First Report of Ilyonectria robusta Causing Rusty Root of Asian Ginseng in China

Abstract

Asian ginseng (Panax ginseng) is an economically important perennial herb, mainly cultivated in Jilin Province, China. In September 2013, Asian ginseng plants in Jilin showed rusty root symptoms. Typical symptoms included rusty superficial lesions of irregular shapes and margins. Ten symptomatic roots were collected from each of five fields for investigation. To isolate the pathogen, root epidermal tissues with typical lesions were excised, surface-sterilized, and placed on potato dextrose agar (PDA) amended with 50 μg/ml tetracycline. After incubation at 20 ± 1°C in the dark for a week, 18 single-spore isolates out of 50 samples were obtained and identified as Ilyonectria robusta (A.A. Hildebr.) A. Cabral & Crous based on morphological characters and DNA sequence analysis (1). After incubating 7 days on PDA in the dark at 20°C, colonies were cottony to felty in texture and orange white to brownish grey in color with average diameters of 60 ± 3 mm. Isolates were cultured on synthetic nutrient-poor agar for conidial measurements. Macroconidia formed on simple conidiophores predominately, with mostly one and occasionally up to three septa, and were cylindrical with both ends broadly rounded. Macroconidia varied in size depending on the number of cells as follows: one-septate, 7.0 ± 0.6 × 27.7 ± 2.7 μm; two-septate, 7.3 ± 0.7 × 33.3 ± 2.1 μm; three-septate, 7.4 ± 0.6 × 33.4 ± 2.2 μm. Microconidia that formed on complex conidiophores were ellipsoid to ovoid and ranged in size from aseptate 3.7 ± 0.5 × 8.7 ± 1.1 μm to one-septate 5.0 ± 0.6 × 13.1 ± 1.6 μm. Brown chlamydospores were abundantly produced on PDA, globose to subglobose in shape, and in size of 10.9 ± 1.3 × 11.8 ± 1.5 μm (n ≥ 30 observations per structure for each measurement). The isolates were further classified by amplifying and sequencing the ITS1-5.8S rRNA-ITS2 region and histone H3 gene with primer pairs ITS5 and ITS4 (4), and H3-1a and H3-1b (3), respectively. Sequences of the two loci (GenBank Accession Nos. KM015300 and KM015299) showed 100% identity among the three examined isolates and the published I. robusta isolates (JF735268 and JF735517). To confirm the pathogenicity, bare roots of 3-year-old Asian ginseng were inoculated with mycelial plugs of three isolates of I. robusta selected randomly. Four roots were inoculated as replicates for each isolate with pathogen-free agar plugs as a control. One week post-inoculation in the dark at 20 ± 1°C, all the inoculated ginseng roots showed light-brown to dark-brown lesions. I. robusta was recovered from symptomatic roots and confirmed by analyzing the DNA sequence of the histone H3 gene. The inoculation experiment was repeated, and both trials showed the same results. The ginseng tissue under the control agar plugs remained symptomless, and no fungi were isolated. To our knowledge, this is the first report of I. robusta causing rusty root of P. ginseng in China (1,2,5). References: (1) A. Cabral et al. Mycol. Prog. 11:655, 2012. (2) I. Erper et al. Eur. J. Plant Pathol. 136:291, 2013. (3) N. L. Glass et al. Appl. Environ. Microbiol. 61:1323, 1995. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990. (5) X. Lu et al. Plant Dis. 98:1580, 2014.

Deciphering Microbiome Related to Rusty Roots of Panax ginseng and Evaluation of Antagonists Against Pathogenic Ilyonectria

“…It is mentioned in some studies that there was a close relationship between rusty roots and rhizosphere soil properties (Liu et al, 2014). Besides, microorganisms also play a key role in this disease as suggested in other studies that ginseng roots could be infected by putative pathogens, including both bacteria (Choi et al, 2005; Lee et al, 2011) and fungi (Rahman and Punja, 2005; Reeleder et al, 2006; Lu et al, 2015, 2019), and showed rusty symptom after inoculation of the isolates. However, there is no systematic research on the microbiome involved in the disease.…”

“…The latter disease, also known as rusted roots or “the rust,” is characterized by small or large reddish-brown spots on the surface of ginseng roots (Supplementary Figure S1). It is widely reported in China, South Korea and Canada as a major cause of ginseng root deterioration at all stages (Hildebrand, 1934; Lee et al, 2011; Lu et al, 2015). Given the fact that ginseng roots are commercially graded according to their sizes, shapes and overall appearances, rusty root disease severely limits the output and quality of ginseng worldwide.…”

Microbial Community Changes in the Rhizosphere Soil of Healthy and Rusty Panax ginseng and Discovery of Pivotal Fungal Genera Associated with Rusty Roots

“…Interestingly, 5 fungal genera were enriched in the rhizosphere soil of rusty root-affected ginseng compared to that of healthy ginseng, whereas the opposite was true for 15 fungal genera (P < 0.05, LDA >3.00). Soil microbial diversity and the composition of microbial communities play important roles in maintaining soil ecosystem function, health, and quality [13,27], and disease occurrence in medicinal plants is governed largely by imbalances in soil microbial diversity and community composition [53,54]. Because rusty root has severely threatened the sustainable development of the P. ginseng industry, numerous studies have investigated the relationship between soil microbes and rusty roots; however, most studies have considered only one or two pathogenic fungi that are involved in the occurrence of rusty root in P. ginseng, such as Alternaria panax Whetz.…”

“…For example, Farh et al reported that the rusty root of P. ginseng was caused by Cylindrocarpon destructans var. destructans [11], while Lu et al investigating the pathogenicity of Ilyonectria robusta against the root of P. ginseng using repeated inoculation, highlighted that I. robusta could be the causative agent of rusty root of P. ginseng in China [27]. Recent studies have also indicated that the Ilyonectria fungus was the most likely cause of rusty root in P. ginseng, as this pathogen was found to be enriched in the rhizosphere soils of plants exhibiting rusty root symptoms [11,25].…”

Unveiling of Dominant Fungal Pathogens Associated With Rusty Root Rot of Panax notoginseng Based on Multiple Methods

“…(Heydari et al 2014;Hu et al 2008), and fungi such as Alternaria spp., Cylindrocarpon spp., Fusarium spp., Phytophthora cactorum, Phoma herbarum, Lasiodiplodia theobromae, and Rhizoctonia solani (Abdel-Monaim and Abo-Elyousr 2012; Latha et al 2011;Miao et al 2006). Cylindrocarpon destructans and Ilyonectria robusta were reported to cause rusty root rot of P. ginseng in China and other nations (Cabral et al 2012;Lu et al 2015).…”