First Report of Colletotrichum fructicola Causing Leaf Anthracnose on Idesia polycarpa in China

Abstract

Idesia polycarpa, a deciduous tree in the Salicaceae family, is used for oil and timber (Wu et al. 2019). In August 2025, a field survey at an I. polycarpa nursery in Zunyi City, Guizhou Province (27.9° N, 106.9° E) revealed leaf disease on seedlings, with 50% disease incidence (n=1000). Early symptoms were small light yellow spots, expanding into irregular brown to black lesions with dried margins, necrotic areas, and defoliation. Lesion edges were yellowish, centers grayish-brown to blackish-brown. Twenty diseased leaves were randomly selected from five plants. Tissue pieces (5 mm × 5 mm) from lesion margins were disinfected with 3% sodium hypochlorite for 30 s and 75% alcohol for 1 min, rinsed with sterile distilled water, placed on PDA medium, and incubated at 28℃ for five days. Three morphologically similar strains, HB-1, HB-2, and HB-869, were isolated. On PDA at 28℃ in the dark, colonies were white on the obverse and light yellow on the reverse. The texture was velvety to floccose; the aerial mycelium was well-developed, dense, and fluffy, evenly covering the entire plate. Acervuli were black, spherical to subspherical, 0.15-0.97 mm in diameter, and embedded in the mycelial layer. All isolates produced asexual structures, with black setae measuring 37.7–39.5 μm × 4.7–4.9 μm, having a cylindrical base and a gradually tapering tip. Appressoria were ellipsoidal, dark brown, measuring 6.90–11.26 μm (mean = 8.14 ± 1.26 μm) × 5.08–7.94 μm (mean = 6.50 ± 0.92 μm) (n = 50). Conidiophores were colorless to light brown, septate, and branched. Conidia were slender ovoid, colorless, unicellular, aseptate, and smooth-walled, with rounded ends and containing oil globules, measuring 8.24–13.24 μm (mean = 11.50 ± 1.35 μm) × 4.51–5.18 μm (mean = 4.87 ± 0.22 μm) (n = 50). Cylindrical, rounded-end conidia were observed, matching the morphology of C. fructicola (Khuna et al. 2025). For identification, representative isolate HB-869 was subjected to multi-locus sequence analysis using ITS, CHS-1, and GAPDH, with primers ITS1/ITS4, CHS-79F/CHS-345R, and GDF/GDR, respectively (Weir et al. 2012). BLAST searches revealed that the ITS (GenBank: PX992721), CHS-1 (PZ094120), and GAPDH (PZ094121) sequences of HB-869 showed 98.89%, 100%, and 98.57% identity to those of the C. fructicola ex-type strain ICMP 18581 (JX010165, JX009866, and JX010033, respectively). Based on morphological and molecular evidence, HB-869 was identified as C. fructicola. For pathogenicity assay, ten 1-year-old I. polycarpa seedlings, approximately 40 cm tall, were selected. Leaves were inoculated by sterile needle-pricking. Five plants received 5 mL of HB-869 conidial suspension (1×10⁶ conidia/mL), while five control plants received sterile distilled water. All were incubated at 25 °C, 70% RH, under a 12 h light/dark cycle. The assay was independently repeated three times. At 14 days post-inoculation, typical anthracnose symptoms appeared on inoculated plants, consistent with field observations, while controls remained asymptomatic. The re-isolated pathogen was identified by morphology and ITS, CHS-1, and GAPDH sequences, fulfilling Koch’s postulates. C. fructicola infects diverse hosts, including Epimedium sagittatum (Hou et al. 2024). Given the economic importance of I. polycarpa, this finding expands the known host range of C. fructicola and offers valuable insights for disease diagnosis and management. This is the first report of anthracnose on I. polycarpa caused by C. fructicola in China.