Ginseng (Panax spp.) has long been a perennial, traditional Chinese medicinal herb. Japanese ginseng (P. japonicus C. A. Mey.) is an important kind of ginseng, and its root is widely used in traditional Chinese medicine for the treatment of inflammation. In recent years, cultivation of Japanese ginseng has increased tremendously in China because of its high value. During the summer of 2013 and 2014, severe necrotic lesions on leaves were observed on two-year-old Japanese ginseng plants in a cultivated field in Enshi City, Hubei Province, China. Approximately 95% of plants in the field were affected. Plants had lesions with elliptical with flavicant margins, with irregular-shaped lesions on the leaf margin or tip. As the infection continued, necrotic leaf spots developed on most leaves, particularly along the leaf margins. At temperatures ranging from 20 to 22°C and relative humidity from 80 to 95%, spots enlarged to form round areas 2 to 7 cm in diameter with well defined, brown margins. Severely infected leaves became chlorotic and abscised. Heavily affected plants were defoliated. Stems and flowers were not affected by the disease. In order to identify the pathogen, infected leaf tissue pieces were surface disinfested with 1% NaOCl solution for 1 min and plated on potato dextrose agar (PDA). Plates were incubated at 22°C with a 12-h photoperiod. After 20 days, subglobose, black pycnidia 145 to 350 μm in diameter developed superficially or immersed in the culture medium. Conidia were cylindrical to oval, hyaline, usually one-celled, and measured 2.5 to 3.4 × 5.8 to 9.3 μm. These morphological characteristics suggested the pathogenic fungus to be a Phoma sp. (Bardas et al. 2008), which was further confirmed by sequencing analysis of the internal transcribed spacer (ITS) region of rDNA. Primers ITS1 and ITS4 were used for PCR (White et al. 1990), and the ITS rDNA sequence was deposited in GenBank (Accession No. KP284148). BLAST analysis of this pathogen’s rDNA showed 100% identity to that of Phoma exigua var. exigua (EU343168.1, EU343160.1, EU343139.1, and EU343119.1). Pathogenicity was tested by spraying the foliage of 30 healthy, two-year-old Japanese ginseng plants with a conidia suspension (106 conidia per ml of sterile distilled water). Each plant received 30 ml of the inoculum. Ten plants sprayed with sterilized water were used as control. Plants were then placed in a growth chamber at 22°C and individually covered with a plastic bag for the first 4 days to maintain high relative humidity. Earliest foliar lesions were observed 10 days after inoculation. Lesions were similar to those observed in the field. No symptoms developed on the control plants. Phoma exigua was consistently reisolated from all artificially inoculated plants. The pathogenicity test was repeated twice with the same results. To our knowledge, this is the first report that P. exigua causes leaf spots and lesions on Japanese ginseng in China. Given the recent tremendous expansion of cultivation area for this herb in China, the economic importance of this disease is likely to increase with the greater prevalence of this host species. © 2016 The American Phytopathological Society.
