<b>HRAC Group 2</b> <font size='2'> (Legacy B) </font> resistant Amaranthus retroflexus from United States, Mississippi

International Survey of Herbicide-Resistant Weeds

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Tuesday, November 4, 2025

HRAC GROUP 2 (LEGACY B) RESISTANT REDROOT PIGWEED
(Amaranthus retroflexus)

Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)

United States, Mississippi

INTRODUCTION		REDROOT PIGWEED
Redroot Pigweed (Amaranthus retroflexus) is a dicot weed in the Amaranthaceae family. In Mississippi this weed first evolved resistance to Group 2 (Legacy B) herbicides in 2010 and infests Railways. Group 2 (Legacy B) herbicides are known as Inhibition of Acetolactate Synthase (Inhibition of Acetolactate Synthase ). Research has shown that these particular biotypes are resistant to imazethapyr, pyrithiobac-sodium, and trifloxysulfuron-Na and they may be cross-resistant to other Group 2 (Legacy B) herbicides. The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

QUIK STATS (last updated Jun 16, 2017 )
Common Name	Redroot Pigweed
Species	Amaranthus retroflexus
Group	Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)
Herbicides	imazethapyr, pyrithiobac-sodium, and trifloxysulfuron-Na
Location	United States, Mississippi
Year	2010
Situation(s)	Railways
Contributors - (Alphabetically)	Vijay Nandula
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NOTES ABOUT THIS BIOTYPE

MECHANISM

Vijay Nandula

DNA sequencing showed the ALS mutation of Trp574Leu.

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ACADEMIC ASPECTS

Confirmation Tests

Greenhouse trials comparing a known susceptible Redroot Pigweed biotype with this Redroot Pigweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on HRAC Group 2 resistant Redroot Pigweed have not been reported to the site. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

Studies on the mechanism of resistance of Group 2 (Legacy B) resistant Redroot Pigweed from Mississippi indicate that resistance is due to an altered target site. There may be a note below or an article discussing the mechanism of resistance in the Fact Sheets and Other Literature

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of Group 2 (Legacy B) resistant Redroot Pigweed from Mississippi please update the database.

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CONTRIBUTING WEED SCIENTISTS

VIJAY NANDULA

Research Plant Physiologist USDA-ARS Crop Production Systems Research Unit 141 Experiment Station Road Stoneville, 38776, Mississippi United States Email Vijay Nandula Web : Web Site Link

ACKNOWLEDGEMENTS
The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Mississippi have been instrumental in providing you this information. Particular thanks is given to Vijay Nandula for providing detailed information.

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Herbicide Resistant Redroot Pigweed Globally
(Amaranthus retroflexus)

Country

FirstYear

Situation

Active Ingredients

Site of Action

Brazil

2011

Cotton

atrazine, prometryne, and trifloxysulfuron-Na

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Brazil

2012

Cotton

pyrithiobac-sodium, and trifloxysulfuron-Na

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Brazil

2014

Cotton, and Soybean

fomesafen

Inhibition of Protoporphyrinogen Oxidase ( HRAC Group 14 (Legacy E)

Bulgaria

1984

Corn (maize), Cropland, Grapes, and Orchards

atrazine, metribuzin, and simazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Bulgaria

1984

Cropland

linuron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Canada (Ontario)

1980

Corn (maize), and Cropland

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Canada (Quebec)

1980

Corn (maize)

atrazine, and metribuzin

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Canada (Ontario)

1998

Corn (maize), Cropland, and Soybean

imazethapyr, and thifensulfuron-methyl

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Canada (Ontario)

2001

Carrots, and Vegetables

linuron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Canada (Quebec)

2001

Carrots

linuron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Canada (Manitoba)

2002

Wheat

florasulam

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Canada (Ontario)

2005

Corn (maize)

bentazon, and bromoxynil

PSII inhibitors - Histidine 215 Binders ( HRAC Group 6 (Legacy C3)

Canada (Quebec)

2009

Soybean

imazethapyr

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Canada (Saskatchewan)

2010

Wheat

thifensulfuron-methyl, and tribenuron-methyl

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Canada (Prince Edward Island)

