First report of Neocosmospora ipomoeae causing root rot on Panax notoginseng in Wenshan, China

Panax notoginseng, a traditional Chinese medicinal herb, is susceptible to various fungi (Li et al. 2024). In Jun 2024, symptoms of black necrotic lesions on root was observed on P. notoginseng in Wenshan City, China (23.41°N, 104.18°E). The disease incidence was approximately 10%, with a total affected planting area of around 100 ha. Ten diseased plants were used to isolate the pathogen. Small tissue segments (5 × 5 mm) were cut from symptomatic roots, surface-sterilized, and incubated on potato dextrose agar (PDA) at 25 °C for 4 days. Eight monosporic strains obtained from six samples were conducted pathogenicity tests and were reisolated. Morphological and molecular identification demonstrated they were consistent with Neocosmospora ipomoeae. The symptom development on P. notoginseng of the strains was highly consistent across three independent pathogenicity trials. Therefore, strain LHGF12-3 was selected for detailed characterization. Cultured on PDA medium for 7 days, the colony was white with dense aerial mycelia, the reverse side was white to buff. After 20 days incubation on carnation leaf agar (CLA) medium, spore characteristics of the isolates were evaluated (Zheng et al. 2024). Macroconidia, falcate, 3 to 6 septa, measured 21.7 to 60.1 × 2.5 to 7.8 μm (n = 60). Microconidia, aseptate or 1-septa, measured 1.5 to 21.0 × 3.0 to 6.8 μm (n = 100). Chlamydospores, globose to subglobose, measured 10.1 to 20.6 × 10.1 to 19.6 μm (n = 50). The morphological features were consistent with the description of N. ipomoeae (Custódio et al, 2022). For molecular identification, partial gene sequences of translation elongation factor 1-α (TEF1), the second subunit region of RNA polymerase (rpb2), and the internal transcribed spacer (ITS) were amplified and sequenced using primers EF1/EF2 (O 'Donnell et al., 1998), fRPB2-6f/fRPB2-7cr (Eddouzi et al., 2013), and ITS1F/ITS4 (White et al., 1990). Based on polyphasic identification, the sequences of TEF1 (GenBank accession PV461722), rpb2 (PV461787), and ITS (PV453061) from the strain LHGF12-3 were compared with all sequences in the FUSARIUM-ID database (O' Donnell et al., 2022). The results demonstrated 99.9% identity with GenBank sequences of N. ipomoeae, including CBS 833.97, NRRL 52699 and CBS 353.87. A concatenated phylogenetic analysis of these loci confirmed the strain as N. ipomoeae. The pathogenicity of the strain LHGF12-3 was confirmed by inoculating onto one-year-old P. notoginseng plants. The roots were washed with sterilized water, and then surface-sterilized with 75% ethanol. Before inoculation the roots were wounded with a sterile needle. Root of each plant was inoculated by brushing 1 mL of a conidial suspension (1 × 106 spores/mL) of the corresponding strain; sterile water was used for controls. Plants were grown in 25 cm plastic pots (five plants per pot) filled with sterilized sand-vermiculite substrate (1:1, v/v) and maintained in the greenhouse at 20 to 26 °C and 80 % RH. After 15 days, inoculated plants developed root rot symptoms similar to those observed in the field, whereas controls remained symptomless. The experiment was repeated twice with similar results. The re-isolated strain was verified as N. ipomoeae via TEF1 sequencing. To date, this is the first report of N. ipomoeae causing root rot in P. notoginseng in Wenshan, China. Given the economic importance of P. notoginseng in China, this finding highlights the need for targeted management strategies against this emerging pathogen.