First Report of Colletotrichum fructicola Causing Leaf Spot on Smilax glabra in China

Abstract

Smilax glabra Roxb is a medicinal plant distributed in 17 countries and used in the production of food and tea (Wu et al. 2022). In May 2021, a leaf spot disease was observed on ~60% of S. glabra plants in a field (∼0.4 ha) in Qinzhou City, Guangxi Province. Initially, small, circular, brown spots appeared on the leaf surfaces, which then gradually expanded into large, sunken, dark brown necrotic areas. As disease progressed, lesions merged into large spots, eventually leading to defoliation. To determine the causal agent, six symptomatic plants were collected from the field. Small pieces (∼5 mm2) were cut from the infected leaves (n = 12), sterilized for two min in 1% NaOCl, and rinsed three times in sterile water. Then, the leaf tissues were placed on potato dextrose agar (PDA) with chloramphenicol (0.1 g/liter) and incubated for 3 days at 28°C (12-h photoperiod). Pure cultures were obtained by transferring hyphal tips from recently germinated spores or colony edges onto PDA. Among the 17 isolates, 15 exhibited similar morphologies. Two single-spore isolates (TFL45.1 and TFL46.2) were subjected to further morphological and molecular characterization. Colonies on PDA were grayish green with a white outer ring and cottony surface, and pale blackish green on the reverse side. Conidia were hyaline, aseptate, straight, and cylindrical, with rounded ends, and 11.4 to 16.5 μm × 4.1 to 6.1 μm (average 13.9 × 4.8 μm, n = 100). Appressoria were brown to dark brown, with a smooth edge and different shapes such as ovoid, elliptical or irregular, and 6.8 to 8.9 μm × 5.9 to 7.8 μm (average 7.7 × 6.6 μm, n = 25). For molecular identification, eight target gene sequences, internal transcribed spacer (ITS), glyceraldehydes-3-phosphate dehydrogenase (GAPHD), calmodulin (CAL), partial actin (ACT), chitin synthase (CHS-1), glutamine synthetase (GS), manganese superoxide dismutase (SOD2), and β-tubulin (TUB) were selected for PCR amplification (Weir et al. 2012). The resulting sequences were deposited in GenBank (OR399160-61 and OR432537-50). BLASTn analysis of the obtained sequences showed 99-100% identity with those of the ex-type strain C. fructicola ICMP:18581 (JX010165, JX010033, FJ917508, FJ907426, JX009866, JX010095, JX010327, JX010405) (Weir et al. 2012). In addition, a phylogenetic analysis confirmed the isolates as C. fructicola. Therefore, based on morphological and molecular characteristics (Park et al. 2018; Weir et al. 2012), the isolates were identified as C. fructicola. To verify pathogenicity, three healthy leaves on each of six two-year-old S. glabra plants were inoculated with ∼5 mm2 mycelial discs or aliquots of 10 μl suspension (106 conidia/ml) of the strain TFL46.2, and six control plants were inoculated with sterile PDA discs or sterile water. All plants were enclosed in plastic bags and incubated in a greenhouse at 25°C (12-h photoperiod). Six days post-inoculation, leaf spot symptoms appeared on the inoculated leaves. No symptoms were detected in the controls. Experiments were replicated three times with similar results. To fulfill Koch's postulates, C. fructicola was consistently re-isolated from symptomatic tissue and confirmed by morphology and sequencing of the eight genes, whereas no fungus was isolated from the control plants. To our knowledge, this is the first report of C. fructicola causing leaf spot disease on S. glabra. Further studies will be needed to develop strategies against this disease based on the identification of this pathogen.

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