2023

Potatoes

linuron, and metribuzin

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

China

1990

Cropland

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

China

2017

Soybean

acifluorfen, fluoroglycofen-ethyl, fomesafen, imazethapyr, and lactofen

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Protoporphyrinogen Oxidase ( HRAC Group 14 (Legacy E)

Czech Republic

1985

Corn (maize), Railways, Roadsides, and Sugar beets

atrazine, cyanazine, metamitron, prometryne, terbuthylazine, and terbutryne

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

France

1980

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Germany

1980

Corn (maize), Railways, and Roadsides

atrazine, and fenuron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Germany

2012

Corn (maize)

nicosulfuron

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Greece

2000

Potatoes

metribuzin

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Israel

1991

Forests

chlorsulfuron

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Italy

1999

Corn (maize), Soybean, and Sugar beets

chloridazon/pyrazon, metamitron, and terbuthylazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Italy

2003

Soybean

imazamox, imazethapyr, nicosulfuron, oxasulfuron, and thifensulfuron-methyl

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Poland

1991

Orchards, and Sugar beets

atrazine, and metamitron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Serbia

2002

Soybean

imazethapyr

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

Spain

1986

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Switzerland

1982

Grapes, Orchards, and Vegetables

simazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

Ukraine

2020

Corn (maize), and Sunflower

florasulam, flumetsulam, foramsulfuron, imazamox, imazethapyr, iodosulfuron-methyl-Na, thiencarbazone-methyl, thifensulfuron-methyl, and tribenuron-methyl

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

United States (Indiana)

1980

Cropland

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Colorado)

1982

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Connecticut)

1984

Corn (maize)

atrazine, and cyanazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (New Hampshire)

1984

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Maine)

1985

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Minnesota)

1991

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Vermont)

1991

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Virginia)

1993

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Oregon)

1994

Mint

terbacil

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Arkansas)

1995

Cropland, and Soybean

imazaquin

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

United States (Kansas)

1995

Corn (maize), and Sorghum

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Maryland)

1998

Unspecified

imazaquin, and imazethapyr

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

United States (Pennsylvania)

1998

Corn (maize), Soybean, and Tomatoes

atrazine, chlorimuron-ethyl, cloransulam-methyl, imazamox, imazaquin, imazethapyr, primisulfuron-methyl, and thifensulfuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (North Dakota)

1999

Soybean

imazethapyr

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

United States (West Virginia)

2000

Corn (maize)

atrazine

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Michigan)

2001

Asparagus

atrazine, and diuron

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Idaho)

2005

Potatoes

metribuzin

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Washington)

2010

Mint, and Potatoes

metribuzin, and terbacil

PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)

United States (Mississippi)

2010

Railways

imazethapyr, pyrithiobac-sodium, and trifloxysulfuron-Na

Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)

United States (North Carolina)

2020

Soybean

fomesafen, imazamethabenz-methyl, lactofen, and thifensulfuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Protoporphyrinogen Oxidase ( HRAC Group 14 (Legacy E)

United States (North Carolina)

2022

Soybean

glyphosate

Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)

United States (North Carolina)

2022

Soybean

fomesafen, imazamethabenz-methyl, lactofen, mesotrione, and thifensulfuron-methyl

Multiple Resistance: 3 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Protoporphyrinogen Oxidase ( HRAC Group 14 (Legacy E)
Inhibition of Hydroxyphenyl Pyruvate Dioxygenase ( HRAC Group 27 (Legacy F2)

Literature about Similar Cases

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Schwartau, V. V., Mykhalska, L. M. . 2022. Herbicide-resistant weed biotypes in Ukraine. Reports of the National Academy of Sciences of Ukraine : 85 - 94.

