(Panicum dichotomiflorum)

Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)

United States, Illinois
Fall Panicum (Panicum dichotomiflorum) is a monocot weed in the Poaceae family.  In Illinois this weed first evolved resistance to Group 2 (Legacy B) herbicides in 2019 and infests Sweet corn.   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 nicosulfuron 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.

If we do not have a picture for this weed, or you can provide a better picture (scan, digital camera etc) please please e-mail it to us.

QUIK STATS (last updated Oct 24, 2022 )

Common NameFall Panicum
SpeciesPanicum dichotomiflorum
GroupInhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)
LocationUnited States, Illinois
Situation(s)Sweet corn
Contributors - (Alphabetically)Patrick Tranel 
Edit this Case  |  Add New Case of Resistance  |  Add Note



Patrick Tranel

Nunes JJ, Raiyemo DA, Arneson NJ, Rosa AT, Tranel PJ, Werle R (2022) Target site resistance to acetolactate synthase inhibitors in a fall panicum (Panicum dichotomiflorum Michx.) accession from Wisconsin and its response to alternative herbicides. Weed Technol. 36: 48–55. doi: 10.1017/wet.2021.104


 Owing to the lack of effective POST herbicide options, producers typically rely on nicosulfuron as the main POST grass herbicide in sweet corn production systems. In 2019, a Wisconsin sweet corn producer reported fall panicum control escapes after spraying nicosulfuron. Seeds from mature plants were collected to (1) measure fall panicum response to acetolactate synthase (ALS)-inhibiting herbicides, (2) elucidate the resistance mechanism, and (3) evaluate its response to alternative POST herbicides. Greenhouse and laboratory investigations were conducted to assess fall panicum response to ALS-inhibiting herbicides and elucidate the resistance mechanism. Dose–response results showed that fall panicum was highly resistant to nicosulfuron with a resistance ratio of >12.9-fold (survived rates >254 g ai ha−1, or 8× the field label rate). Molecular and genetic studies indicated that there are multiple ALS gene copies in fall panicum and that resistance was due to a mutation in one copy, resulting in an Asp-376Glu amino acid substitution. Additional greenhouse experiments indicate that clethodim (105 g ai ha−1), quizalofop-p-ethyl (70 g ae ha−1), glyphosate (864 g ae ha−1), and glufosinate (650 g ai ha−1) are effective POST options to manage the ALS-resistant fall panicum (>90.0% control and 96.8% biomass reduction) in rotational years. The 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides isoxaflutole (105 g ai ha−1), mesotrione (105 g ai ha−1), tembotrione (92 g ai ha−1), and tolpyralate (39 g ai ha−1) did not provide effective POST fall panicum control. Because these herbicides are commonly used for POST weed control in sweet corn, more investigations are required to evaluate combinations of HPPD-inhibiting herbicides with herbicides from other sites of action for POST fall panicum control. Herein we confirm the first case of herbicide resistance in fall panicum in the United States.

Edit this Note  |  Add New Note


Confirmation Tests

Field, and Laboratory trials comparing a known susceptible Fall Panicum biotype with this Fall Panicum 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 Fall Panicum indicate that the inheritance is determined by a one gene, nuclear trait.  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 Fall Panicum from Illinois 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 Fall Panicum from Illinois please update the database.
Edit Case  |  Add Note  |  Add Article  |  Add New Case  |  


University Of Illinois
Dept. Of Crop Sciences
1201 W. Gregory Dr.
Urbana, 61801, Illinois
United States
Email Patrick Tranel
Web   : Web Site Link


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Illinois have been instrumental in providing you this information. Particular thanks is given to Patrick Tranel for providing detailed information.
Herbicide Resistant Fall Panicum Globally
(Panicum dichotomiflorum)
Herbicide Resistant Fall Panicum Globally
(Panicum dichotomiflorum)
Drag a column header and drop it here to group by that column
Herbicide Resistant Fall Panicum Globally
(Panicum dichotomiflorum)
#CountryFirstYearSituationActive IngredientsSite of Action
1 SpainSpain 1981 Corn (maize), and Cropland atrazine 38 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
122Panicum dichotomiflorumFall Panicum527
2 United States (Illinois) United StatesIllinois2019 Sweet corn nicosulfuron 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
122Panicum dichotomiflorumFall Panicum24246
Literature about Similar Cases

Page size:
Page: of 3
Items 1 to 5 of 13

Page size:
Page: of 3
Items 1 to 5 of 13

PERMISSION MUST BE OBTAINED FIRST if you intend to base a significant portion of a scientific paper on data derived from this site. Citation:
Heap, I.  The International Herbicide-Resistant Weed Database.  Online.  Thursday, July 18, 2024 .  Available Copyright © 1993- 2024 All rights reserved. Fair use of this material is encouraged. Proper citation is requested.