Persistence of PPO mutations and dynamics of Amaranthus palmeri resistance to PPO ‐inhibitor herbicides in a decade

BACKGROUND: Amaranthus is a highly troublesome weed genera globally. Most cotton, corn, and soybean fields in the US mid-south have Amaranthus palmeri populations resistant to protoporphyrinogen oxidase (PPO)-inhibiting herbicides. We evaluated the persistence of resistance to PPO-inhibiting herbicides in A. palmeri populations from Greene (GRE-A) and Crittenden (CRI-D) counties, Arkansas, collected in 2015 and approximately 8 years later. RESULTS: Fomesafen, the primary selector for resistance to PPO-inhibiting herbicides, was the least effective herbicide in both post- and pre-emergence tests. Soil-applied sulfentrazone was more effective on GRE-A than CRI-D populations. Fomesafen was less effective on CRI-D than GRE-A populations. GRE-A populations were more resistant to foliar-applied PPO-inhibiting herbicides (62-76% control) than CRI-D populations. Resistance to fomesafen in CRI-D increased significantly with time, declining from 91% control in 15-CRI-D to 48% in 22-CRI-D. GRE-A populations harbored high frequencies of ΔG210 and A399 mutations. CRI-D populations predominantly harbored ΔG210 and G128 mutations. Increased resistance to fomesafen was strongly linked to increased frequency of single mutations, primarily ΔG210. Resistance to foliar-applied flumioxazin lacked a clear association with target-site mutations, while resistance to saflufenacil was associated with enrichment of double mutations. CONCLUSIONS: PPO2 mutations remained the primary factor conferring resistance to PPO-inhibiting herbicides over 8 years, underscoring their evolutionary advantage in maintaining A. palmeri resistance. The weak relationship between resistance level and mutation frequency indicates enrichment of additional resistance-conferring mechanisms. Stronger, contemporary PPO-inhibitor herbicides are still effective on resistant populations, but rare escapes carry nontarget-site resistance (NTSR) mechanisms which should be mitigated with other management tools. © 2026 Society of Chemical Industry.