A European biotype of Amaranthus retroflexus cross-resistant to ALS inhibitors and response to alternative herbicides

Abstract

An acetolactate synthase (ALS)-resistant Amaranthus retroflexus biotype was collected in a soyabean crop after repeated exposure to imazethapyr and thifensulfuron- methyl in north-eastern Italy. Studies were conducted to characterise the resistance status and determine alternative post-emergence herbicides for controlling this biotype. Whole-plant bioassay revealed that the GR50 values were 1898- and 293-fold higher than those observed for the biotype susceptible to imazethapyr and imazamox respectively. The biotype also displayed high cross-resistance to sulfonylureas. Molecular analysis demonstrated that a single nucleotide substitution had occurred in domain B (TGG to TTG at position 574), conferring a change from the amino acid tryptophan to leucine in the resistant biotype. However, herbicides with other modes of action (PSII, 4-HPPD and PPO inhibitors) provided excellent control. The GR50 ratios for metribuzin, terbuthylazine and mesotrione were close to 1 and treatments with fomesafen gave 100% control of both susceptible and resistant biotypes at the recommended field dose. This study documents the first case of an imidazolinone and ALS-resistant biotype in European crops and identifies the post-emergence herbicide options available for managing this troublesome weed in soyabean crops. Alternative management strategies are also discussed