First report of root rot caused by Fusarium commune on American ginseng (Panax quinquefolium L.) in China.

Abstract

American ginseng (Panax quinquefolium L.) is one of the most valuable herb crops because of its unique pharmacological effects. In 2019, American ginseng plants withered and root rot with incidences of 20-45% were observed in about 70000m2 of ginseng production field located in mountainous valley of Benxi city (41º23'32" N, 124º04'27" E), Liaoning Province in China. Disease symptoms included chlorotic leaves with dark brown discoloration extending gradually from the basal to the apical part of the leaves. Water-soaked, irregular lesions appeared on the surface of roots and rotten at later stage. Twenty-five symptomatic roots were surface-sterilized by immersion in 2% sodium hypochlorite (NaOCl) for 3 min, followed by rinsing three times in sterilized water. The sections healthy tissues bordered rotten tissues, i.e. the leading edge, were cut into 4-5 mm pieces with a sterile scalpel and 4 pieces were placed on each PDA plate. After 5 days incubation at 26°C, total of 68 single spores were obtained from the colonies with an inoculation needle under stereomicroscope. Colonies from single conidia were white to greyish white, densely floccose to fluffy, and the reverse grayish yellow with dull violet pigmentation. Single-celled and ovoid microconidia in false heads were borne on aerial monophialidic or polyphialidic conidiophores on Carnation Leaf Agar (CLA) media, and measured 5.0 -14.5 × 3.0 -4.8 μm (n=25). Macroconidia were two to four septa, slightly curved, apical and basal cells were also curved, and they measured 22.5 - 45.5 × 4.5 - 6.3 μm (n=25). Chlamydospores were singly or in pairs, circular or subcircular, smooth, and measuring 5 - 10.5 μm (n=25) in diameter. Morphologically, the isolates were identified as Fusarium commune (Skovgaard et al. 2003; Leslie and Summerell 2006 ). To confirm the identity, the rDNA partial translation elongation factor1 alpha (TEF-a) gene and the internal transcribed spacer (ITS) region of ten isolates were amplified and sequenced (O'Donnell et al. 2015; White et al. 1990). Identical sequences were obtained, and one representative sequence of isolate BGL68 was submitted to GenBank. BLASTn analysis of both the TEF-α (MW589548) and the ITS (MW584396) sequences, revealed 100% and 99.46 % sequence identity to F. commune MZ416741 and KU341322, respectively. The pathogenicity test was conducted under greenhouse conditions. The surface of healthy 2-year-old American ginseng roots was washed and disinfested in 2% NaOCl for 3 min before rinsing in sterilized water. Twenty roots were wounded with a toothpick, resulting in tiny perforations (1.0 × 1.0×3.0 mm), 3 perforations were wounded on each root. Inoculums was prepared from the culture of isolate BGL68 incubate in potato dextrose broth (PD) for 5 days at 26°C,140 rpm. Ten wounded roots were immersed in a conidial suspension (2 × 105 conidia/ml) for four hours in a plastic bucket, and planted in five containers (two roots per container) filled with sterile soil. Another ten wounded roots were immersed in sterilized distilled water and planted in five containers as controls. The containers were incubated for four weeks in a greenhouse at temperature between 23°C and 26°C, under a 12-hr light and dark regime, and irrigate with sterile water every 4 days. Three weeks after inoculation, all inoculated plants exhibited chlorotic leaves, wilting and root rot. The taproot and the fibrous roots showed brown to black root rot and no symptoms in non-inoculated controls. The fungus was reisolated from the inoculated plants, but not from any of the control plants. The experiment was repeated two times with similar results. This is the first report of root rot caused by F. commune on American ginseng in China. The disease might bring a threat to this ginseng production and should be implemented effective control measures to reduce losses.