Abstract: Three studies were conducted in 1999 and 2000 to determine whether acetolactate synthase (ALS)–resistant common ragweed and giant ragweed biotypes were present in Ohio. Results of field studies indicated that biotypes of both species had cross-resistance to three chemical families of ALS-inhibiting herbicides. Cloransulam-methyl applied postemergence at 9, 18, and 36 g/ha controlled more than 85% of two susceptible populations of common and giant ragweed 28 d after treatment, whereas less than 35% control of resistant populations was achieved at the same rates. Fomesafen, lactofen, and glyphosate applied alone at the recommended rates provided the most effective control of ALS-resistant common and giant ragweed. Mixtures of cloransulam-methyl with either fomesafen or lactofen did not significantly increase ALS-resistant common and giant ragweed control compared with each diphenylether herbicide used alone. Dose–response bioassays conducted in the greenhouse indicated that susceptible common and giant ragweed tended to be more sensitive to cloransulam-methyl and chlorimuron than to imazamox. ALS-resistant common ragweed demonstrated a high level of resistance to all the herbicides tested because GR50 values were not reached with rates 1,000 times higher than the recommended rate. ALS-resistant giant ragweed treated with 13,000 g/ha of chlorimuron and 18,000 g/ha of cloransulam-methyl was not inhibited enough to obtain a GR50 value, thus also demonstrating a high level of resistance. The GR50 for ALS-resistant giant ragweed treated with imazamox was 1,161 g/ha. Results of these studies confirmed the presence of ALS–cross-resistant populations of common and giant ragweed in Ohio and suggest that herbicides with different mechanisms of action will be required to manage these weeds effectively.
Nomenclature: Chlorimuron; cloransulam-methyl; fomesafen; glyphosate; imazamox; lactofen; common ragweed, Ambrosia artemisiifolia L. #3 AMBEL; giant ragweed, Ambrosia trifida L. # AMBTR.
Additional index words: Acetolactate synthase, herbicide resistance.
Abbreviations: ALS, acetolactate synthase; COC, crop oil concentrate; DAT, days after treatment; NIS, nonionic surfactant; POST, postemergence; PPF, photosynthetic photon flux; PRE, preemergence; UAN, urea ammonium nitrate; 1× rate, the recommended label rate; 2× rate, twice the recommended label rate.