Tobacco Fusarium root rot is caused by various Fusarium species, with eleven species reported, among which F. oxysporum and F. solani are main responsible in China (Yang et al., 2024). Notably, Fusarium concentricum has not been previously reported until now at a global level in tobacco. In June 2022, tobacco (Nicotiana tabacum L.) root rot symptoms were observed in tobacco fields (approximately 1500 m2) in Xuanen County, Enshi City, Hubei Province (29.99°N, 109.44°E). About 20% of tobacco plants showed symptoms including dwarfing of plants, chlorosis in the lower leaves and black roots or root rot, and some plants had died. The roots of twenty diseased tobacco plants were randomly collected from this field and subjected to surface sterilization with 75% ethanol for 5 min and 1% sodium hypochlorite for 5 min, followed by rinsing with sterile distilled water. The roots were then cut into small pieces (around 0.5 cm), placed onto potato dextrose agar (PDA) medium, and incubated at 28℃ in darkness for 3 to 5 days. Three representative isolates 40-LD, 41-LD and 42-LD with typical Fusarium spp. characters were obtained using single-spore purification method (Xiao et al. 2023). After 4 days of culture at 28°C on PDA, the colonies were circular, and by the next day, they began to produce red pigment. On the seventh day, the pigment turned purple, and the mycelium appeared cottony and wool-like. On PDA, microconidia were obovate to fusoid, with the size of 4.6 to 8.7×1.6 to 2.9 μm (n=50), 0 or 1 septate. Macroconidia observed on synthetic low nutrient medium (SNA) were falcate, with the size of 13.6 to 41.7×2.9to 4.8 μm (n=50), 3 to 5 septa. The morphological characteristics of the fungi were consistent with the description of Fusarium species(Liu et al. 1998). The internal transcribed spacer region (ITS) of rDNA, the elongation factor 1α (TEF-1α), and RNA polymerase II second largest subunit (RPB2) of the isolates were amplified using primers ITS1/ITS4(Kim and Choi 2020), EF-1H/EF-2T (Wang et al. 2013) and RPB2-5F/RPB2-7CR(Qiu et al. 2023). The obtained ITS (PQ056894), TEF-1α (PQ118393), and RPB2 (PQ329359) sequences showed 99.6% (490/514 bp), 99.7% (667/667 bp), and 100% (940/940 bp) similarity to F. concentricum NFMJ-1 (GenBank accession nos. ON184033, ON212051 and ON212052, respectively)(Xiao et al. 2023). A maximum likelihood phylogenetic tree, constructed by concatenating ITS, TEF1-α and RPB2 sequences using ACOPTools v2 software, showed the three isolates and F. concentricum were clustered into one branch. Pathogenicity was assessed by coculturing potted tobacco plants with 42-LD-infected wheat grains (Shen et al. 2023). Strain 42-LD was inoculated onto sterilized wheat grains and cultured at 28℃ for 7 days. Subsequently, the roots of ten healthy seedlings of Yunyan 87 at the 4-leaf stage were wounded with sterile scissors and then transplanted into sterile soil containing 30 wheat grains infected with 42-LD. In contrast, the wheat grains without 42-LD inoculation were used as controls. All seedlings were placed in a greenhouse at 25°C with 16 h of light and 8 h of darkness. Seven days later, all seedlings inoculated with 42-LD showed symptoms of yellowing and wilting of leaves, blackening of roots, and slow plant growth, while control plants showed no symptoms. The pathogenicity test was repeated three times. The pathogens reisolated from the root and stem of the infected plants were morphologically and molecularly identified as F. concentricum. This is the first report on F. concentricum inducing tobacco root rot in China. This finding implies that F. concentricum may pose a potential threat to tobacco production and thus reduce the income of tobacco farmers. Consequently, it is essential to develop appropriate prevention and control measures for this pathogen to ensure the sustainability of cultivation in our region.
Keywords: China; Fusarium concentricum; Nicotiana tabacum; root rot.