First report of Postharvest Tuber Rot disease caused by <i>Fusarium solani</i> species complex on <i>Polygonatum sibiricum</i> Redouté in Hunan Province of China

Polygonatum sibiricum Redouté, a perennial plant of the Asparagaceae family, is a traditional Chinese medicine, which has antioxidant, anti-inflammatory, immunomodulatory, and other pharmacological properties (Liu et al. 2023). In 2023, a new post-harvest tuber rot emerged in the P. sibiricum tuber storage facility of Huaihua Linquan Pharmaceutical Co., Ltd (27°17′30″N, 110°20′00″E). Occurring during the tuber storage period, this disease affected nearly 40% of the tubers in the storage facility. In the early stages of the disease, light brown spots appeared on the surface of P. sibiricum tubers. These spots gradually turned dark brown, merged with each other and formed large areas of rot. Finally, the infected tubers decomposed. In order to isolate the pathogen, six symptomatic P. sibiricum tubers were collected from the company and rinsed with sterile water. Small infected parts were cut from tubers and were surface-sterilized in 75% alcohol for 90 s, then in 3% hydrogen peroxide for 30 s. After five rinses in sterile water, they were placed on water agar (WA) plates and incubated at 28℃ for 2-3 days. Upon mycelial development, the hyphae were aseptically transferred onto potato dextrose agar (PDA) plates and maintained at 28°C for 3-5 days. Ten isolates were obtained, and nine of them were Fusarium sp. (isolation frequency 90%). Three representative isolates (HJZS2-1, HJZS2-2, and HJZS2-3) were selected for further study. The colonies were white, producing abundant white aerial mycelium with cottony aerial hyphae and the back is yellowish. Microconidia were reniform and fusiform, with 0 or 1 septa, and occasionally 2 septa, and measured 7.04–13.24 μm × 3.00–4.72 μm (n = 100). Macroconidia have 3–6 septa, were straight to slightly curved, with broad central cells and slightly short, obtuse apical cells, measuring 24.00–41.87 μm × 5.12–7.33 μm (n = 100). To further confirm the identity of the isolates, internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF-1a) and RNA polymerase II second largest subunit (RPB2) were amplified and sequenced using the ITS4/ITS5, TEF-R/TEF-F and RPB2-5f2/RPB2-7cr (O’Donnell et al. 2022) primers, respectively. The sequences were submitted to GenBank (ITS: PQ198646, PQ198647, PQ198648; RPB2: PQ201013, PQ201014, PQ201015; TEF: PQ201016, PQ201017, PQ201018). A joint phylogenetic tree of the three genes constructed using the maximum likelihood method showed that the three isolates belonged to the same clade as Fusarium solani species complex. Pathogenicity experiments were carried out on healthy P. sibiricum tubers using three representative strains. Healthy tubers were soaked in equal amounts of sterile water and conidial suspension (1×10 6 conidia/mL) for 20 min, respectively. There were three replicates for each treatment. After 6 days of inoculation at 25°C and 80% relative humidity, the surface of the inoculated tubers showed water-soaked spots and mycelial layers, which were consistent with the symptoms that appeared on the tubers of P. sibiricum during storage. The noninoculated tubers were asymptomatic. Additionally, pathogenicity experiments were conducted on P. sibiricum plants; however, no infection symptoms were observed. F. solani was re-isolated from the infected tubers and identified based on morphological and molecular characteristics. Koch's postulate is thus fulfilled. F. solani has been shown to be the pathogenic fungus causing tuber rot disease on Gastrodia elata (Li et al. 2022) and Cockscomb (Shehan et al. 2023). This research will lay the foundation for controlling this disease.