First Report of Root Rot Caused by <i>Fusarium oxysporum</i> on American Ginseng ( <i>Panax quinquefolius</i> ) in Tennessee and the United States

American ginseng (Panax quinquefolius L.) is a highly valued medicinal plant native to the United States. In December 2024, root rot symptoms were observed on 5-year-old container-grown American ginseng plants in Rutherford Co., Tennessee. Affected roots exhibited brown to black lesions with disease severity averaging 40% of the root surface and disease incidence of 50% out of 50 plants (Fig. 1a). Symptomatic root tissues were surface-sterilized with 0.525% NaOCl for 1 min, followed by 75% ethanol for 30 s, and rinsed three times with sterile water. Small tissue sections were plated on potato dextrose agar (PDA) and incubated at 24°C under an 8 h photoperiod. After 14 days, colonies with white to violet, fluffy aerial mycelium developed (Fig. 1b). Abundant macroconidia and microconidia were produced. Macroconidia were straight to slightly curved, 14.0–40.7 × 2.46–5.17 µm (n=50), typically with three to five septa, a hook-shaped apical cell, and a foot-shaped basal cell. Microconidia were oval to kidney-shaped; one and/or two cells were measured 5.14–11.63 × 2.22–4.86 µm (n=50) and borne on monophialides on sparsely branched conidiophores (Fig. 1c and e). Chlamydospores were observed after 20 days in simple intercalary in short chains (Fig. 1d). Based on morphological characteristics, the isolates were preliminarily identified as Fusarium oxysporum (Leslie and Summerell 2006). Genomic DNA was extracted (DNeasy PowerLyzer Microbial Kit) from 7-day-old isolates FBG8759, FBG8760, and FBG8761 and amplified using the ITS1/ITS4 (White et al., 1990), T1F/T222 (Stefańczyk et al., 2016), and EF1/EF-2 (O’Donnell et al., 1998) primer pairs. Isolates were sequenced using the ribosomal internal transcribed spacer (ITS), beta-tubulin (TUB), and translation elongation factor 1-α (EF1-α) gene regions. The sequences (GenBank accession nos. PV939276, PV939277, PV939278 for ITS, PV980317, PV980318, PV980319 for TUB, PV980320, PV980320, PV980322 for EF1-α) were 100%, 99.02%, and 100% identical to the ITS, TUB, and EF1-α genetic markers of F. oxysporum reference sequences in GenBank (ITS: KX058546, TUB: KP964910, EF1-α: ON601774). Phylogenetic tree constructed using the concatenated sequence of ITS, TUB, and EF1-α of F. oxysporum and other closely related taxa retrieved from GenBank further confirmed the isolates as F. oxysporum (Fig. 2, Table 1). To fulfill Koch’s postulate, pathogenicity tests were performed on 5-year-old American ginseng seedlings grown in 1-gal containers. Roots were wounded and dipped in a conidial suspension (1 × 105 conidia/ml) of each isolate (three plants per isolate) for 30 min before transplanting into sterile soil (Wang et al., 2022). Control plants were treated with sterile distilled water. Plants were maintained in a greenhouse (21–23°C; 12 h light/dark). After 14 days, inoculated plants developed leaf yellowing and brown necrotic root lesions (Fig. 3), while the control plants remained asymptomatic. The fungus was reisolated and identified as F. oxysporum based on morphology and sequencing. F. oxysporum has been reported to cause root and basal stem rot of P. ginseng in China (Ma et al., 2020; Wang et al., 2022). To our knowledge, this is the first report of F. oxysporum causing root rot on American ginseng in Tennessee and the United States. Identification of the pathogen is essential to support the development of effective management strategies of F. oxysporum on American ginseng.