions in Guangxi province. The pathogen infected from the edges and internal of leaves. Internal lesions were nearly circular or irregular in shape. Initial lesions were brown with a light yellow halo around the edges, and then developed into light brown or grayish-brown necrotic spots. Immature leaves infected by the pathogen develop malformed blades bearing reddish-brown lesions encircled by reddish halos. Diseased leaves were collected from wild and cultivated golden camellia stands, with disease incidence spanning 60% to 100%. Symptomatic leaf tissues were excised, surface sterilized and placed onto potato dextrose agar (PDA) for fungal isolation. Four target isolates originated from Nanning (1), Wuzhou (1) and Fangchenggang (2). Representative isolates C520 and C580-3 were subjected to morphological characterization, molecular identification, pathogenicity assays and phylogenetic reconstruction. On PDA, colonies initially appeared white, gradually turning grayish-white, and developed orange conidial masses at the center. The reverse side was colorless to pale pink, often with dark flecks. Conidia were aseptate, hyaline, cylindrical, smooth-walled, straight, and featured an obtuse apex, 11.8 to 15.2 × 4.5 to 6.7 μm (n = 50). Appressoria were irregular in shape, solitary, and sepia to dark brown, 6.9 to 9.6 × 4.3 to 7.2 μm (n = 30). These morphological traits were consistent with those previously described for C. karstii (Yang et al. 2011). For molecular identification, internal transcribed spacer (ITS), chitin synthase (CHS-1), and beta-tubulin (TUB2) were amplified and sequenced using the primer pairs ITS5/ITS4, CHS-79F/CHS-345R and TUB2Fd/TUB4Rd, respectively (Weir et al. 2012; White et al. 1990). The resulting sequences of ITS (accessions nos. PX931342, PX931344), CHS-1 (PZ520731, PZ520733) and TUB2 (PZ520734, PZ520735) were upoaded in GenBank. BLAST searches revealed that six sequences of two isolates all shared 100% sequence identity to Colletotrichum karstii reference sequences in GenBank library. Pathogenicity assays were performed on ten 2-year-old cutting-propagated seedlings. Leaves of five plants were inoculated by conidial suspension (1 × 10⁶ conidia/ml) of isolate C580-3; the other five served as controls and received sterile water. All plants were maintained in a greenhouse at 28 ± 2°C. Two weeks post-inoculation, typical anthracnose symptoms identical to those observed in the field developed, while control leaves remained asymptomatic. To fulfill Koch’s postulates, the fungus was consistently re-isolated from symptomatic tissues of inoculated plants but not from controls, confirming its pathogenicity. Although C. karstii has previously documented as the causal agent of anthracnose on tea (Camellia sinensis) in China (Wang et al., 2016), this study represents the first record of the pathogen infecting golden camellia. This new host record complements the pathogen diversity of golden camellia anthracnose, furnishes pathogen evidence for epidemic surveillance, targeted management and resistance evaluation, and bears theoretical and practical significance for golden camellia and germplasm conservation.