INTERNATIONAL HERBICIDE-RESISTANT WEED DATABASE

MULTIPLE RESISTANT HORSEWEED
(Conyza canadensis)


Multiple Resistance: 2 Sites of Action
PS I Electron Diversion HRAC Group 22 (Legacy D)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase HRAC Group 9 (Legacy G)

United States, California
INTRODUCTION HORSEWEED
Horseweed (Conyza canadensis) is a dicot weed in the Asteraceae family.  In California this weed first evolved multiple resistance (to 2 herbicide sites of action) in 2014 and infests Almonds.  Multiple resistance has evolved to herbicides in the Groups 22 (Legacy D), and Inhibition of Enolpyruvyl Shikimate Phosphate Synthase HRAC Group 9 (Legacy G).  These particular biotypes are known to have resistance to glyphosate, and paraquat and they may be cross-resistant to other herbicides in the Groups 22 (Legacy D), and Inhibition of Enolpyruvyl Shikimate Phosphate Synthase HRAC Group 9 (Legacy G).

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 Dec 17, 2014 )

Common NameHorseweed
SpeciesConyza canadensis
GroupPS I Electron Diversion HRAC Group 22 (Legacy D)

Inhibition of Enolpyruvyl Shikimate Phosphate Synthase HRAC Group 9 (Legacy G)
Herbicidesglyphosate, and paraquat
LocationUnited States, California
Year2014
Situation(s)Almonds
Contributors - (Alphabetically)Brad Hanson, and Marcelo L. Moretti 
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ACADEMIC ASPECTS

Confirmation Tests

Greenhouse, and Laboratory trials comparing a known susceptible Horseweed biotype with this Horseweed 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 22, 9 resistant Horseweed 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

The mechanism of resistance for this biotype is either unknown or has not been entered in the database.  If you know anything about the mechanism of resistance for this biotype then please update the database.
 
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 multiple resistant Horseweed from California please update the database.
 
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CONTRIBUTING WEED SCIENTISTS

BRAD HANSON
Cooperative Extension Weed Specialist
University Of California, Davis
Department Of Plant Sciences
MS-4, One Shields Ave
276 Robbins Hall
Davis, 91616, California
United States
Email Brad Hanson
Web   : Web Site Link
MARCELO L. MORETTI
Assistant Professor
Oregon State University
Horticulture
4017 ALS 2750 SW Campus Way
Corvallis, 97331, Oregon
United States
Email Marcelo L. Moretti
Web   : Web Site Link