In Ukraine, resistant to the action of herbicides — acetolactate synthase inhibitors of the imidazolinone class — imazapyr and imazamox biotypes of monocot Echinochloa crus-galli var. crus-galli, and dicot Amaranthus retroflexus, and Chenopodium album have been identified. Cross-resistance to herbicides: monocot to sulfonylurea (nicosulfuron), and triazole pyrimidines (penoxsulam); dicots to sulfonylurea foramsulfuron, iodosulfuron-methyl-sodium, thifensulfuron-methyl, tribenuron-methyl; to sulfonylaminocarbonyl triazolinone derivative — thiencarbazone-methyl; to t riazole pyrimidine derivatives — florasulam, and flumetsulam were established. The multiple resistance of Echinochloa crus-galli, Amaranthus retroflexus, and Chenopodium album to herbicides of the classes of glycine derivatives — glyphosate, monocot to graminicide pinoxaden, and dicot species to phenoxy carboxylic derivatives — 2,4-D, benzoic acid — dicamba; triketones — topramezone; diphenyl ethers — aclonifen have not been established. It was shown for the first time that herbicide compositions under the influence of the ammonium pool can increase the level of controlling resistant weed biotypes effectiveness. The identification of highly harmful ALS-resistant Echinochloa crus-galli, Amaranthus retroflexus, and Chenopodium album in the south and central part of the “grain belt” of Ukraine shows the limited effectiveness of weed control exclusively with herbicides with one mechanism of action and requires a significant revision of the principles of crop rotation formation and methods of weed control in the state to maintain high levels of profitability and productivity of agrophytocenoses. Solving these issues is urgent in order to preserve Ukraine’s potential as one of the guarantors of world food security..

Mykhalska, L. M., Schwartau, V. V.. 2022. Identification of acetolactate synthase resistant Amaranthus retroflexus in Ukraine. Regulatory Mechanisms in Biosystems 13 : 231 - 240.

The problem of weed resistance to herbicides has become very important in the last decade and threatens to dramatically reduce the productivity and profitability of modern crop production. Herbicides – ALS inhibitors dominate among current herbicides and are used annually on large areas of sunflower, wheat, corn, soybean, and rapeseed. Also, in recent years, Clearfield seeds of sunflower, corn, canola, soybean and wheat have been sown in large areas. In recent years, there has been a sharp decrease in Amaranthus retroflexus L. control levels by imidazolinone class herbicides. Thus, the effects of herbicides with different modes of action on the development of A. retroflexus on sunflower after imidazolinone application were investigated in field research. In the conditions of the Cherkasy region of Ukraine, the biotype A. retroflexus was identified, which is resistant to the post-emergence application of herbicides – acetolactate synthase
(ALS) inhibitors of the imidazolinone class – imazapyr and imazamox. Weed plants treated with imidazolinone derivatives in the maximum doses registered in Ukraine did not differ from untreated control plants. Also, in the conditions of field experiments, cross resistance of the weed biotype to herbicides – ALS inhibitors of the sulfonylurea class – foramsulfuron and iodosulfuron-methyl-sodium, thifensulfuron-methyl, tribenuron-methyl, nicosulfuron was established; and also, to the triazolinone derivative – thiencarbazone-methyl; to triazolpyrimidine derivatives – florasulam and flumetsulam. Multiple resistance of the A. retroflexus biotype to herbicides of the classes of glycine derivatives – glyphosate, phenoxycarboxylates – 2,4-D, benzoic acid – dicamba has not been established; compositions of dicamba with triketone – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates – clopyralid, picloram and aminopyralid. It was shown for the first time that herbicide compositions with selected nutrients (ammonium pool) can increase the level of effectiveness of controlling resistant weed biotypes. Thus, the addition of ammonium sulfate increases the effectiveness of controlling ALS-resistant A. retroflexus with herbicides – a derivative of benzoic acid (dianate) and a derivative of benzoic acid with a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor (stellar – dicamba + topramesone). Thus, the A. retroflexus biotype resistant to ALS-herbicides of the imidazolinone class was identified for the first time in Ukraine, which is cross-resistant to other ALS-inhibitors of the sulfonylureas, triazolinones, and triazolpyrimidine classes. Multiple resistance of A. retroflexus to herbicides of the classes of glycine derivatives – glyphosate; phenoxycarboxylates – 2,4-D; benzoic acid – dicamba, triketones – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates
– clopyralid, picloram and aminopyralid has not been established. The identification of a highly harmful weed species resistant to widely used herbicides – ALS inhibitors in the central part of the "grain belt" of Ukraine requires a significant revision of the principles of crop rotation formation and ways of controlling weeds in the country in order to maintain high levels of profitability and productivity of agrophytocenoses..

