Sweet viburnum [Viburnum odoratissimum Ker-Gawl. var. awabuki (K. Koch) Zabel ex Rumpl.] belonging to the family Adoxaceae, is a medical and landscape plant, native to Korea (Jeju Island), Taiwan, and Japan (Edita 1988). In June and September 2019, leaf spots were observed on approximately 65% to 80% of sweet viburnum plants in a hedgerow located in Fenghe Xincheng District (28°41'52.9"N 115°52'14.3"E) in Nanchang, China. Initial symptoms of disease appeared as dark brown spots surrounded by red halos (Figure 1 A), which expanded irregularly. Finally, the center of the lesions desiccated and became light-brown, surrounded by a deep-red halos (Figure 1 B). Ten leaf samples with typical symptoms were collected and washed with tap water for about 15 min. The tissue between the healthy and necrotic area (ca. 4 mm × 4 mm) was cut with a sterile scalpel and surface sterilized with 70% alcohol for 45 s, 2% NaClO for 2 min, washed in sterile deionized water three times, dried on sterilized filter paper, then placed in Petri dishes and incubated at 25℃ in the dark. After 3 to 5 days, the hyphal tips from the edges of growing colonies were transferred to fresh PDA dishes. Eventually, 54 fungal isolates were obtained and, of these, 39 isolates were identical in their morphological characteristics. Morphological analysis was performed according with Ellis (1971). The isolate S18, chosen as representative, formed a gray to grayish brown colony with concentric circleson PDA, and a diameter of 8.5 to 9 cm after 7 days incubation at 25℃ (Figure 1 G). Conidia were hyaline, straight or slightly curved, needle shaped, truncate at the base, and acuminate at the tip, with 2 to 6 pseudosepta, 18.90 to 38.38 µm (avg. = 27.51 µm) × 1.64 to 4.50 µm (avg. = 2.60 µm) (n = 36) (Figure 1 H). The genes of fungal isolates (i.e., ITS, tub2 and ACT) were amplified with ITS4/ITS5 for ITS (White, Bruns et al. 1990), Bt2a/Bt2b for tub2 (Glass and Donaldson 1995) and ACT783R/ACT512F for ACT (Carbone and Kohn 1999) and sequenced. The sequences were deposited in GenBank (MW165772 for ITS, MW175900 for ACT and MW168659 for tub2), which showing greater than 99.1% similarity to multiple C. cassiicola accessions, respectively. Pathogenicity tests were performed on healthy leaves in field by inoculating surface-sterilized mature leaves with puncture wound (Figure C) and non-wounded young leaves with 20 µL of a conidial suspension (105 conidia ml-1) (Figure F and G) at 26℃. After 4 to 7 days, all inoculated leaves reproduced similar symptoms as observed initially in the field (Figure 1 C, E and F). To fulfill Koch's postulates, the fungus was isolated on PDA from the margins of leaf spots on inoculated leaves and confirmed as C. cassiicola by morphological characters and ITS gene sequencing. Previously, C. cassiicola was reported as an endophyte on Viburnum spp. and Viburnum odoratissimum (Alfieri et al. 1994). More recently, C. cassiicola has been reported as a pathogen of many plant species in China, such as kiwifruit (Cui, Gong et al. 2015), American sweetgum (Mao, Zheng et al. 2021), castor bean (Tang, Liu et al. 2020), and holly mangrove (Xie, He et al. 2020). To our knowledge, this is the first report of leaf spot disease on sweet viburnum caused by C. cassiicola in China and the precise identification of the causal agent will be useful for its management.
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Feb 28, 2022
Dong-Pil Park 
Open Access