First report of Stagonosporopsis pogostemonis causing leaf blight on Panax notoginseng in Wenshan, China

Panax notoginseng, a valuable Chinese medicinal plant, is susceptible to various diseases (Li et al. 2024). In September 2024, leaf blight symptoms were observed in P. notoginseng in Wenshan City, China (23.41°N, 104.18°E). Disease incidence ranged from 30% to 45%, affecting approximately 2 ha of production in 3 plantations. The disease resulted in the emergence of irregular black necrotic lesions on the surface of the leaves. These lesions started from the middle and eventually enlarged to the margin of the leaf. Six to seven samples were randomly collected in each plantation. Twenty samples were cut into 5 mm × 5 mm tissue pieces, sterilized with 75% ethanol for 30 s, and followed by 1% sodium hypochlorite for 30 s, rinsed with sterile water three times, and transferred onto potato dextrose agar (PDA) with streptomycin sulfate (100 mg/L). They were then incubated at 25±1 ℃ in darkness. Six strains obtained from four samples were single spore purified and examined but given their high consistency (species identity, colony morphology, pathogenicity), only strain LSLS01 is discussed. After 10 days on PDA, the colony was margin regular, cottony, white toward the periphery, brownish-grey in the colony center. The reverse side was white, becoming tawny then dark brown–black from the center. Conidia were oblong, cylindrical to ellipsoidal, aseptate, measured 3.6 to 8.0 × 2.0 to 4.6 μm (n = 100) in diameter. Chlamydospores, subhyaline to dark brown, solitary or in chains, measured 7.4 to 21.6 × 6.1 to 12.1 μm (n = 100). The morphological characteristics of the strains were identical to those of Stagonosporopsis pogostemonis (Dong et al. 2021). For molecular identification, primers Bt2a/Bt2b (Glass and Donaldson, 1995), ITS1F/ITS4 (White et al. 1990), LROR/LR7 (Rehner and Samuels, 1994) and fRPB2-5F/fRPB2-7CR (Liu et al. 1999) were used to amplify tub2, ITS, LSU, and rpb2 gene fragments, respectively, from the genomic DNA. BLASTn results showed the sequences of tub2 (PX138674), ITS (PX129041), LSU（PX129048), and rpb2 (PX138668) >99% identity with those sequences in GenBank (tub2, OQ221890; ITS, MZ156571; LSU, PP355180; rbp2, MZ203135). A concatenated phylogenetic analysis of these loci conformed the strain as S. pogostemonis. To confirm its pathogenicity, two-year-old, healthy P. notoginseng plants were selected and transplanted into pots for inoculation experiments. Sterile needles were utilized to create micro-incisions on the leaves of the P. notoginseng plants, and mycelium plugs (8-mm diameter) that had undergone a 10-day cultivation period were inoculated at the incision sites. Three leaves per plant were selected for inoculation (18 plugs total), while 8 mm blank plugs were established as controls (Lyu et al. 2024). All pots were incubated at 25℃, with an 8-hour light and 16-hour darkness alternation, and 80% relative humidity. Three days post-inoculation, infected leaves showed notable lesions, and the symptoms were in accordance with those observed in the field, whereas the control plants exhibited no disease symptoms. The experiment was repeated twice with similar results. The pathogen was reisolated and confirmed as S. pogostemonis via rpb2 sequencing. As far as is known, this represents the first report of S. pogostemonis causing leaf blight on P. notoginseng in China. This finding highlights a potentially destructive threat to the cultivation of the economically important herb in China, emphasizing the need for monitoring and management strategies.