Confirmation of herbicide resistance mutations Trp574Leu, G210, and EPSPS gene amplification and control of multiple herbicide-resistant Palmer amaranth (Amaranthus palmeri) with chlorimuron-ethyl, fomesafen, and glyphosate

Authors: Spaunhorst, Douglas; NIE, HAOZHEN - Purdue University; Todd, James; YOUNG, JULIE - Purdue University; YOUNG, BRYAN - Purdue University; JOHNSON, WILLIAM - Purdue University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2019
Publication Date: 3/26/2019
Citation: Spaunhorst, D.J., Nie, H., Todd, J.R., Young, J.M., Young, B.G., Johnson, W.G. 2019. Confirmation of herbicide resistance mutations Trp574Leu, G210, and EPSPS gene amplification and control of multiple herbicide-resistant Palmer amaranth (Amaranthus palmeri) with chlorimuron-ethyl, fomesafen, and glyphosate. PLoS One. 14(3):e0214458.
DOI: https://doi.org/10.1371/journal.pone.0214458

Interpretive Summary: Palmer amaranth has become a problematic weed in numerous agricultural commodities in the United States. Some Palmer amaranth plants are resistant to herbicides commonly used in corn, soybean, and cotton production. The objectives of this study were to determine if chlorimuron-ethyl, fomesafen, and glyphosate applied separately and in mixtures control Palmer amaranth, identify the genotype of plants that survived the herbicide treatment to report on the genetic diversity, and confirm the genotypes that survive two- and three-way herbicide mixtures. Fifteen percent of Palmer amaranth treated with the three-way herbicide mixture survived. Mixing fomesafen with chlorimuron-ethyl or fomesafen with glyphosate to create a two-way mixture reduced Palmer amaranth survival 22 to 24% and 60 to 62% more than glyphosate and chlorimuron-ethyl alone, respectively. Genetic mutations previously reported to cause herbicide resistance were found in plants that survived the herbicide treatment. However, one mutation that is associated with resistance to chlorimuron-ethyl was not present in 37% of plants that survived and suggested a different mutation or herbicide metabolism was responsible for plant survival. In a separate instance one Palmer amaranth plant treated with the three-way mixture had genes known to cause survival to chlorimuron-ethyl and fomesafen, but not glyphosate. This result suggested a different glyphosate-resistant mechanism or herbicide antagonism was responsible for plant survival. Indiana soybean fields infested with Palmer amaranth possessed diverse genotypes and herbicide surviving plants are likely to produce seed and spread if alternative control measures are not implemented.

Technical Abstract: Herbicide-resistant weeds, especially Palmer amaranth (Amaranthus palmeri S. Watson), are problematic in row-crop producing areas of the United States. The objectives of this study were to determine if chlorimuron-ethyl, fomesafen, and glyphosate applied separately and in mixtures control A. palmeri, genotype surviving plants to document the genetic diversity, and confirm the presence of various genotypes surviving two- and three-way herbicide mixtures. Fifteen percent of A. palmeri treated with the three-way herbicide mixture survived. Mixing fomesafen with chlorimuron-ethyl or fomesafen with glyphosate to create a two-way mixture reduced A. palmeri survival 22 to 24% and 60 to 62% more than glyphosate and chlorimuron-ethyl alone, respectively. Previously characterized mutations associated with A. palmeri survival to chlorimuron-ethyl, fomesafen, and glyphosate Trp574Leu, a missing glycine codon at position 210 of the PPX2L gene ('G210), and 5-enolpyruvylshikimate-3-phosphase synthase (EPSPS) gene amplification; respectively, were present in surviving plants. However, 37% of plants treated with chlorimuron-ethyl did not contain heterozygous or homozygous alleles for the Trp574Leu mutation, suggesting alternative genotypes contributed to plant survival. All surviving A. palmeri treated with fomesafen or glyphosate possessed genotypes previously documented to confer resistance. A single A. palmeri plant that survived the three-way mixture possessed the genotype heterozygous, homozygous, and wild type for Trp574Leu, 'G210, and EPSPS gene amplification, respectively, and suggested a glyphosate-resistant mechanism other than gene amplification or herbicide antagonism resulted. Results demonstrated repeated post-emergence applications of fomesafen could result in a fomesafen-induced genetic shift towards more plants with 'G210 genotypes. Indiana soybean [Glycine max (L.) Merr] fields infested with A. palmeri possessed diverse genotypes and herbicide surviving plants are likely to produce seed and spread if alternative control measures are not implemented.