Abstract
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.
Highlights
The visual estimation of biomass (VEB) of the ALS-treated plants was high, with values ranging from 78% to 94% and from 86% to 93% for imazamox and thifensulfuron-methyl, indicating a negligible effect of both herbicides on biomass production
Our results show that all three herbicides should be considered valuable options to control these A. palmeri populations
Amaranthus palmeri had not been recorded in Italy before 2015
Summary
A. palmeri occurs as alien species in all continents. In Europe it is considered as a casual alien in most countries and naturalized in a few (Belarus, Greece, Italy, Lithuania, Spain and Sweden), where it grows on human-made habitats, especially along roads [2]. In Italy, A. palmeri was found for the first time in 2014 in the Emilia Romagna region (NE-Italy) and recorded as casual along a major route [3]. The species was recorded in Piedmont region (NW-Italy) on a riverbank and under a railway bridge [4]. In 2018, A. palmeri was found in a soybean field in the Veneto region (NE-Italy) [5]. In 2020, the species was scouted for the first time in Lombardy, in a harvested corn field [6]
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