Tobacco (Nicotiana tabacum) is one of the most important industrial crops in the world. Its leaves are the main raw material for cigarettes, but they are often threatened by fungal pathogens in the production process (Wang et al. 2022). From May to June 2022, a disease of tobacco (cv K326) (15% of plants) in a 0.3-ha field in Jingxi of Guangxi Province showed symptoms of local necrosis and perforation of middle and basal leaves (Fig S1). Pieces of leaf tissue (3 × 3 mm) were excised from the edge of the necrotic lesion of each plant, treated with 75% ethanol for 10 s, soaked in 2% NaClO solution for 1-2 min, rinsed with sterile water for three times, and then plated on potato dextrose agar（PDA）medium and incubated at 28°C. Isolate TJYA13 was used for subsequent studies. After 8 days, the colony margin was yellowish brown and irregular, the center was black and plicated. The isolate TJYA13 was incubated on oatmeal agar medium at 28°C for 4 days, and many pseudothecia were observed embedded on the surface of the medium. Pseudothecium was globose or subglobose, dark brown, and size was 184.7-304.7 µm × 187.5-340.5 µm (n=20). Ascospores were usually wrapped by the saccate ascus in pseudothecium, cylindrical or ellipsoidal, with 5-6 transverse septa, and size was 12.2-18.5 µm × 35.6-51.8 µm (n=80). The morphological characteristics of ascospores were consistent with a Leptosphaerulina species (Hou et al. 2020). For accurate identification, the genomic DNA of isolate TJYA13 was extracted with Ezup Column Fungi Genomic DNA Purification Kit (Sangon, Shanghai, China). The ITS region, 28s ribosomal RNA (LSU), β-tubulin (TUB), and RNA polymerase II second largest subunit (RPB2) were amplified with primers ITS1/ITS4 (Gardes and Bruns 1993; White et al. 1990), LROR/LR7 (Rehner and Samuels 1994), Btub2Fd/Btub4Rd (Woudenberg et al. 2009), and RPB2-5F2/fRPB2-7cR (Liu et al. 1999), respectively and sequenced at Sangon Biotech (Sichuan, China). The sequences were deposited in GenBank (accession nos. OP926927, OP926933, OP939419, OP939422). The phylogenetic analysis grouped the isolate TJYA13 within the L. americana clade (Fig S2) (Hou et al. 2020). Pathogenicity of the isolate TJYA13 was verified on four healthy tobacco plants (cv K326). The mycelial plugs were inoculated on leaves sterilized with 75% ethanol, and control plants were inoculated with sterile PDA plugs. Plants were incubated at 28 ℃ and 78% humidity. After 10 days, the leaves inoculated with mycelial plugs had symptoms similar to those in the field, but there were no symptoms on the control leaves. L. americana were reisolated from the leaves inoculated with the mycelial plugs. To the best of our knowledge, this is the first report of L. americana causing holing disease on tobacco in China. This disease may reduce yields and lower quality of flue-cured tobacco leaf. Therefore, the emergence of tobacco holing disease should be noted to prevent potential damage to tobacco production in Guangxi. Reference 1. Hou L. W., et al. 2020. Stud. Mycol. 96: 309-396 2. Liu, Y. J., et al. 1999. Mol. Biol. Evol. 16:1799. 3. Rehner, S. A., and Samuels, G. J. 1994. Mycol. Res. 98:625. 4. Wang H. et al. 2022. Microorganisms. 10: 1890. 5. White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. 6. Woudenberg, J. H. C., et al. 2009. Persoonia 22:56. The author(s) declare no conflict of interest. Funding: Funding was provided by Guangxi Zhuang Autonomous Region Tobacco Monopoly Bureau (grant no. 202,145,000,024,006). Tobacco (Nicotiana tabacum) is one of the most important industrial crops in the world. Its leaves are the main raw material for cigarettes, but they are often threatened by fungal pathogens in the production process (Wang et al. 2022). From May to June 2022, a disease of tobacco (cv K326) (15% of plants) in a 0.3-ha field in Jingxi of Guangxi Province showed symptoms of local necrosis and perforation of middle and basal leaves (Fig S1). Pieces of leaf tissue (3 × 3 mm) were excised from the edge of the necrotic lesion of each plant, treated with 75% ethanol for 10 s, soaked in 2% NaClO solution for 1-2 min, rinsed with sterile water for three times, and then plated on potato dextrose agar（PDA）medium and incubated at 28°C. Isolate TJYA13 was used for subsequent studies. After 8 days, the colony margin was yellowish brown and irregular, the center was black and plicated. The isolate TJYA13 was incubated on oatmeal agar medium at 28°C for 4 days, and many pseudothecia were observed embedded on the surface of the medium. Pseudothecium was globose or subglobose, dark brown, and size was 184.7-304.7 µm × 187.5-340.5 µm (n=20). Ascospores were usually wrapped by the saccate ascus in pseudothecium, cylindrical or ellipsoidal, with 5-6 transverse septa, and size was 12.2-18.5 µm × 35.6-51.8 µm (n=80). The morphological characteristics of ascospores were consistent with a Leptosphaerulina species (Hou et al. 2020). For accurate identification, the genomic DNA of isolate TJYA13 was extracted with Ezup Column Fungi Genomic DNA Purification Kit (Sangon, Shanghai, China). The ITS region, 28s ribosomal RNA (LSU), β-tubulin (TUB), and RNA polymerase II second largest subunit (RPB2) were amplified with primers ITS1/ITS4 (Gardes and Bruns 1993; White et al. 1990), LROR/LR7 (Rehner and Samuels 1994), Btub2Fd/Btub4Rd (Woudenberg et al. 2009), and RPB2-5F2/fRPB2-7cR (Liu et al. 1999), respectively and sequenced at Sangon Biotech (Sichuan, China). The sequences were deposited in GenBank (accession nos. OP926927, OP926933, OP939419, OP939422). The phylogenetic analysis grouped the isolate TJYA13 within the L. americana clade (Fig S2) (Hou et al. 2020). Pathogenicity of the isolate TJYA13 was verified on four healthy tobacco plants (cv K326). The mycelial plugs were inoculated on leaves sterilized with 75% ethanol, and control plants were inoculated with sterile PDA plugs. Plants were incubated at 28 ℃ and 78% humidity. After 10 days, the leaves inoculated with mycelial plugs had symptoms similar to those in the field, but there were no symptoms on the control leaves. L. americana were reisolated from the leaves inoculated with the mycelial plugs. To the best of our knowledge, this is the first report of L. americana causing holing disease on tobacco in China. This disease may reduce yields and lower quality of flue-cured tobacco leaf. Therefore, the emergence of tobacco holing disease should be noted to prevent potential damage to tobacco production in Guangxi.
Keywords: China; Fungal pathogens; Leptosphaerulina americana; tobacco.