Taxonomy of fungal complex causing red-skin root of Panax ginseng in China

Abstract

Background: Red-skin root of Asian ginseng (Panax ginseng) significantly reduces the quality and limits the production of ginseng in China. The disease has long been thought to be a noninfectious physiological disease, except one report that proved it was an infectious disease. However, the causal agents have not been successfully determined. In the present study, we were to reveal the pathogens that cause red-skin disease.

Methods: Ginseng roots with red-skin root symptoms were collected from commercial fields in Northeast China. Fungi were isolated from the lesion and identified based on morphological characters along with multilocus sequence analyses on internal transcription spacer, β-tubulin (tub2), histone H3 (his3), and translation elongation factor 1α (tef-1α). Pathogens were confirmed by inoculating the isolates in ginseng roots.

Results: A total of 230 isolates were obtained from 209 disease samples. These isolates were classified into 12 species, including Dactylonectria sp., D. hordeicola, Fusarium acuminatum, F. avenaceum, F. solani, F. torulosum, Ilyonectria mors-panacis, I. robusta, Rhexocercosporidium panacis, and three novel species I. changbaiensis, I. communis, and I. qitaiheensis. Among them, I. communis, I. robusta, and F. solani had the highest isolation frequencies, being 36.1%, 20.9%, and 23.9%, respectively. All these species isolated were pathogenic to ginseng roots and caused red-skin root disease under appropriate condition.

Conclusion: Fungal complex is the causal agent of red-skin root in P. ginseng.

Keywords: Cylindrocarpon; Fusarium; Ilyonectria; Panax ginseng; Root disease.

Fig. 1

Symptoms of red-skin disease of Panax ginseng. (A and B) On 6-year-old roots. (C and D) On 3-year-old roots. (E) Cross-section of a 3-year-old root (F) Comparison with a 6-year-old healthy ginseng root.

Fig. 2

Phylogenetic tree of Cylindrocarpon-like isolates based on the analysis of combined 4 genes. Branches with BS = 100% and PP = 1.00 are thickened and in red. Braches with BS ≥ 80% and PP ≥ 0.95 are thickened and in green. The phylogram is rooted with Campylocarpon fasciculare (CBS 112613) and C. pseudofasciculare (CBS 112679).

Fig. 3

Morphological characters of Ilyonectria changbaiensis (CGMCC 3.18789). (A–C) Macroconidia and microconidia. (D and E) Conidiophores. (F) Chlamydospores. Bar = 10 μm.

Fig. 4

Morphological characters of Ilyonectria communis (CGMCC 3.18788). (A–C) Microconidia and macroconidia. (D and E) Chlamydospores. (F and H) Conidiophores. Bar = 10 μm.

Fig. 5

Morphological characters of Ilyonectria qitaiheensis (CGMCC 3.18787). (A–C) Macroconidia and microconidia. (D and E) Conidiophores. (F and G) Chlamydospores. Bar = 10 μm.

Fig. 6

Symptoms of red-skin root disease induced by in vitro inoculation on detached Panax ginseng roots with the fungi. (A) Dactylonectria sp. (B) D. hordeicola. (C) Ilyonectria changbaiensis. (D) I. communis. (E) I. mors-panacis. (F) I. qitaiheensis. (G) I. robusta. (H) Fusarium acuminatum. (I) F. avenaceum. (J) F. solani. (K) F. torulosum. (L) Mock-inoculated control. On each tap root, two to four inoculum plugs were placed in a line with same distance between each other. From the top and the tip of a root, the first and fourth plugs were directly placed on root surface and the second and the third plugs were placed on a punctured tissue, which was poked with an inoculation needle.

Fig. 7

Symptoms of red-skin root disease of Panax ginseng roots inoculated with the fungi under greenhouse conditions. (A) Dactylonectria sp. (B) D. hordeicola. (C) Ilyonectria changbaiensis. (D) I. communis. (E) I. mors-panacis. (F) I. qitaiheensis. (G) I. robusta. (H) Fusarium acuminatum. (I) F. avenaceum. (J) F. solani. (K) F. torulosum. (L) sterilized water.

Fig. S1

Colony morphology of Dactylonectria sp. (A), Ilyonectria changbaiensis (B), I. communis (C), and I. qitaiheensis (D) isolates on PDA.