A family affair: resistance mechanism and alternative control of three Amaranthus species resistant to acetolactate synthase inhibitors in Italy

BACKGROUND: several soybean fields in Italy were found to be infested by multiple species of Amaranthus spp. not adequately controlled by acetolactate (ALS) inhibitor herbicides. The objectives of this research were a) to create a simplified botanical key to identify weedy amaranths b) to determine the number and type of sites of action the accession are resistant to, i.e. resistance pattern c) to determine the main resistance mechanisms involved d) to evaluate the efficacy of herbicides with different site of action. RESULTS: An easy-to-use botanical key was devised and successfully used in the infested sites and results were confirmed through a species-specific molecular marker. Amaranthus retroflexus L. (redrood pigweed) was found in three sites; plants with Asp376Glu substitution at the ALS gene were resistant to thifensulfuron-methyl. Amaranthus tuberculatus (Moq.) J.D.Sauer (waterhemp) and Amaranthus hybridus L. (smooth pigweed) accessions were cross-resistant to thifensulfuronmethyl and imazamox; most ALS-resistant plants had a point mutation at position 574. One A. hybridus accession had the substitution Trp574Met, new for Amaranthus genus. All ALS-resistant accessions were controlled by glyphosate and metribuzin. A. retroflexus accessions were controlled by bentazon, instead an A. hybridus and some A. tuberculatus accession were not. CONCLUSIONS: the simplified botanical key proposed herein could be a useful tool for farmers and weed scientists to reliably identify Amaranthus species in the field. The main resistance mechanism in the three Amaranthus species is target-site mediated. This is the first evidence of ALS-resistant A. tuberculatus outside its native North American range