Cross-resistance ofBidens subalternansto acetolactate synthase inhibitors in Brazil

Abstract

Summary Weeds resistant (R) to herbicides are widespread worldwide. Bidens subalternans is one of the most troublesome weeds in conventional soyabean fields in Brazil, and in a crop rotation system of cotton/soyabean and maize/soyabean some populations had evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides. Bidens subalternans plants suspected of resistance were observed in soyabean fields where the main ALS-inhibiting herbicide sprayed is chlorimuron-ethyl. To confirm and characterise the resistance of B. subalternans to ALS inhibitors, whole-plant bioassays were conducted in 2006 and 2008. ALS in vivo enzyme bioassays were also conducted in 2007. In both bioassays, the R biotype showed cross-resistance to four chemical families of ALS-inhibiting herbicides. According to whole-plant level tests the R biotype showed 498-, 797-, 726- and >877-fold resistance to chlorimuron-ethyl, imazethapyr, cloransulam-methyl and pyrithiobac-sodium herbicides respectively. The R biotype was also 17-, 166-, 436- and 516-fold R to chlorimuron-ethyl, imazethapyr, cloransulam-methyl and pyrithiobac-sodium herbicides, respectively, based on the enzyme assay. Therefore, the herbicide-R B. subalternans can no longer be controlled by any ALS-inhibitor herbicides. Integrated control methods involving alternative herbicide with different modes of action are needed, to avoid yield losses in conventional soyabean fields in Brazil that are infested by ALS-R B. subalternans populations.