Abstract
<jats:title>Abstract</jats:title> <jats:sec> <jats:title>BACKGROUND</jats:title> <jats:p> Reduced control of giant ragweed ( <jats:italic>Ambrosia trifida</jats:italic> L.) with protoporphyrinogen oxidase (PPO)-inhibiting herbicides was recently reported in two southern Illinois populations, VRC and TMS. The objectives of this study were to assess resistance to postemergence-applied PPO inhibitors in VRC and TMS, evaluate VRC response to acetolactate synthase (ALS)- and enolpyruvyl shikimate phosphate synthase (EPSPS)-inhibiting herbicides, and identify target-site mechanisms associated with PPO-inhibitor resistance. </jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> Based on LD estimates, VRC resistance ratios ranged from 1.1- to 3.7-fold for lactofen and 2.2- to 6.9-fold for fomesafen relative to PPO-sensitive populations SIU and DSO. In TMS, LD estimates were 257.4 g ai ha ¹ for lactofen and 318.4 g ai ha ¹ for fomesafen. Glyphosate LD estimates exceeded three times the labeled field rate in VRC, SIU, and DSO, whereas VRC and SIU had higher cloransulam-methyl LD estimates than DSO. Whole-transcriptome sequencing identified polymorphisms in <jats:italic>PPX1</jats:italic> and <jats:italic>PPX2</jats:italic> ; however, only PPO2 R98Q altered a catalytic-domain binding-pocket residue and was considered likely to contribute to resistance in VRC. R98Q was absent in TMS, and <jats:italic>PPX1</jats:italic> and <jats:italic>PPX2</jats:italic> expression did not differ among populations. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> VRC and TMS have evolved resistance to lactofen and fomesafen, and VRC also exhibited reduced sensitivity to cloransulam-methyl and glyphosate. PPO2 R98Q is novel in <jats:italic>A. trifida</jats:italic> and, to our knowledge, represents the first report of this mutation associated with PPO-inhibitor resistance in plants. The absence of target-site alterations in TMS suggests a potential non-target-site basis for resistance. These findings highlight the need for integrated, diversified management to further reduce herbicide selection pressure. </jats:p> </jats:sec>