Protoporphyrinogen oxidase (PPO) inhibitor-resistant waterhemp (Amaranthus tuberculatus) from Nebraska is multiple herbicide resistant: Confirmation, mechanism of resistance, and management

A waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] biotype (designated as NER) collected from a soybean [Glycine max (L.) Merr.] production field in eastern Nebraska survived the POST application of fomesafen at the labeled rate. The objectives of this study were to (1) quantify the level of resistance to protoporphyrinogen oxidase (PPO) inhibitors (acifluorfen, fomesafen, and lactofen) applied POST, (2) determine the mechanism of PPO-inhibitor resistance in the NER biotype, (3) determine whether NER possessed multiple resistance to acetolactate synthase (ALS)-, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-, and photosystem II (PSII)-inhibiting herbicides, and (4) control NER with POST soybean herbicides. A whole-plant dose-response bioassay revealed that the NER biotype was 4- to 6-fold resistant to PPO-inhibiting herbicides depending on the known susceptible biotype (S1 or S2) used for comparison. A Kompetitive Allele Specific PCR (KASP™) assay was developed and performed for rapid and robust detection of the ΔG210 mutation (deletion of a codon) in the PPX2L gene. All samples of the NER biotype tested positive for the ΔG210 mutation. Dose-response bioassays confirmed that the NER biotype was resistant to three additional herbicide sites of action. Chlorimuron and imazethapyr, both ALS inhibitors, applied at 32X the labeled rate resulted in <80% reduction in the aboveground biomass of the NER biotype. The same biotype was 3- and 7-fold resistant to glyphosate (EPSPS inhibitor) and atrazine (PSII inhibitor), respectively. Glufosinate, 2,4-D choline plus glyphosate, and dicamba were the only soybean POST herbicides that controlled NER effectively (≥92% aboveground biomass reduction). Amaranthus tuberculatus is the first confirmed weed species in Nebraska to evolve resistance to four distinct herbicide sites of action, leaving no POST herbicide choice for effective control in glyphosate-resistant and conventional (non-transgenic) soybean.