First Report of <i>Aucklandia costus</i> Root Rot Caused by <i>Fusarium avenaceum</i> in China

Aucklandia costus (syn. Saussurea costus) belongs to the Asteraceae family and has been cultivated in India and China, where its roots were used in traditional Chinese medicine. In December 2023, root rot symptoms were observed in Chengkou County (31.69°N 108.58°E), Chongqing Municipality of China. The infection started with brown roots and the overground parts showing dull leaf color and the roots gradually rotted and turned black, and the overground parts completely withered. Ten A. costus samples with a disease incidence of 10% to 30%, were collected from fields spanning about five acres. Rotted roots were disinfested with 1% NaOCl for 1 min, 75% ethanol for 30 s, and washed in sterile water. Pieces (1 cm3) were cut from the disinfested root, placed on potato dextrose agar (PDA), and incubated at 25°C for 1 week. Ten isolates with similar morphology were obtained and the isolate YMX was selected for further characterization. Colonies maintained on PDA in the dark had an average radial growth rate of 8-12 mm/d at 25°C. The colony was white to jasmine, presented velvety to felty mycelia, with a beige reverse lacking diffusible pigments (Fig. 1). On carnation leaf agar, sporodochia appeared as slimy dots, macroconidia were 3 to 7-septate, 35-85 × 3-6 μm, falcate, while microconidia and chlamydospores were absent. These morphological characteristics were consistent with descriptions of Fusarium sp. The isolate YMX was further characterized using the internal transcribed spacers (ITS) (White et al. 1990), RNA polymerase II largest subunit (RPB1) (Senatore et al. 2021), RNA polymerase II second largest subunit (RPB2) (Xia et al. 2019) and translation elongation factor 1-α gene (TEF1) (Carbone and Kohn 1999; O'Donnell et al. 2010) which were amplified using the primer pairs ITS5/ITS4, Amp3f/Amp3r, 5f2/7cr and EF1-728F/EF2, respectively. Sequences were deposited in GenBank (ITS: PQ381655; RPB1: PQ404771; RPB2: PQ404772; TEF1: PQ404773). BLASTn queries of ITS, RPB1, RPB2 and TEF1 sequences showed 100%, 99.69%, 99.04% and 98.74% identity with OL832320, OL772910, MH582370 and OL772758, respectively, of the reference strains of F. avenaceum NRRL 54939 or NRRL 36069. The isolate YMX clustered with Fusarium avenaceum (100% bootstrap) in a concatenated phylogenetic tree (Fig. 2). Pathogenicity tests were conducted on roots of ten A. costus healthy plants. A wound was created using a sterile toothpick on each root and one mycelial plug (5 mm diameter) from a 7-day-old colony of the isolate YMX was placed on the wound. Root samples inoculated with plugs of noncolonized PDA served as a control. The test was repeated three times. After 7 days of incubation at 25°C, all inoculated roots showed symptoms like those in the field while control roots remained healthy (Fig. 1). The fungi isolated from the experimental plant roots were confirmed as F. avenaceum using morphological and four-gene sequence analyses. F. avenaceum causes root rot in several species including Coptis chinensis (Mei et al. 2021), Lepidium meyenii (Wei et al. 2017) and Rubus corchorifolius (Niyongabo Turatsinze et al. 2024) in China. To our knowledge, this is the first report of A. costus root rot caused by F. avenaceum in China. Considering the economic value of this crop, accurate identification will facilitate disease management.