Huang, Z., J. Chen, C. Zhang, H. Huang, S. Wei, X. Zhou, J. Chen, and X. Wang. 2016. Target-site basis for resistance to imazethapyr in redroot amaranth (Amaranthus retroflexus L.).. Pesticide Biochemistry and Physiology 128 : 10 - 15.

Experiments were conducted to confirm imazethapyr resistance in redroot amaranth (Amaranthus retroflexus L.) and study the target-site based mechanism for the resistance. Whole-plant response experiments revealed that the resistant (R) population exhibited 19.16 fold resistance to imazethapyr compared with the susceptible (S) population. In vitro ALS activity assay demonstrated that the imazethapyr I₅₀ value of the R population was 21.33 times greater than that of the S population. However, qRT-PCR analysis revealed that there is no difference in ALS gene expression between the R and S populations. Sequence analysis revealed an Asp-376-Glu substitution in ALS in the R population. In order to verify that the imazethapyr resistance was conferred by Asp-376-Glu mutation, the ALS-R and ALS-S genes were fused to the CaMV 35S promoter and introduced into Arabidopsis respectively. The expression of ALS-R in transgenic Arabidopsis plants exhibited 13.79 fold resistance to imazethapyr compared to ALS-S transgenic Arabidopsis..

Chen, J., Z. Huang, C. Zhang, H. Huang, S. Wei, J. Chen, and X. Wang. 2015. Molecular basis of resistance to imazethapyr in redroot pigweed (Amaranthus retroflexus L.) populations from China. . Pesticide Biochemistry and Physiology 124 : 43 - 47.

Three putative resistant Amaranthus retroflexus L. populations were collected in Heilongjiang province in China. Whole plant bioassays indicated high resistance (RI > 10) to imazethapyr in the three populations. In vitro acetolactate synthase (ALS) assays revealed that ALS from populations H3, H17 and H39 was less sensitive to imazethapyr inhibition compared to the susceptible population H76. The half-maximal inhibitory concentration (I₅₀) values for H3, H17 and H39 were 14.83, 15.27 and 268 times greater, respectively, than that of the susceptible population H76. Three nucleotide mutations resulted in three known resistance-endowing amino acid substitutions, Ala-205-Val, Trp-574-Leu and Ser-653-Thr in the three resistant populations respectively. Therefore, ALS target-site mutations in resistant A. retroflexus could be responsible for imazethapyr resistance..

Krga, I. ; Pavlovic´, D. ; Anđelkovic´, A. ; Đurovic´, S. ; Marisavljevic´, D.. 2013. The resistance of orchard weed populations to glyphosate. Zaštita Bilja 64 : 125 - 133.

Nowadays, both worldwide and in Serbia, for weed eradication in orchards mostly herbicides based on glyphosate, glufosinate-ammonium, diquat and others are used. Intensive glyphosate application has led to the development of resistant weed species, which has consequently resulted in a decrease in its effectiveness. In our country, areas under orchards amount to 224.000 hectares, which certainly points to a significant herbicide use and a possibility that weed resistant populations have developed. For this reason, seeds of several weed species from areas where glyphosate has been intensively used for years were collected (localities: Indjija, Brestovac, Šabac, Vršac, Sombor, Glogonjski Rit, Padinska Skela and Surčin). Plants were grown in controlled conditions and in the open field. Plant material was then crushed using liquid nitrogen, and the extraction of shikimic acid was performed using hydrochloric acid (1 g of plant material+5 ml 1M HCl). 24 hours later the amount of shikimic acid was detected using high-pressure liquid chromatography. The analysis of the obtained results showed that species Amaranthus retroflexus (loc. Šabac), Abutilon teophrasti (loc. Brestovac) and wild Helianthus annuus (loc. G. Rit) have developed a certain degree of glyphosate resistance..

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