Resistance to protoporphyrinogen oxidase inhibitors applied preemergence or postemergence to waterhemp (Amaranthus tuberculatus)

Abstract In Wisconsin, herbicide resistance in waterhemp [Amaranthus tuberculatus (Moq.) Sauer] has been confirmed to five herbicide sites of action, including protoporphyrinogen oxidase (PPO) inhibitors. Following a report of a suspected PPO inhibitor–resistant A. tuberculatus population (A92 accession), our objective was to characterize resistance to PPO inhibitors applied preemergence or postemergence to this accession, along with two PPO inhibitor–susceptible control accessions (A66 and A82). We hypothesized that PPO-inhibitor resistance in A92 was driven by target site–resistance mechanisms. According to our results, the A92 accession is resistant to sulfentrazone (3.1-fold; P-value = 0.0278) and fomesafen (3.1-fold; P-value = 0.0745) preemergence and to lactofen (18.6-fold; P-value = 0.0003) and fomesafen (5.9-fold; P-value <0.0001) postemergence. Resistance to PPO inhibitors was not explained by the presence of any known target-site mutations in PPX1 or PPX2 genes. Our study represents the first confirmed case of an A. tuberculatus accession resistant to PPO inhibitors applied preemergence in Wisconsin. Consistent with previous research, our results demonstrate that the A92 accession, compared with control accessions, is less sensitive to fomesafen regardless of the application timing. Further research is necessary to identify other potential PPO-inhibitor resistance mechanisms in the A92 accession, including potential non–target site resistance mechanisms associated with cytochrome P450 monooxygenases or glutathione S-transferases.