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First Report of Corynespora cassiicola Causing Leaf Spot on Kadsura coccinea in China

Zhong, J., Wang, Y., Li, C. X., Chen, J., He, A. G., Gao, B. D.
Plant disease 2019 v.103 no.2 pp. 366
Corynespora cassiicola, Kadsura coccinea, conidia, conidiophores, container-grown plants, culture media, defoliation, ethanol, fungi, gardens, genes, greenhouses, internal transcribed spacers, leaf spot, leaves, medicinal plants, mercuric chloride, mycelium, neoplasms, oligodeoxyribonucleotides, pain, pathogenicity, pathogens, peptide elongation factors, photoperiod, plastic bags, ribosomal DNA, skin diseases, tissues, tubulin, China
Kadsura coccinea (Lem.) A. C. Smith, belonging to Schisandraceae, is a traditional Chinese medicinal plant that is distributed widely in southwest China. This plant was effectively used for treatment of cancer and dermatosis and as an anodyne to relieve pain (Song et al. 2010). In March 2018, a leaf spot disease was observed on K. coccinea in a nursery garden of Changsha (28°13′ N; 112°56′ E), in Hunan Province, China. The symptoms occurred on leaves as small brown spots initially and then developed into suborbicular or irregular-shaped brown necrotic lesions, which often displayed irregular concentric rings. Severely affected plants showed defoliation. The incidence was about 30 to 40% of the plants. To isolate the pathogen, symptomatic leaves were collected and excised at the border of lesions, surface disinfected with 70% ethanol and 0.1% HgCl₂, and rinsed in sterile distilled water. The tissues were then plated onto potato dextrose agar (PDA) and incubated at 26°C in the dark. Colonies cultured on PDA for 7 days were dense, grayish green or grayish brown, with an average radial growth rate of 8.8 mm/day. Conidiophores were upright, unbranched, brown, single or clustered, 50 to 400 × 4 to 10 µm (n = 50). Conidia formed singly or in chains, obclavate to cylindrical, straight or slightly curved, pale olivaceous to brown, with 3 to 12 pseudoseptate, 50 to 230 × 8 to 18 μm (n = 50). These morphological characteristics conformed to the descriptions of Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei (Ellis 1971). The internal transcribed spacer (ITS) region of rDNA, β-tubulin, and translation elongation factor (EF-1α) genes were amplified and sequenced with the primer pairs ITS4/ITS5, Bt2a/Bt2b, and EF728F /EF986R, respectively (Li et al. 2013). BLAST searches showed that the ITS, β-tubulin, and EF-1α (GenBank accession nos. MH255527, MH263734, and MH263735, respectively) were 99% identical to the C. cassiicola strains (GenBank accession nos. FJ852576, KP735615, and KP735616, respectively) (Zheng et al. 2015). Pathogenicity tests were performed by placing mycelial discs, obtained from colonies actively growing on PDA for 7 days, on the surface of sterile-needle-wounded, detached leaves of K. coccinea, and sterile PDA plugs were used as controls. In addition, two sets of potted plants, whose leaves were either wounded or unwounded, were also inoculated by dropping conidial suspensions (1 × 10⁶ spores/ml) on the leaves. Each kind of control plant was treated with sterile water. The inoculations of detached leaves were kept in humid chambers in the dark at 25°C, and the plotted plants were sheathed with plastic bags for 24 h and then maintained in a greenhouse at 25°C with a 12-h/12-h light/dark cycle. After 4 to 6 days, typical symptoms of dark brown spots were observed on all the inoculated leaves, whereas the control leaves remained asymptomatic. C. cassiicola was specifically reisolated and identified from the inoculated, symptomatic leaves, thus confirming Koch’s postulates. To our knowledge, this is the first report of C. cassiicola causing leaf spot on K. coccinea in China. Because C. cassiicola is a common fungal pathogen infecting many other plants, its occurrence in K. coccinea should be taken into consideration to mitigate possible outbreaks of this disease in the country’s medicinal plant fields.