Recent Discovery of Amaranthus palmeri S. Watson in Italy: Characterization of ALS-Resistant Populations and Sensitivity to Alternative Herbicides

Amaranthus palmeri S. Watson (Amaranthaceae Juss.) is a dioecious noxious weed, native to the Americas, which infests summer crops. It causes high crop losses, and rapidly evolves resistance to herbicides. In Europe, A. palmeri was recorded mostly as a casual alien, but in 2018 it was reported infesting a soybean field in Italy, and the next year two more populations were found in the same area. Experiments were conducted on these three populations to evaluate the resistance to ALS-inhibiting herbicides, to determine the main resistance mechanisms involved and assess the efficacy of alternative herbicides with different sites of action than ALS. The three populations were confirmed cross-resistant to ALS-inhibiting herbicides (thifensulfuron-methyl and imazamox). Gene sequencing identified a Trp to Leu substitution at position 574 of ALS gene in resistant plants, proving that the main resistance mechanism for the three populations is target-site related. The presence of other resistance mechanisms cannot be excluded. Metobromuron, metribuzin and glyphosate are still effective on these populations.