First Report of Leaf Spot Caused by Alternaria alternata on Cyclocodon lancifolius in China

Cyclocodon lancifolius Bunge in the family Campanulaceae, and commonly known as Hong Guo Ginseng, is found in the Indonesia, Philippines, Vietnam, Japan, and China. The leaves and roots of C. lancifolius are widely used as tonics by ethnic minorities in Guizhou and Hunan Provinces in China. In addition, the fruit is edible, and it is a new resource for both medicine and food. In June 2022, symptoms of leaf spot (Fig 1 A and B.) were observed on C. lancifolius plants in the medicinal plant greenhouse of Guizhou University of Traditional Chinese Medicine (106°61'E, 26°39'N), Guizhou Province. The incidence of leaf spot on C. lancifolius was approximately 40 to 70% of all leaves in canopy. Early symptoms on leaves were small circular or irregular brown spots. As the disease progressed the lesions gradually expanded, and multiple lesions coalesced to form large irregular brown spots. Eventually the seedlings died and leaves of mature plants wilted. In order to isolate the pathogen, ten leaf pieces (5×5 mm) were cut from the junction of the diseased and the healthy tissues, surface sterilized with 75% ethanol for 30 s, 0.1% mercuric chloride (HgCl2) solution for 60 s, rinsed in sterile water three times, finally dried and placed on potato dextrose agar (PDA) and cultured in the dark at 27°C for 4 days. Five purified fungal isolates were obtained by single spore isolation. The colonies were olivaceous to dark olive with white margins and abundant aerial mycelia. On potato carrot agar (PCA) medium, these fungi produced septate conidiophores. Conidia were obclavate or ellipsoid, brown, with one to four transverse septa and one to two longitudinal septa. Spores measured 7.64 to 14.20 × 3.32 to 6.38 µm (n=50). These morphological characteristics are consistent with Alternaria alternata (S. P. Wiltshire. 1933). To further confirm the identification, four genomic DNA regions including the ribosomal DNA internal transcribed spacer (ITS), translation elongation factor 1-a gene (TEF), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), RNA polymerase II second largest subunit (RPB2), and Alternaria major allergen gene (Alt a1) were amplified and sequenced with primers ITS1/ITS4 (White et al. 1990), TEF1-728F/TEF1-986R (Carbone and Kohn 1999), gpd1/gpd2 (Berbee et al. 1999), RPB2-5F/RPB2-7cR (Liu et al. 1999), and Alt-for/Alt-rev (Hong et al. 2005), respectively. Sequences were deposited in GenBank with accession Nos. ITS: OQ128111, OQ690707, and OQ690708; TEF: OQ200380, OQ700996, and OQ700998; GAPDH: OQ200378, OQ700993, and OQ700995; RPB2:OQ200379, OQ701002, and OQ701004; Alt: OQ675614, OQ700999, and OQ701001. In a BLAST search, the sequences were 99-100% identical with corresponding sequences of A. alternata. A maximum likelihood phylogenetic tree was constructed with the combined sequence data sets of ITS, TEF, GAPDH, RPB2, and Alt a1 using MEGA 11. The isolate DHY0, DHY1, and DHY3 clustered with A. alternata (J. H. C. Woudenberg et al. 2015) (Fig. 2). To fulfill Koch's postulates, leaves on three healthy 3-month-old potted C. lancifolius seedlings were wounded with sterile needles and inoculated with 5 mm diameter mycelium, which was covered moist by sterile cotton for 24 h. Sterile water was used as the control. After inoculation, the plants were incubated at 27°C, 85% relative humidity, and a 12 h photoperiod. The experiment was repeated three times. Fifteen days after inoculation, all the leaves showed leaf spot symptoms that were similar to those observed in the greenhouse, while control leaves were asymptomatic (Fig. 1). A. alternata was successfully re-isolated from the symptomatic leaves and identified by morphology and the molecular methods described above. This pathogen has been reported to cause a leaf disease in a wide range of vegetables (Zhang et al. 2021), flowers (Zhang et al. 2022), and medicinal plants (Xing et al. 2020). To the best of our knowledge, this is the first report of A. alternata causing leaf spot on C. lancifolius in China. The accurate identification of this pathogen will provide a basis for the prevention and control of C. lancifolius leaf spot disease in the future.