First Report of <i>Serratia plymuthica</i> Causing Root Rot of American Ginseng ( <i>Panax quinquefolius</i> ) in Tennessee and the United States

American ginseng (Panax quinquefolius L.) is a valuable medicinal plant native to eastern North America and grows as an understory plant in deciduous and mixed forests in regions of the northern states of the United States. In December 2024, bacterial infections were observed on 5-year-old container-grown American ginseng plants in Rutherford Co., Tennessee. Root symptoms were brown and soft rots characterized by moist and watery decay of the ginseng root (Fig. 1a). Disease severity averaged 25% of root area, and incidence was 30% out of 50 plants. Bacterial streaming from the infected tissue was observed under the microscope. Symptomatic root tissues were surface-sterilized with 0.525% sodium hypochlorite for 1 min and washed twice with sterile water. Bacterial colonies were cream-white on King’s B (KB) medium after 3 days of incubation at 28°C. Cells were rod-shaped when observed under microscopy, gram-negative based on the KOH test, and non-fluorescent on KB medium. Catalase and gelatin hydrolysis were positive, but oxidase, arginine dihydrolase, and H2S production were negative. Bacterial identity was confirmed by sequencing of DNA from pure cultures (strains FBG8317, FBG8318, and FBG8741). The small subunit ribosomal RNA (16S rRNA), RNA polymerase sigma factor (rpoD), and DNA gyrase (gyrB) genes were amplified and sequenced using the primers 8F/1492R (Galkiewicz and Kellogg 2008), rpoDpF/R, and gyrBF/R, respectively (Sarkar and Guttman 2004). The sequences (GenBank accession nos. PX147167, PX147168, PX147169 for 16S rRNA; PX204288, PX204289, and PX204290 for rpoD; PX204291, PX204292, and PX204293 for gyrB) were 100%, 99.38%, and 99.52% identical to the 16S rRNA, rpoD, and gyrB genetic markers of Serratia plymuthica reference sequences in GenBank (16S rRNA: KX394779, rpoD: CP065747, gyrB: AJ300532). A phylogenetic tree was constructed using the concatenated sequence of 16S rRNA, rpoD, and gyrB of S. plymuthica (Fig. 2). Pathogenicity tests were conducted on 5-year-old healthy American ginseng plants grown in 1-gal containers to fulfill Koch’s postulates. Roots were wounded with a sterile needle and inoculated with 20 μl of bacterial suspension (1 × 106 CFU/ml) of each isolate (three plants per isolate) (Jung et al. 2018). Control plants were treated with sterile water. Plants were kept in a greenhouse at 21 to 23°C, and a 12-h photoperiod. All inoculated plants developed leaf yellowing and brown to dark brown root lesions within 10 days (Fig. 1b), whereas control plants remained asymptomatic (Fig. 1c). The bacteria were reisolated from inoculated plants and confirmed as S. plymuthica by DNA sequencing. S. plymuthica was previously reported as the causal agent of bacterial root rot of ginseng in Korea (Jung et al. 2018). To our knowledge, this is the first report of S. plymuthica causing root rot of American ginseng in Tennessee and in the United States. Identification of this pathogen provides a foundation for future research and the development of effective disease management strategies.