Buckhorn plantain populations purportedly resistant to 2,4-D were identified in Pennsylvania following long-term, continual applications of the active ingredient to turfgrass. The research objectives of this study were to 1) confirm 2,4-D resistance with dose-response experiments, 2) confirm field resistance of buckhorn plantain to 2,4-D in Pennsylvania, and 3) evaluate alternative herbicides for 2,4-D-resistant buckhorn plantain. Greenhouse dose-response experiments evaluated the sensitivity of buckhorn plantain biotypes that were resistant or susceptible to 2,4-D, and to halauxifen-methyl, two synthetic auxin herbicides from different chemical families. The resistant biotype was ≥11.3 times less sensitive to 2,4-D than the susceptible biotype and required a 2,4-D dosage ≥4.2 times greater than the standard application rate to reach 50% necrosis. No cross-resistance was observed to halauxifen-methyl because both resistant and susceptible populations demonstrated similar herbicide sensitivity. Field experiments confirmed previous reports of ineffectiveness (≤30% reduction) with 2,4-D and other phenoxycarboxylic herbicides in potentially resistant buckhorn plantain biotypes. Treatments containing halauxifen-methyl resulted in a ≥70% reduction in resistant biotypes. This is the first known report of synthetic auxin herbicide resistance in any weed species in Pennsylvania and highlights emerging herbicide resistance challenges in turfgrass systems.

Nomenclature: 2, 4-D; halauxifen-methyl; buckhorn plantain, Plantago lanceolata L. PLALA