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

An 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 SUs. 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 paper documents the first case of an IMI and ALS-resistant biotype in European crops and identifies the post-emergence herbicide options available for managing this troublesome weed in soyabean crop. Alternative management strategies are also discussed.