Date of Award

Fall 2013

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Botany and Plant Pathology

First Advisor

Steven G. Hallett

Second Advisor

William G. Johnson

Committee Chair

William G. Johnson

Committee Member 1

Steven G. Hallett

Committee Member 2

Gurmukh S. Johal

Committee Member 3

Kiersten A. Wise


Continuous glyphosate use has contributed to an increasing number of problematic glyphosate-resistant (GR) weeds. The mechanism of resistance in many GR weeds is poorly understood, in part, due to a poor understanding of how exactly glyphosate kills a plant. In previous research, the efficacy of glyphosate was demonstrated to be strongly influenced by root invading soil-borne microorganisms. However, this interaction among plants, glyphosate, and soil microorganisms has only been studied in a number of crop plants, but not in weed species. This is surprising since the soil biotic environment has a strong impact on the activity of this important herbicide. Gaining a better understanding of these interactions may shed more light on the performance of glyphosate in the field and the evolution of glyphosate resistance. The objective of this research was to determine the role of soil microbes in the resistance to glyphosate of three problematic weeds of the midwestern United States: giant ragweed (Ambrosia trifida L.), horseweed [(Conyza canadensis (L.) Cronq.], and common lambsquarters (Chenopodium album L.). Through a series of greenhouse and lab experiments we determined that root colonization by soil microorganisms increased the activity of glyphosate in glyphosate-resistant (GR)

and -susceptible (GS) biotypes of giant ragweed and a GS common lambsquarters biotype, but not in horseweed biotypes. The GS biotypes of each weed species were colonized by a greater number of soil microorganisms, specifically oomycete (e.g. Pythium spp. and Phytophthora spp.) pathogens, when treated with glyphosate, compared to the GR biotypes. These data suggested that the ability of giant ragweed to tolerate a glyphosate application may involve differences in the susceptibility to soil microbial colonization, specifically Pythium spp. However, the degree of giant ragweed susceptibility to two Pythium species, P. ultimum and P. aphanidermatum, did not differ between the GR and GS biotypes grown in disease conducive conditions, in the absence of glyphosate. Utilizing next-generation sequencing revealed that the rhizosphere microbial community of giant ragweed is different between the GR and GS biotypes, and the microbial community is perturbed by a glyphosate treatment. Glyphosate does cause changes to the diversity of the rhizosphere microbial community 3 days after treatment, and this needs to be investigated further. Overall, the results of this research demonstrate that rhizosphere interactions are important to the mode of action of glyphosate, in the weed species selected. These findings suggest that the range of tolerance to glyphosate observed in weeds and the evolution of resistance in weed biotypes may also be influenced by rhizosphere interactions. Understanding these interactions are crucial in understanding the biology of these weed species and the resistance to this herbicide.