First Report of Alternaria sp. Causing Leaf Blight on Polygonatum cyrtonema in China
TL;DR
Strain AB3-4 featured brownish-green centers, white peripheral mycelium, darker central reverse fading radially, and based on morphology and phylogenetic pattern, AB3-4 was identified as Alternaria sp.
Abstract
Polygonatum cyrtonema, a traditional Chinese medicinal plant, is predominantly cultivated in southeastern and southwestern China (Tang et al. 2026). From July 2022 to August 2023, leaf blight symptoms were observed on P. cyrtonema in Yanshan County (23.52°N, 104.32°E). Brown necrotic lesions appeared at leaf apices and margins, accompanied by yellowing and withering. Disease incidence ranged from 45 to 95% among four surveyed plantations. Twenty-two symptomatic samples were randomly collected. Leaf tissue segments (5 × 5 mm) were cut from symptomatic leaves, surface-sterilized, and incubated on potato dextrose agar (PDA) at 25 °C. Among the obtained 54 fungal isolates, 34 isolates were identified as Alternaria alternata molecularly and morphologically. The remaining 20 fungal isolates differed morphologically and molecularly; pathogenicity assays and identification collectively confirmed them as A. sp. The symptom development on P. cyrtonema of the isolates was highly consistent across three independent pathogenicity assays. Therefore, strain AB3-4 was selected for detailed characterization. Strain AB3-4 featured brownish-green centers, white peripheral mycelium, darker central reverse fading radially. On potato carrot agar, conidiophores were branched or occasionally unbranched. On branched conidiophores, conidia were in dwarf tree-like branched chains of 2 to 5 conidia. On unbranched conidiophores, conidia were simple or in chains of 3 to 11 conidia. After 15 days of incubation on PDA, conidia were observed; they were light to dark brown, beakless or short-beaked, obclavate to ellipsoid, with distinct 1–3 transverse and 0–2 longitudinal septa, and measured 8.1–39.3 × 7.0–17.3 μm (n = 100). For molecular identification, partial sequences of the Alternaria major allergen gene (Alt a 1), translation elongation factor 1-α (TEF1), endopolygalacturonase (EndoPG), RNA polymerase second largest subunit (rpb2), internal transcribed spacer (ITS), and glyceraldehyde-3-phosphate dehydrogenase (G3P) gene were amplified and sequenced using primers Alt-for/Alt-rev, EF-728F/EF-986R, EPG-F/EPG-R, fRPB2-6f/fRPB2-7cr, ITS1/ITS4, gpd1/gpd2 (Woudenberg et al. 2015). BLAST analysis of AB3-4 sequences (GenBank PV624379, OR001992, PV624433, PV624495, PV651598, and OR052172) showed 99% identity with Alternaria spp. A maximum likelihood phylogenetic tree based on the combined six-locus dataset placed AB3-4 on a branch adjacent to A. gossypina and A. longipes. Based on morphology and phylogenetic pattern, AB3-4 was identified as Alternaria sp. Pathogenicity was tested on healthy 3-year-old P. cyrtonema plants. Fifteen healthy leaves from three individual plants were abraded to create micro-wounds with sterile sandpaper, followed by droplet application of 0.1 mL conidial suspension (106 conidia/mL) with sterile water as control. All treatments were incubated at 25±1℃, 80% RH. Five days postinoculation, inoculated leaves developed typical blight symptoms identical to field observations, whereas controls remained symptomless. The experiment was replicated twice with consistent results. The pathogen was successfully re-isolated from diseased tissues and confirmed as A. sp. via morphological observation and DNA sequencing. A. alternata has been already reported to cause leaf blight on P. cyrtonema in Hunan Province (Fan et al. 2024). But to our knowledge, this is the first report of A. sp. causing leaf blight on P. cyrtonema in China.