First Report of Bacterial Root Rot of Ginseng Caused by Pseudomonas aeruginosa in China

Ginseng (Panax ginseng) is an economically valuable medicinal herb mainly planted in Jilin Province, China. In September 2013, during harvest, suspected bacterial rots were observed on ginseng roots with about 10% incidence in Fusong County, Jilin Province, China (127°29.48' N, 42°11.12' E). Rotted roots completely lost their economic value. Symptoms on roots began as water-soaked lesions, and developed rapidly into a soft, watery, decayed mass within 3 to 5 days. Three diseased root tissues were surface-sterilized in 70% ethanol for 30 s, rinsed 3 times in sterilized water and cut into small pieces (2 to 3 mm). Tissues were then macerated for 5 min in sterilized water, streaked onto nutrient agar (NA) medium, and incubated at 28°C for 2 days. Representative colonies were selected from each plate and further purified by sub-culturing onto NA medium. Five strains of the bacteria were gram-negative, short straight rods, 0.5 to 1.0 × 1.5 to 3.0 μm with a single, polar flagellum. Colonies were round, smooth, translucent, and yellowish green on NA medium. The bacteria were identified based on physiological and biochemical tests as follows (3): They were levan and potato rot negative, oxidase, aerobic, and arginine dihydrolase positive, converted nitrate to N2, hydrolyzed gelatin, produced nitrites from nitrates, produced pyocyanin, and grew at 41°C. Bacterial identity was further confirmed by amplifying the 16S rRNA (1,461 bp), gyrB (1,134 bp), and 16S-23S ITS genes (523 bp) with 27F/1492R, UP1/UP2, and L1/L2 primer sets, respectively. The 16S rRNA gene sequence (NCBI Accession No. KJ156527), gyrB gene sequence (KJ748373), and 16S-23S ITS gene sequence (KJ748374) had 99% identity to that of Pseudomons aeruginosa strain BS01 (JQ229778), ATCC25011 (FJ652721), and ATCC15522 (AB547908), respectively. The strains were also identified by using BD Phoenix-100 Automated Microbiology System (BD Ltd., New Jersey) as P. aeruginosa with 99% confidence. A pathogenicity test was conducted by spraying a suspension of five strains individually (108 CFU/ml) onto 4-year-old ginseng roots (cv. Damaya) wounded with a sterilized needle. Five ginseng roots were inoculated with each strain and five ginseng roots were inoculated with sterilized water as controls. All inoculated plants were maintained at 28°C with 80 to 85% relative humidity. Soft, watery tissue rot symptoms developed 3 to 5 days after inoculation, and were similar to those observed on the diseased plants under natural conditions. In contrast, no symptoms developed on control plants. The bacteria were readily re-isolated from inoculated plants and identified as P. aeruginosa using bacterial colony morphology, physiological and biochemical tests, as well as sequence analysis of the 16S rRNA gene, fulfiling Koch's postulates. The bacterium was not isolated from control plants. P. aeruginosa has been reported to cause diseases in a variety of plants including onion (1,2), arabidopsis, and sweet basil (4). To our knowledge, this is the first report of P. aeruginosa causing ginseng root rot in China. References: (1) E. J. Cother et al. Phytopathology 66:828, 1976. (2) X. J. Hao, and G. L. Xie. J. Plant Pathol. 88:340, 2006. (3) N. W. Schaad et al. Laboratory Guide for the Identification of Plant Pathogenic Bacteria, 3rd edition. APS Press, St. Paul, MN, 2001. (4) T. S. Walker et al. Plant Physiol. 134:320, 2004.