Tetrapanax papyrifer (Hook.) K. Koch, widely utilized in clinical practices in traditional Chinese medicine, is a medicinal plant whose dried stem pich exhibits activities such as lactation induction, diuresis, and anti-inflammatory effects. The species is native to the southwest of China, such as Guizhou and Yunnan provinces. It thrives in sunlight and warmth and is planted in fertile valleys in the region (Zhang et al. 2023). In July 2021, a leaf spot-like disease was observed on approximately 15% of T. papyrifer (Big T. papyrifer) in a field in Shibing County (127.2°E, 25.2°N), Guizhou Province, China. The symptomatic leaves displayed irregular, watery dark brown lesions with black conidiomata in gray centers and surrounded by yellow halos. To identify the causal agent leading to the disease, 15 symptomatic leaves from five trees in one field were collected. These leaves underwent surface sterilization, which included 30s in 75% ethanol, 2 min in 3% NaOCl, and three times of washing with sterilized distilled water. Thereafter, small pieces of the symptomatic leaf tissues (0.2 × 0.2 cm) were plated on PDA and incubated at 25°C for seven days (Fang 2007). Three isolates were obtained based on the improved single spore isolation method proposed by Gong et al. (2010), and named as GUTC 321, GUTC 523 and GUTC 873. The fungal colonies on PDA were villiform, creamy-white, whorled, and sparse aerial mycelium on the surface with black, gregarious conidiomata. The conidia were ellipsoid, mid brown to dark brown, mainly with 3-4 transverse septa, usually divided by longitudinal septum, often constricted at the septa, 21.8 (12.6-34.5) × 13.9 (8.8-19.8) µm (n=50). The morphological features were consistent with the descriptions of Pseudopithomyces chartarum (Ariyawansa et al. 2015). All three isolates exhibited identical morphological properties. The potential pathogen was confirmed as P. chartarum by amplification and sequencing of the internal transcribed spacer regions (ITS), large subunit ribosomal (LSU) and translation elongation factor 1 alpha (TEF1) genes with primers ITS4/ITS5, LROR/LR7 and EF-983F/EF-2218R, respectively (Ariyawansa et al. 2015; Jayasiriet al. 2019). BLASTn analyses of the sequences showed 100% identity among the three isolates and a high homology (ITS, 99.8%: 598/599; LSU, 100%: 853/853; and TEF1, 100%: 871/871) with those of P. chartarum sequences in GenBank (MT123059, OK655822, and MK360080, respectively). The sequences of the genes from isolate GUTC321 were deposited in GenBank under accession numbers OP269599 (ITS), OP237015 (LSU), and OR069689 (TEF1). Phylogenetic analyses of the concatenated ITS-LSU-TEF1 sequence (2,685 bp) of GUTC 321 using PhyloSuite 1.2.2 with PartitionFinder model revealed that the isolate clustered closely with P. chartarum isolate CBS 329.86T (Cecilia 1986). The pathogenicity of GUTC 321 was tested thereafter on ten healthy T. papyrifer plants grown in pots in growth chamber. The plants were inoculated by spraying with spore suspension (106 spores mL-1) of GUTC 321 or sterile water (control) onto leaves that had been slightly injured with sterilized SiO2 (0.1-0.25 mm) until runoff. The plants were maintained at 25°C in the growth chamber, and monitored for symptom development. Local lesions began to appear on all GUTC 321-inoculated leaves, but not on those of the control plants, 48 hours after inoculation. Seven days after the inoculation, lesions similar to those observed on field plants occurred on GUTC321-inoculated plants but not on the control plants, the lesions observed only in inoculated leaves. The same fungus was reisolated and identified based on the morphological characterization and molecular analyses (ITS, LSU and TEF1) from the infected leaves thus fulfilling Koch's postulates. To our knowledge, this is the first report of leaf spot on T. papyrifer caused by P. chartarum in China. Considering the significance of T. papyrifer in Chinese medicine, approximate management measures need to be developed and applied to control the disease in the crop.
Keywords: Leaf spot; Pseudopithomyces chartarum; Tetrapanax papyrifer; pathogenicity.