Characterisation of resistance to ALS inhibitors in Chenopodium album (L.) populations in Ontario

Four Chenopodium album L. (common lambsquarters) populations, two susceptible (S) and two putative resistant (R) were screened for resistance to four post-emergent acetolactate synthase (ALS)-inhibiting herbicides using whole plant spraying and enzyme inhibition assays. Thifensulfuron-methyl (sulfonylurea), imazethapyr (imidazolinone), thiencarbazone-methyl (triazolinone) and pyrithiobac-sodium (pyrimidinylthiobenzoate) were all applied at varying doses. Above ground biomass and visible injury data were taken nine days after treatment. Resistance to thifensulfuron-methyl was confirmed in the two putative resistant populations and cross resistance was discovered to the three other classes of ALS inhibitors. ALS inhibition assay revealed 157- to 729-fold-resistance to thifensulfuron and up to 6930-fold to pyrithiobac, confirming resistance due to enzyme target site modification.