In 1997, farmers in Ontario, Canada, reported failure of some ALS-inhibiting herbicides to provide adequate control of pigweed species. Growth room experiments were conducted to confirm resistance to ALS inhibitors in populations of Powell amaranth and redroot pigweed. Twenty-two out of 35 collected seed samples were able to grow in the presence of soil-applied imazethapyr or flumetsulam. Dose–response curves were generated for 11 and 9 populations of Powell amaranth and redroot pigweed, respectively, using foliar-applied imazethapyr and thifensulfuron. Resistance to ALS inhibitors was confirmed in nine and five populations of Powell amaranth and redroot pigweed, respectively. Within each species, comparison of the herbicide rate required to reduce plant dry weight 50% (GR50) between the resistant populations and a susceptible population was conducted to obtain resistance factors. For imazethapyr, resistance factors ranged from 4.2 to 3,438 and from 33 to 168 for Powell amaranth and redroot pigweed, respectively. High-level cross-resistance to thifensulfuron was found in two populations of each species, with resistance factors ranging from 270 to 2,416. In both species, populations could be grouped according to their cross-resistance patterns: some populations were resistant to imazethapyr only, whereas others expressed resistance to both imazethapyr and thifensulfuron. The observed patterns of cross-resistance were not correlated with known herbicide exposure history of the fields where these populations originated.
Nomenclature: Flumetsulam; imazethapyr; thifensulfuron; Powell amaranth = green pigweed, Amaranthus powellii S. Wats. AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE.