Project Code: PRR14-050
Project Lead
Deena Errampalli - Agriculture and Agri-Food Canada
Objective
To generate knowledge on the replant disease complex and develop management options to improve ginseng re-cropping opportunities for old gardens
Results Summary
Background
American ginseng (Panax quinquefolius L.) is cultivated for its highly valued roots which provide health benefits such as boosting the immune system in humans. With about 3,500 hectares of ginseng crops, Canada is the world’s largest producer of American ginseng exporting to more than 10 countries. About 99% of Canadian ginseng is grown in southwestern Ontario generating close to $240 Million in revenue annually. Ginseng is a perennial crop and usually remains in the ground for 3 or 4 years before the roots are harvested and marketed.
Soil-borne pests are a serious threat to viable ginseng production. Ginseng replant disease, a complex issue resulting in crop failure when ginseng is planted in the same garden where a previous ginseng crop was grown, is thought to originate from several fungal plant pathogens causing root lesions and root rot. This disease poses a major challenge for growers, and in Ontario most fields are cropped only once to ginseng as a result. This leaves growers with limited options for high quality, clean agricultural soils to establish new gardens. Moreover, the industry has experienced the loss of major soil fumigants in the wake of regulatory review of these compounds. Identification of alternative approaches to address replant disease has become a priority for the ginseng industry.
The main goal of this 3-year project was to identify and characterize the pathogen complex and key biochemical interactions responsible for replant disease in ginseng. It also explored management methods to identify alternatives to the chemical fumigation which has been the standard disease management tactic in ginseng production.
Approach
The studies under this project were conducted at the Vineland Station of Agriculture and Agri-Food Canada's (AAFC) London Research and Development Centre, and at commercial ginseng fields in Simcoe, Ontario. The project pursued three objectives. First, screening studies were conducted in the fall of 2014, 2015, and 2016 to identify and characterize pathogens associated with the replant disease complex in ginseng roots and soils where replant disease was observed. Soil samples from old ginseng gardens (0, 1, 2, 3, 4, 5, 10, 11, 22 and 25 years after harvest), from current ginseng gardens, from wild ginseng stands in forests, as well as from clean fields (which had not been cultivated to ginseng before) were collected, cultured on selective media and incubated to allow growth of any fungi which were present. Root samples were also obtained from ginseng plants in 2-3 year old gardens. Fungi isolated from both root and soil samples were identified using traditional (based on morphological characteristics) and advanced molecular diagnostic methods, including deoxyribonucleic acid (DNA) sequence analysis of fungal internal transcribed spacer (ITS) regions. A multiplex polymerase chain reaction (PCR) capable of detecting the pathogenic DNA directly from soil was developed and optimized for testing DNA extracts from soils and fungi isolated from diseased and healthy ginseng roots.
Pathogenicity assays on detached roots and as well as in soil, using 2-year old potted ginseng plants, were conducted for fungal species isolated from ginseng replant soil: Alternaria alternata, Epicoccum nigrum, Fusarium oxysporum, Fusarium solani, and Ilyonectria radicicola. Clonostachys rosea isolated from wild ginseng soil was also subjected to these assays.
Secondly, a lab study was conducted in 2016-2017 and 2017-2018 to determine the role of ginsenosides, byproducts exuded by ginseng roots into the rhizosphere, in replant disease development. Whole ginseng crude root extracts and selected major ginsenosides were examined for their effect on the above-mentioned six fungal isolates.
Thirdly, laboratory and greenhouse studies were conducted in 2015 and 2016 to test the efficacy of alternative replant disease management approaches including currently available biofungicide (Trichoderma harzianum, Rootshield®) and fungicides (Fludioxonil and Cyprodinil, Switch®; Fludioxonil, Scholar®; Propiconazole, Topas®) as well as soil cover with leaf litter mulch from the maple tree forest floor.
Results
A total of 1,752 fungal isolates were collected from all root and soil samples over three years of the screening study. Morphological identification of fungal isolates was not reliable and so selected isolates were identified by either PCR or DNA sequencing. Molecular identification of a subset of 164 isolates (122 from soils and 42 from roots) were further identified based on DNA sequencing of ITS region of the fungi. The analysis of symptomatic roots collected from existing replant gardens was conducted. In 2014, 45%, 11% and 7% of symptomatic roots were associated with F. solani, E. nigrum and A. alternata, respectively. In 2015, I. radicicola (also known as Cylindrocarpon destructans) was the most commonly isolated fungus at 32% followed by F. solani at 14%.
The DNA sequence based analysis of the 122 selected soil fungal isolates collected at 0, 1, 2, 3, 4, 5, 10, 11, 22 and 25 years after ginseng was planted (old gardens) was compared to soils with no record of ginseng planting (control) and those of forests where wild ginseng was growing. During 2014, 2015 and 2016, Fusarium spp. were present across all soil samples from 0, 1, 2, 3, 4, 5, 10, 11, 22 and 25 years after ginseng was planted and also in the wild ginseng soils. Fusarium species were found at 3.6%, 6.6%, and 20% in 2014, 2015 and 2016, respectively. A comparison of the soils from ginseng gardens and soils where wild ginseng was grown showed there is little overlap between the species found in cultivated and wild soils. Clonostachys rosea was present only in the soils where wild ginseng was grown but not in old gardens. The molecular diagnosis provided more accurate identification than the morphological assessment.
The pathogenicity assay on ginseng roots showed that of the six fungi tested, F. solani and A. alternata were highly pathogenic, F. oxysporum was weakly pathogenic and C. rosea was not pathogenic.
The ginsenosides study indicated that some tested fungi suffered growth inhibition against ginsenosides derived from whole roots extracts. For instance, E. nigrum and F. oxysporum were the most inhibited by the ginsenoside whole root extract, whereas F. solani and C. rosea showed slight inhibition of growth. Two fungi, I. mors-panacis and A. alternata were not inhibited, suggesting that ginsenosides may not play a role in stopping these pathogens from infecting the roots. Ilyonectria mors-panacis is a soil-borne pathogenic fungus that has already been shown, in other studies, to cause root rot symptoms in ginseng.
Due to failure of seed germination in 2014, the management component of the study was postponed. The treatments were applied in 2015 and 2016 and data will be collected in December 2018.
This project has generated new knowledge on fungi associated with ginseng replant disease present in the soils and on symptomatic ginseng roots, and on the role of ginsenosides in this pathosystem. Select fungal isolates from this study were found to be pathogenic on ginseng roots. A difference in fungal populations between old ginseng garden soils and wild soils shows that potential microorganisms with biocontrol properties against replant diseases may be present in the soils where wild ginseng grows. The project results were shared with ginseng growers, provincial specialists and industry stakeholders throughout the course of the project. Results from the management trials will be made available to growers and are expected to contribute towards the development of an integrated management system for replant disease in ginseng production.