First Report of Fusarium oxysporum Causing Root Rot on Salvia yunnanensis in China.

The roots of Salvia yunnanensis, an herbaceous perennial widely distributed in Southwest China, is often used as a substitute for S. miltiorrhiza, a highly valued plant in traditional Chinese medicine (Wu et al. 2014). In June 2023, wilted plants of S. yunnanensis were observed in Wenshan City (23.20°N, 104.01°E), China. The average disease incidence was 40% to 65% and the total area affected by the disease was approximately 50 ha. The infected plants displayed wilted leaves, black necrotic lesions on roots, and eventually plant death. Fungal colonies with similar morphology were consistently isolated from the symptomatic roots. Eighteen monosporic isolates were individually cultured on potato dextrose agar (PDA) in separate petri dishes at 25 ± 1°C in darkness. After 7 days, the mycelia within the colonies exhibited a cottony texture and the colors ranged from white to pink or purple, while their reverse sides were white to purple. After 20 days incubation on carnation leaf agar (CLA) medium, spore characteristics of the isolates were evaluated (Zheng et al. 2024). On CLA medium, macroconidia had 2 to 5 septa, usually 3 septa, measuring 21.7 to 39.8 × 4.0 to 6.5 μm (n = 100). Microconidia were falciform, slightly curved or straight, measuring 6.8 to 15.4 × 2.5 to 5.4 μm (n = 100), with 0 to 1 septa. Chlamydospores were globose to subglobose, intercalary or terminal, with an average diameter of 8.9 μm (n = 100). Morphologically, the isolates were identified as Fusarium oxysporum (Lombard et al. 2019; Zheng et al. 2024). To confirm the identification, the translation elongation factor 1-α (EF1α) region was amplified with the primers EF-1/EF-2 (O'Donnell et al. 1998) and the RNA polymerase second largest subunit region (RPB2) was amplified with the primers fRPB2-6f/fRPB2-7cr (Eddouzi et al. 2013). The EF1α (GenBank accession no. PP805676) and RPB2 (PQ383276) sequences of isolate DS10-1 were compared with all sequences in the FUSARIUM ID database (O'Donnell et al. 2022) using polyphasic identification. The highest similarity (100%) was with F. oxysporum isolates, including the ex-epitype of Fusarium cugennagense isolate InaCC F984 (100% similarity). To further assess the phylogenetic relationships, a phylogenetic tree was constructed based on the Neighbor-Joining method in MEGA-X (Kumar et al. 2018). The tree confirmed that the isolate DS10-1 was closely related to F. oxysporum. Pathogenicity tests of the 18 isolates were conducted on five healthy one-year-old S. yunnanensis plants per isolate. Inoculum (1 ml of 106 conidia/ml) of each strain was brushed onto the roots of individual plants with a paintbrush. As controls, five plants were inoculated with sterile water. All plants were potted in plastic containers (diameter = 25 cm, five plants per pot) filled with a sterilized substrate mixture of sand and vermiculite (1:1, v/v), and maintained in the greenhouse at 20 to 26°C with 80% relative humidity. After 45 days, symptoms similar to those observed in the field were present on the roots of all plants inoculated with the 18 isolates, whereas the controls remained symptomless. The experiment was repeated two times with similar results. The same pathogens were consistently reisolated from inoculated roots and confirmed as F. oxysporum based on morphological and molecular analyses. F. oxysporum has been previously reported to cause root disease in various hosts, and to our knowledge, this is the first report of F. oxysporum causing root rot on S. yunnanensis in China. Therefore, subsequent crops should be grown to circumvent this disease.