International Survey of Herbicide-Resistant Weeds

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(Amaranthus spinosus)

Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)

United States, Mississippi
Spiny Amaranth (Amaranthus spinosus) is a dicot weed in the Amaranthaceae family.  In Mississippi this weed first evolved resistance to Group 2 (Legacy B) herbicides in 2013 and infests Cotton, and Soybean.   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, nicosulfuron, 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.

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QUIK STATS (last updated Jun 28, 2017 )

Common NameSpiny Amaranth
SpeciesAmaranthus spinosus
GroupInhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)
Herbicidesimazethapyr, nicosulfuron, pyrithiobac-sodium, and trifloxysulfuron-Na
LocationUnited States, Mississippi
Situation(s)Cotton, and Soybean
Contributors - (Alphabetically)Vijay Nandula 
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Vijay Nandula

Weed Science 2016 64:240–247

Transfer and Expression of ALS Inhibitor Resistance from Palmer Amaranth (Amaranthus palmeri) to an A. spinosus 6 A. palmeri Hybrid

William T. Molin, Vijay K. Nandula, Alice A. Wright, and Jason A. Bond

Transfer of herbicide resistance among closely related weed species is a topic of growing concern. A spiny amaranth 6 Palmer amaranth hybrid was confirmed resistant to several acetolactate synthase (ALS) inhibitors including imazethapyr, nicosulfuron, pyrithiobac, and trifloxysulfuron. Enzyme assays indicated that the ALS enzyme was insensitive to pyrithiobac and sequencing revealed the presence of a known resistance conferring point mutation, Trp574Leu. Alignment of the ALS gene for Palmer amaranth, spiny amaranth, and putative hybrids revealed the presence of Palmer amaranth ALS sequence in the hybrids rather than spiny amaranth ALS sequences. In addition, sequence upstream of the ALS in the hybrids matched Palmer amaranth and not spiny amaranth. The potential for transfer of ALS inhibitor resistance by hybridization has been demonstrated in the greenhouse and in field experiments. This is the first report of gene transfer for ALS inhibitor resistance documented to occur in the field without artificial/human intervention. These results highlight the need to control related species in both field and surrounding noncrop areas to avoid interspecific transfer of resistance genes.

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Confirmation Tests

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

Genetic studies on HRAC Group 2 resistant Spiny Amaranth 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 Spiny Amaranth 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 Spiny Amaranth from Mississippi please update the database.
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Research Plant Physiologist
Crop Production Systems Research Unit
141 Experiment Station Road
Stoneville, 38776, Mississippi
United States
Email Vijay Nandula
Web   : Web Site Link


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.
Herbicide Resistant Spiny Amaranth Globally
(Amaranthus spinosus)
Herbicide Resistant Spiny Amaranth Globally
(Amaranthus spinosus)
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Herbicide Resistant Spiny Amaranth Globally
(Amaranthus spinosus)
#CountryFirstYearSituationActive IngredientsSite of Action
1 United States (Mississippi) United StatesMississippi2012 Cotton glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
278Amaranthus spinosusSpiny Amaranth5682
2 United States (Mississippi) United StatesMississippi2013 Cotton, and Soybean imazethapyr, nicosulfuron, pyrithiobac-sodium, and trifloxysulfuron-Na 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
278Amaranthus spinosusSpiny Amaranth16081
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