Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Tai, Vera

Abstract

Ginseng (Panax quinquefolius) is susceptible to ginseng replant disease (GRD), resulting in root rot and decreased yield when ginseng is grown in soils previously used for ginseng crops. Although fungal and oomycete pathogens are implicated, GRD is a complex disease and the changes in the soil when initially cultivating ginseng that lead to GRD remain unclear. This thesis reported changes in the soil microbiome from three new gardens in Norfolk County, Ontario over three years starting from seeding with ginseng. Over this period of cultivation, metabarcoding of the V4 region of the 16S ribosomal RNA gene clearly showed a decrease in alpha diversity of the microbiome and a shift in microbial community composition. Families of microbes that are known to perform nitrogen fixation, ammonia oxidation, and toxin degradation increased in relative abundance. Future investigations should confirm whether any of the changes in microbial taxa during ginseng cultivation contribute to GRD.

Summary for Lay Audience

Ginseng is grown for its roots, which have been used in traditional Chinese and Korean medicine for thousands of years. However, ginseng cultivation is prone to ginseng replant disease (GRD) which results in poor growth of ginseng and lower yields when planted in soils previously used for ginseng crops. This is a major concern for ginseng growers. Pathogens in the fungi and oomycetes are known to contribute to GRD. In addition, ginseng roots can produce ginsenosides which can accumulate over time and cause changes to beneficial or pathogenic bacteria in the soil and contribute to GRD. However, the specific changes in the soil microbiome when ginseng is first grown that end up causing GRD are unclear.

This study aimed to investigate how microorganisms in new ginseng garden soils changed over a period of three years. Between October 2018 and November 2021, soil samples were collected every season from three new gardens in Norfolk County, Ontario. They were compared to see how their microbial communities changed as ginseng grew. The study found that microorganisms known to perform nitrogen fixation, ammonia oxidation, and toxin degradation increased after three years.

In summary, the diversity of the soil microbiome where ginseng was grown had evidently changed. These changes in community composition could be directly due to the growth of ginseng or be affected by the farmers’ cultivation practices, such as fertilization, and should be further examined to determine whether the bacterial changes are related to the development of GRD, and to develop specific tests to diagnose the soil.

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