ACKNOWLEDGEMENTS

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in California have been instrumental in providing you this information. Particular thanks is given to Brad Hanson, and Marcelo L. Moretti for providing detailed information.
Herbicide Resistant Horseweed Globally
(Conyza canadensis)
Herbicide Resistant Horseweed Globally
(Conyza canadensis)
Drag a column header and drop it here to group by that column
Herbicide Resistant Horseweed Globally
(Conyza canadensis)
#CountryFirstYearSituationActive IngredientsSite of Action
1 BelgiumBelgium 1989 Corn (maize), Nurseries, and Roadsides atrazine 3 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed35
2 BelgiumBelgium 1998 Nurseries paraquat 3 PS I Electron Diversion ( HRAC Group 22 (Legacy D)
61Conyza canadensisHorseweed1145
3 BrazilBrazil 2005 Fruit, Orchards, and Soybean glyphosate 5 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5273
4 Canada (Ontario) CanadaOntario1993 Peaches paraquat 7 PS I Electron Diversion ( HRAC Group 22 (Legacy D)
61Conyza canadensisHorseweed48
5 Canada (Ontario) CanadaOntario2010 Soybean glyphosate 7 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5665
6 Canada (Ontario) CanadaOntario2011 Soybean cloransulam-methyl, and glyphosate 7 Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5666
7 ChinaChina 2006 Orchards glyphosate 9 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5277
8 Czech RepublicCzech Republic 1987 Corn (maize), Nurseries, Orchards, Pastures, Railways, and Roadsides atrazine 12 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed83
9 Czech RepublicCzech Republic 2007 Railways glyphosate 12 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5317
10 FranceFrance 1981 Cropland atrazine 17 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed460
11 FranceFrance 1988 Orchards linuron 17 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed451
12 FranceFrance 2019 Grapes glyphosate 17 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed18198
13 GreeceGreece 2012 Grapes, and Orchards glyphosate 19 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5692
14 HungaryHungary 2016 Grapes glyphosate 20 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed15070
15 IsraelIsrael 1993 Forests, Industrial sites, and Roadsides atrazine, chlorsulfuron, imazapyr, metribuzin, pyrithiobac-sodium, and sulfometuron-methyl 23 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)
61Conyza canadensisHorseweed109
16 ItalyItaly 2011 Orchards glyphosate 24 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5617
17 JapanJapan 1980 Grapes, Orchards, Railways, and Roadsides paraquat 25 PS I Electron Diversion ( HRAC Group 22 (Legacy D)
61Conyza canadensisHorseweed119
18 JapanJapan 2014 Railways glyphosate 25 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed10999
19 PolandPoland 1983 Orchards simazine 33 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed168
20 PolandPoland 2000 Railways imazapyr 33 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed5081
21 PolandPoland 2010 Railways glyphosate 33 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5622
22 PortugalPortugal 2011 Olive glyphosate 34 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed7824
23 South KoreaSouth Korea 2017 Orchards glyphosate 27 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed16077
24 SpainSpain 1987 Corn (maize), Orchards, and Roadsides atrazine, and simazine 38 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed177
25 SpainSpain 2006 Orchards glyphosate 38 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5319
26 SwitzerlandSwitzerland 1982 Corn (maize), Nurseries, Orchards, Pastures, Railways, and Roadsides atrazine, and simazine 41 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed203
27 SwitzerlandSwitzerland 2021 Grapes glyphosate 41 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed24248
28 United KingdomUnited Kingdom 1982 Orchards simazine 44 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed362
29 United States (Mississippi) United StatesMississippi1994 Soybean paraquat 45 PS I Electron Diversion ( HRAC Group 22 (Legacy D)
61Conyza canadensisHorseweed1109
30 United States (Delaware) United StatesDelaware2000 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5086
31 United States (Tennessee) United StatesTennessee2001 Cotton, and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5122
32 United States (Ohio) United StatesOhio2001 Soybean chlorimuron-ethyl, and cloransulam-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed5157
33 United States (Kentucky) United StatesKentucky2001 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5208
34 United States (Ohio) United StatesOhio2002 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5127
35 United States (Indiana) United StatesIndiana2002 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5128
36 United States (Maryland) United StatesMaryland2002 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5129
37 United States (New Jersey) United StatesNew Jersey2002 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5130
38 United States (Michigan) United StatesMichigan2002 Corn (maize), and Soybean chlorimuron-ethyl, and cloransulam-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed5172
39 United States (Michigan) United StatesMichigan2002 Blueberries atrazine, diuron, and simazine 45 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed5173
40 United States (Michigan) United StatesMichigan2002 Blueberries diuron 45 PSII inhibitors - Serine 264 Binders ( HRAC Group 5 (Legacy C1 C2)
61Conyza canadensisHorseweed5174
41 United States (Missouri) United StatesMissouri2002 Corn (maize), Cotton, and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5260
42 United States (Ohio) United StatesOhio2003 Soybean chlorimuron-ethyl, cloransulam-methyl, and glyphosate 45 Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5158
43 United States (Arkansas) United StatesArkansas2003 Cotton glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5187
44 United States (Mississippi) United StatesMississippi2003 Corn (maize), Cotton, Rice, and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5195
45 United States (North Carolina) United StatesNorth Carolina2003 Cotton glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5200
46 United States (Delaware) United StatesDelaware2003 Soybean paraquat 45 PS I Electron Diversion ( HRAC Group 22 (Legacy D)
61Conyza canadensisHorseweed5257
47 United States (Pennsylvania) United StatesPennsylvania2003 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5258
48 United States (California) United StatesCalifornia2005 Corn (maize), Fencelines, Grapes, Orchards, Roadsides, and Wheat glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5250
49 United States (Illinois) United StatesIllinois2005 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5276
50 United States (Kansas) United StatesKansas2005 Cotton, and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5308
51 United States (Virginia) United StatesVirginia2005 Corn (maize), and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5559
52 United States (Indiana) United StatesIndiana2005 Soybean chlorimuron-ethyl, and cloransulam-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed7836
53 United States (Nebraska) United StatesNebraska2006 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5496
54 United States (Michigan) United StatesMichigan2007 Nurseries glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5312
55 United States (Mississippi) United StatesMississippi2007 Soybean glyphosate, and paraquat 45 Multiple Resistance: 2 Sites of Action
PS I Electron Diversion ( HRAC Group 22 (Legacy D)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5384
56 United States (West Virginia) United StatesWest Virginia2007 Corn (maize), and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed7774
57 United States (Oklahoma) United StatesOklahoma2009 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5528
58 United States (South Dakota) United StatesSouth Dakota2010 Corn (maize), and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5634
59 United States (Delaware) United StatesDelaware2010 Soybean, and Wheat glyphosate, thifensulfuron-methyl, and tribenuron-methyl 45 Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5710
60 United States (Iowa) United StatesIowa2011 Corn (maize), and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed5567
61 United States (Kansas) United StatesKansas2011 Corn (maize), Cotton, Soybean, and Wheat chlorsulfuron, iodosulfuron-methyl-Na, metsulfuron-methyl, rimsulfuron, thiencarbazone-methyl, thifensulfuron-methyl, and tribenuron-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed7806
62 United States (Alabama) United StatesAlabama2013 Corn (maize), Cotton, and Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed7783
63 United States (Wisconsin) United StatesWisconsin2013 Soybean glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed7823
64 United States (Illinois) United StatesIllinois2013 Soybean cloransulam-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed7865
65 United States (California) United StatesCalifornia2014 Almonds glyphosate, and paraquat 45 Multiple Resistance: 2 Sites of Action
PS I Electron Diversion ( HRAC Group 22 (Legacy D)
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed9943
66 United States (Montana) United StatesMontana2015 Roadsides glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed11011
67 United States (Oklahoma) United StatesOklahoma2018 Cotton, Soybean, and Wheat chlorsulfuron, and metsulfuron-methyl 45 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B)
61Conyza canadensisHorseweed17149
68 United States (Oregon) United StatesOregon2023 Fallow, and Wheat glyphosate 45 Inhibition of Enolpyruvyl Shikimate Phosphate Synthase ( HRAC Group 9 (Legacy G)
61Conyza canadensisHorseweed27286
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