First Report of Panax notoginseng Leaf Spot Disease Caused by Boeremia exigua in China

Panax notoginseng a perennial herb native to China, is widely grown in the Yunnan Province. (Yang et al. 2022). From July to August 2022, a new leaf spot disease was observed on fully expanded leaves of P. notoginseng from a planting base in the Xundian, Yunnan Province, China. Approximately 250 ha. of P. notoginseng is the cultivated area, and the incidence of leaf spot disease was around 10-15%. Round spots appeared on the infected leaves and as the disease progressed these leaves fell off the plant. A total 21 symptomatic leaves were randomly collected from the planting base to isolate the pathogens and further study in the laboratory. The surface of infected leaves were sanitized with 0.5% sodium hypochlorite for 2 min. and 75% alcohol for 1 min., and then rinsed thrice with sterile water. Once drying, the samples were placed on potato dextrose agar (PDA), plates and incubated at 25 °C for 5 days. The fungus was isolated from the symptomatic tissue, but only three isolates were preserved for further identification. Pure cultures of the representative strain Zhaochanglin 118 were obtained using the singlespore method, and the colonies obtained were dark-green to dark-black in appearance. The pycnidia were dark brown, solitary, or congregated with an inconspicuous neck. The conidia were colorless, ellipsoidal, and measured between 4.5 to 7 × 2 to 3 μm (n = 30). These morphological characteristics were similar to those described for Boeremia exigua (Valenzuela-Lopezi et al. 2018). The genomic DNA of the isolate was extracted using the DN14 cetyltrimethylammonium bromide rapid plant genome extraction kit. The internal transcribed spacer (ITS), RNA polymerase second largest subunit (RPB2) and translation elongation factor 1-alpha (TEF1) genes were amplified via polymerase chain reaction using the primers ITS1/ITS4 (White et al. 1990), Af/Cf (Matheny et al. 2002), and EF1-983F/EF1-2218R (Chen et al. 2015), respectively. All sequences were deposited in GenBank (OQ996531 for ITS; OR291158 for RPB2 and OR291159 for TEF1). A BLASTN homology search using the ITS nucleotide sequence indicated that this has 99.6% identity with the sequence MH859059, named B. exigua from CBS culture collection (517/519 bp); the RPB2 sequence has 97.5% identity with sequence GU371780, named B. exigua from CBS culture collection (704/722 bp); and the TEF1 sequence has 98.4% identity with sequence GU349080, named B. exigua from CBS culture collection (871/885 bp). To test Koch's postulates, a pathogenicity test was carried out on the leaves of six fully expanded P. notoginseng plants in the Xundian planting base. Conidial suspensions were prepared for one isolates at a concentration of 106 spores per milliliter. Three leaflets on different plants were applied with 20µl spore suspension and the other three leaflets were drop of 20 µl sterile distilled water. The whole experiment was repeated three times. The P. notoginseng plants were incubated under sterile conditions at 25°C for 7 days. Inoculated leaves showed the characteristic brown round spots, while control leaves were asymptomatic so, Koch's postulates were fulfilled by re-isolating the pathogen from symptomatic tissue, which was subsequently confirmed as B. exigua through morphological and molecular analyses. Koch's postulates were fulfilled. To our knowledge, this is the first report of B. exigua causing leaf spot disease in P. notoginseng in China, which lays a foundation for further study and developing disease control methods.