Date of Award
Master of Science (MS)
Plant and Environmental Science
Palmer amaranth is a troublesome weed for growers to control, not only due to its aggressive growth characteristics that limit row-crop production, but because of its resistance to different herbicide modes of action. The first case of herbicide resistance in Palmer amaranth was detected in 1989 and has since grown to nine different herbicide classes throughout the United States. New herbicide modes of action have not been developed since the 1980s, so proper stewardship of the remaining modes of action is important for effective control of Palmer amaranth. Increased herbicide resistance from states bordering South Carolina have been reported; therefore, a statewide survey of Palmer amaranth was conducted to screen for resistance to nine selected herbicides. Results from the survey showed that 2,4-D, glufosinate, and dicamba were effective postemergence herbicides for control Palmer amaranth in South Carolina. Isoxaflutole and fomesafen were effective in controlling Palmer amaranth, but a few populations showed evidence of potential resistance. Glyphosate accounted for most of the resistance as many populations were not controlled. The dose response study for glyphosate showed that five biotypes with resistance levels of 6.66, 17.45, 29.87, 12.42, and 13.13 times greater to a known susceptible biotype. Palmer amaranth response to thifensulfuron-methyl varied which indicated the populations were mixed. Results from the thifensulfuron dose-response study confirmed thifensulfuron sensitivity was present in the five selected biotypes. Populations also showed resistance to s-metolachlor and atrazine and is a concern for future use of these herbicides for South Carolina growers. Results from these studies demonstrated high levels of resistance to glyphosate, whereas thifensulfuron-sensitive populations were higher than anticipated in South Carolina. Atrazine and s-metolachlor showed signs of reduced efficacy on Palmer amaranth. Fomesafen, isoxaflutole, 2,4-D, glufosinate, and dicamba were the most effective herbicides for Palmer amaranth management.
Williams, Mitchell, "Survey and Prevalence of Palmer Amaranth Herbicide Resistance in South Carolina" (2023). All Theses. 4099.
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