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Background: Amaranthus palmeri is an aggressive annual weed native to the United States, which has become invasive in some European countries. Populations resistant to acetolactate synthase (ALS) inhibitors have been recorded in Spain and Italy, but the evolutionary origin of the resistance traits remains unknown. Bioassays were conducted to identify cross-resistance to ALS inhibitors and a haplotype-based genetic approach was used to elucidate the origin and distribution of resistance in both countries., Results: Amaranthus palmeri populations were resistant to thifensulfuron-methyl and imazamox, and the 574-Leu mutant ALS allele was found to be the main cause of resistance among them. In two Spanish populations, 376-Glu and 197-Thr mutant ALS alleles were also found. The haplotype analyses revealed the presence of two and four distinct 574-Leu mutant haplotypes in the Italian and Spanish populations, respectively. None was common to both countries, but some mutant haplotypes were shared between geographically close populations or between populations more than 100 km apart. Wide genetic diversity was found in two very close Spanish populations., Conclusion: ALS-resistant A. palmeri populations were introduced to Italy and Spain from outside Europe. Populations from both countries have different evolutionary histories and originate from independent introduction events. ALS resistance then spread over short and long distances by seed dispersal. The higher number and genetic diversity among mutant haplotypes from the Spanish populations indicated recurrent invasions. The implementation of control tactics to limit seed dispersal and the establishment of A. palmeri is recommended in both countries. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., (© 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.)

Background: Before 2010, Amaranthus tuberculatus (Moq.) J. D. Sauer was barely known to farmers and stakeholders in Italy. Since then, several populations resistant to acetolactate synthase (ALS)-inhibiting herbicides have been collected. In most populations, a known target site resistance-endowing mutation was found, a Trp to Leu substitution at position 574 of the ALS gene, but it was unclear whether they had evolved resistance independently or not. The aims of the work were (i) to elucidate the population structure of Italian ALS-resistant A. tuberculatus populations, and (ii) to analyze the ALS haplotypes of the various populations to determine whether resistance arose multiple times independently., Results: In order to determine the population structure of eight A. tuberculatus populations, eight previously described microsatellite loci were used. Two ancestors were found: three populations derived from one, and five from the other. In the 4-kb ALS region of the genome, including the 2-kb coding region, 389 single nucleotide polymorphisms were found. In silico haplotype estimation was used to reconstruct the sequence of three distinct haplotypes carrying the Trp574Leu mutation. In addition, no mutation was found in 83% of plants of a single population., Conclusions: (i) Resistance must have arisen independently at least three times; (ii) at least one population was already resistant to ALS inhibitors when introduced in Italy; (iii) a single haplotype with a Trp574Leu mutation was shared among six populations, probably because of broad seed dispersal; and (iv) one population likely evolved nontarget site ALS inhibitors resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., (© 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.)

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 (i) to create a simplified botanical key to identify weedy amaranths; (ii) to determine the number and type of sites of action the accession are resistant to, i.e. resistance pattern; and (iii) 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 Asp 376 Glu 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 thifensulfuron-methyl and imazamox; most ALS-resistant plants had a point mutation at position 574. One A. hybridus accession had the substitution Trp 574 Met, 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. © 2019 Society of Chemical Industry., (© 2019 Society of Chemical Industry.)

Background: The repeated use of acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides to control grass weeds has selected for resistance in Lolium spp. populations in Italy. The efficacy of pinoxaden, a recently marketed phenylpyrazoline herbicide, is of concern where resistance to ACCase inhibitors has already been ascertained. ACCase mutations associated with pinoxaden resistance were investigated, and the cross-resistance pattern to clodinafop, haloxyfop, sethoxydim, clethodim and pinoxaden was established on homo/heterozygous plants for four mutant ACCase alleles., Results: Seven different mutant ACCase alleles (1781-Leu, 1999-Leu, 2041-Asn, 2041-Val, 2078-Gly, 2088-Arg and 2096-Ala) and 13 combinations with two types of mutation were detected in the pinoxaden-resistant plants. The 1781-Leu allele appears to confer a dominant resistance to pinoxaden, clodinafop, haloxyfop, sethoxydim and clethodim at 60 g AI ha(-1) . The 2041-Asn and 2041-Val alleles are associated with dominant or partially dominant resistance to FOPs, no substantial resistance to DIMs and a moderate resistance to pinoxaden. The 2088-Arg allele endows a partially dominant resistance to clodinafop, sethoxydim and most likely to pinoxaden. In addition, non-target-site resistance mechanisms seem to be involved in pinoxaden resistance., Conclusion: Almost all the ACCase mutations selected in the field by other ACCase inhibitors are likely to confer resistance to pinoxaden. Although pinoxaden is sometimes able to control FOP-resistant populations, it should not be considered as a sustainable ACCase resistance management tool. The presence of non-ACCase-based resistance mechanisms that could confer resistance to herbicides with different modes of action further complicates the resistance management strategies., (Copyright © 2011 Society of Chemical Industry.)

Background: The polyploid weed Schoenoplectus mucronatus (L.) Palla has evolved target-site resistance to ALS-inhibiting herbicides in Italian rice crops. Molecular and genetic characterisation of the resistance mechanism is relevant to the evolution and management of herbicide resistance. The authors aimed (a) to study the organisation of the target-site loci in two field-selected S. mucronatus populations with different cross-resistance patterns, (b) to identify the mutations endowing resistance to ALS inhibitors and determine the role of these mutations by using transgenesis and (c) to analyse the implications for the management of the S. mucronatus populations., Results: Two complete ALS genes (ALS1 and ALS2) having an intron and a third partial intronless ALS gene (ALS3) were identified. The presence of multiple ALS genes was confirmed by Southern blot analyses, and ALS loci were characterised by examining cytosine methylation. In S. mucronatus leaves, the transcripts of ALS1, ALS2 and ALS3 were detected. Two mutations endowing resistance (Pro(197) to His and Trp(574) to Leu) were found in both resistant populations, but at different frequencies. Tobacco plants transformed with the two resistant alleles indicated that the Pro(197)-to-His substitution conferred resistance to SU and TP herbicides, while the allele with the Trp(574)-to-Leu substitution conferred cross-resistance to SU, TP, IMI and PTB herbicides., Conclusion: Schoenoplectus mucronatus has multiple ALS genes characterised by methylated sites that can influence the expression profile. The two mutated alleles proved to be responsible for ALS resistance. At population level, the resistance pattern depends on the frequency of various resistant genotypes, and this influences the efficacy of various ALS-inhibiting herbicides.