Evaluation of three Manitoba redroot pigweed (Amaranthus retroflexus L.) populations resistant to acetolactate synthase-inhibitors

Redroot pigweed (Amaranthus retroflexus L.) is a common broadleaf weed species in Manitoba, Canada. Producers in this region have been reporting reduced response of this weed to commonly used Acetolactate synthase (ALS) inhibiting herbicides. Three suspected resistant populations of redroot pigweed were compared with known susceptible populations in a greenhouse study to determine the level of resistance to the ALS-inhibiting herbicides imazethapyr and thifensulfuron-methyl. A laboratory experiment was conducted to identify the mechanism of herbicide resistance of these three populations. Dose response curves to these herbicides indicated that all three pigweed populations were resistant to imazethapyr (~30 to >170-times), but only one of these populations showed a low level (7-fold) of cross resistance to thifensulfuron-methyl. Acetolactate synthase (ALS) gene sequence analysis detected a Ser653Asn amino acid substitution that was consistent in the two populations ArMB1 & ArMB2 which were highly resistant to imazethapyr. A Ser653Asn/Ile substitution was found in population ArMB3. That population showed high resistance to imazethapyr and low levels of resistance to thifensulfuron-methyl. A malathion treatment study was conducted to determine the presence of non-target site resistance through enhanced metabolism among the three resistant populations. Cytochrome P450 induced metabolism based non-target site resistance (NTSR) did not contribute to resistance to imazethapyr or thifensulfuron-methyl among the three resistant populations. Target site resistance alone was the mechanism conferring resistance to these two ALS inhibitors among the three resistant redroot pigweed populations and that facilitates the development of rapid detection markers to identify the resistant redroot pigweed with these mutations.