First Report of Fusarium oxysporum Causing Root Rot on Polygonatum sibiricum in Chizhou, China

Polygonatum sibiricum, which produces active ingredients including steroidal saponins, polysaccharide, and flavones, is used as a Chinese traditional medicine to alleviate palpitation, dry cough, and diabetes mellitus, and is widely cultivated in China. During the past 2 years, a root rot disease was identified that seriously affected the production of P. sibiricum, resulting in approximately 20 to 50% yield loss each year, in Chizhou city of China. To investigate the causal agents of this disease, root rot samples were collected from a field in this area in March 2019. Most infected plants had reddish brown to dark brown lesions on the roots, which thereafter expanded in the depth of root tuber and make them putrid. The diseased tissues were surface disinfested in 2% sodium hypochlorite for 2 min, rinsed three times using sterilized water, and plated on potato dextrose agar (PDA) at 28°C in darkness for 3 to 4 days. Fungal colonies were obtained from infectious tissues and nine single-spore isolates were purified, with white to violet fluffy aerial hyphae growth on PDA. On these colonies, two types of conidia developed; the slightly falcate macroconidia (3.96 to 5.93 × 10.26 to 31.25 μm) have 3 to 5 septa, while the ovoid or ellipse microconidia (2.05 to 4.56 × 4.55 to 9.96 μm) have 0 to 1 septum. Chlamydospores were terminal or intercalary, in pairs or in chains, and measured 5.63 to 9.46 µm in diameter. Such typical morphological characteristics suggests the identification of the nine single-spore isolates to be Fusarium oxysporum (Leslie and Summerell 2006). To confirm pathogenicity, Koch’s postulate was carried out. Five 1-year-old P. sibiricum seedlings with healthy growth were planted together in a pot with sterilized soil (diameter = 30 cm), and a 20 ml conidial suspension of each isolate was irrigated into the soil near the roots (1 × 10⁵ conidia/ml, 4 ml for each plant). As a control, five 1-year-old P. sibiricum seedlings were treated with distilled water. Inoculated seedlings were grown in a greenhouse at 28°C with an 8-h photoperiod of fluorescent light. Similar lesions were observed on the inoculated roots but not on control ones after 3 weeks. To further confirm the identification of this pathogen, we reisolated the F. oxysporum fungus from the infected roots and characterized them by DNA sequencing. The internal transcribed spacer (ITS) and translation elongation factor (TEF-1α) gene regions were amplified using the primers ITS4/ITS5 (Guo et al. 2012; Wang et al. 2016) and EF-1/EF-2 (O’Donnell et al. 1998), respectively, and sequences were deposited in GenBank (ITS, accession no. MK811209; TEF-1α, accession no. MK830060). The sequences shared 99.61% and 100% similarity with F. oxysporum for ITS (MK681347 and 31 others) and TEF-1α (MH828020 and 99 others), respectively. Based on the above data, all nine isolates were identified as F. oxysporum. To our knowledge, this is the first report of F. oxysporum as causal agent of root rot on P. sibiricum in Chizhou, China. F. oxysporum infections pose a serious threat to P. sibiricum production and thus need to be considered when developing effective control strategies.