First Report of <i>Fusarium asiaticum</i> Causing Fusarium Root Rot on Tobacco in China

Fusarium root rot is recognized as a critical root disease responsible for significant reductions in tobacco yield and postharvest quality. It was caused by soil-borne pathogenic fungi of the genus Fusarium, with 11 species reported, among which F. oxysporum and F. solani are the main pathogens in China (Yang et al. 2024), but Fusarium asiaticum has not been previously reported until now at a global level in tobacco. In May 2024, field-grown tobacco (Nicotiana tabacum L.) plants exhibiting Fusarium root rot symptoms (approximately 5,000 m2 of 60,000 m2 surveyed) were observed in Xuancheng, Anhui Province, China (30.77°N, 118.84°E), with disease incidence of 2.52% to 14.35%, which was investigated in specific villages including Lianshumiao (3.71%), Xicun (4.62%), and Xizha (14.35%) in Huangdu Township; Yangliu Village (11.67%) in Yangliu Town; and Futian (2.52%) and Yanhe(3.82%) in Wenchang Town. Affected tobacco plants exhibited characteristic symptoms including brown to black lesions on the roots, epidermal sloughing in the root collar region, browning of xylem tissue, and chlorosis progressing to necrosis accompanied by wilting in mid-to-lower leaves. Under high soil moisture, root rot progressed rapidly, leading to plant death. To isolate the pathogen, thirty symptomatic roots (cv. Yunyan87) with typical and severe symptoms which were selected as representative isolates from a total of 242 diseased plants collected across 12 different fields were surface-sterilized (75% ethanol for 40 s, 5% NaClO for 1 min), rinsing with sterile distilled water. The roots were plated on PDA after cut into small pieces(5 mm×5 mm), and incubated at 28°C. Three isolates were obtained from three samples. Each isolate was obtained from a single spore and examined for morphological, molecular analyses and pathogenicity (The three strains used in this study are independent in origin but genetically closely related and phenotypically consistent isolates of Fusarium asiaticum. Given their high consistency in key traits such as species identity, colony morphology, and pathogenicity, so only strain A3 is covered in this report). Colonies on PDA displayed white aerial mycelium with white sporodochia, and began to produce red pigment after 7 days of culture. On carnation leat agar (CLA) medium, strain A3 produced macroconidia, which were falcate, 4–6 septate, measuring 45.4–50.6 × 3.7–4.5 μm (n=30) but no microconidia. Morphology matched Fusarium spp. (Leslie and Summerell. 2006). The total DNA of strain A3 was extracted from mycelium of the 7-day-old cultures by CTAB method (White et al. 1990). The ribosomal DNA internal transcribed spacer (ITS) region, elongation factor 1α (TEF-1α) and RNA polymerase II second largest subunit (RPB2) were amplified using primer pairs ITS1/ITS4 (White et al. 1990), EF1-728F/EF2R (Carbone and Kohn. 1999), and RPB2-5F/RPB2-7CR (Qiu et al. 2023) respectively, and sequenced. Neighbor-Joining analysis constructed by concatenating three sequences was carried out using MEGA 11. The obtained ITS (PV459639), TEF-1α (PV521975), and RPB2 (PV521974) sequences were 99.1 (511/516), 99.6% (757/760) and99.61% (1108/1112) identical to the corresponding DNA sequences of F. asiaticum strains (GenBank: ITS=KY272817, TEF-1α=PP404039, RPB2=PQ281488). Phylogenetic analysis confirmed clustering with F. asiaticum. Pathogenicity was verified by inoculating four-leaf-stage tobacco seedlings (cv. Yunyan 87) grown in sterile soil under controlled greenhouse conditions (25±2°C, 70% RH) with a 16-h light/8-h dark photoperiod. A total of 40 seedlings were used in the experiment. Twenty seedlings were inoculated via soil drenching with 20 mL of a conidial suspension (1×106 spores/mL) of Fusarium asiaticum prepared from a 7-day-old culture grown on PDA medium. Another twenty seedlings, serving as controls, were treated similarly with 20 mL of sterile water. The experiment was repeated third. Inoculated plants developed wilting and root discoloration within 7 days, while controls remained healthy. The pathogen was reisolated and confirmed as F. asiaticum via EF-1α sequencing. Although F. asiaticum has been reported on wheat(Zhang et al. 2025) , Ligusticum (Zhu et al. 2022), maize (Dong et al. 2021), corn (Jiang et al. 2023), Foxtail Millet (Kong et al. 2022) and rice(Chen et al. 2022) in Asia, this is the first report on tobacco. Given the economic importance of tobacco in China, this finding highlights the need for targeted management strategies against this emerging pathogen.