<b>HRAC Group 5</b> <font size='2'> (Legacy C1 C2) </font> resistant Conyza canadensis from United States, Michigan

International Survey of Herbicide-Resistant Weeds

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Tuesday, December 3, 2024

HRAC GROUP 5 (LEGACY C1 C2) RESISTANT HORSEWEED
(Conyza canadensis)

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

United States, Michigan

INTRODUCTION		HORSEWEED
Horseweed (Conyza canadensis) is a dicot weed in the Asteraceae family. In Michigan this weed first evolved resistance to Group 5 (Legacy C1 C2) herbicides in 2002 and infests Blueberries. Group 5 (Legacy C1 C2) herbicides are known as PSII inhibitors - Serine 264 Binders (Inhbition of Photosynthesis at PSll - Serine 264 Binders ). Research has shown that these particular biotypes are resistant to atrazine, diuron, and simazine and they may be cross-resistant to other Group 5 (Legacy C1 C2) 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 May 12, 2003 )
Common Name	Horseweed
Species	Conyza canadensis
Group	PSII inhibitors - Serine 264 Binders HRAC Group 5 (Legacy C1 C2)
Herbicides	atrazine, diuron, and simazine
Location	United States, Michigan
Year	2002
Situation(s)	Blueberries
Contributors - (Alphabetically)	Steven Gower
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NOTES ABOUT THIS BIOTYPE

GENERAL

Steven Gower

Mature horseweed seedheads were collected from four blueberry fields in southwest Michigan suspected to be resistant to triazine herbicides. Herbicide resistance was evaluated using a greenhouse plant bioassay. Horseweed plants received a PRE application of simazine at 3.96 and 11.88 lbs. ai/A (1 and 3X standard field rate, respectively) and a POST application of atrazine at 2 and 6 lbs. ai/A and diuron at 1.6 and 4.8 lbs. ai/A (1 and 3X standard field rate, respectively). Other treatments included sensitive and untreated controls. All four horseweed samples treated with a 1 and 3X rate of simazine contained a 100% survival rate and no injury/growth reduction. Considering the POST application, all horseweed samples were highly resistant to both rates of atrazine. Averaged across the four samples, 86 and 92% of the horseweed survived applications of atrazine at the 1 and 3X rate, respectively, with little to no injury/growth reduction compared with the untreated controls. Resistance to diuron at the 1X rate was detected in 2 of the 4 horseweed samples, with a 16% average survival rate and 75% average injury/growth reduction. All horseweed plants treated with diuron at the 3X rate were completely controlled. Results indicate that horseweed collected from four blueberry locations is highly resistant to triazines (atrazine and simazine) and somewhat cross-resistant to phenylureas (diuron).

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ACADEMIC ASPECTS

Confirmation Tests

Greenhouse 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 5 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 Group 5 (Legacy C1 C2) resistant Horseweed from Michigan please update the database.

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CONTRIBUTING WEED SCIENTISTS

STEVEN GOWER

Former Weed Science Diagnostician (Now with Monsanto) Michigan State University Diagnostic Services 578 Wilson Rd. East Lansing, 48824, Michigan United States Email Steven Gower

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

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Herbicide Resistant Horseweed Globally
(Conyza canadensis)

Country

FirstYear

Situation

Active Ingredients

Site of Action

Belgium

1989

Corn (maize), Nurseries, and Roadsides

atrazine

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

Belgium

1998

Nurseries

paraquat

PS I Electron Diversion ( HRAC Group 22 (Legacy D)

Brazil

2005

Fruit, Orchards, and Soybean

glyphosate

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

Canada (Ontario)

1993

Peaches

paraquat

PS I Electron Diversion ( HRAC Group 22 (Legacy D)

Canada (Ontario)

2010

Soybean

glyphosate

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

Canada (Ontario)

2011

Soybean

cloransulam-methyl, and glyphosate

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)

China

2006

Orchards

glyphosate

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

Czech Republic

1987

Corn (maize), Nurseries, Orchards, Pastures, Railways, and Roadsides

atrazine

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

Czech Republic

2007

Railways

glyphosate

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

France

1981

Cropland

atrazine

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

France

1988

Orchards

linuron

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

France

2019

Grapes

glyphosate

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

Greece

2012

Grapes, and Orchards

glyphosate

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

Hungary

2016

Grapes

glyphosate

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

Israel

1993

Forests, Industrial sites, and Roadsides

atrazine, chlorsulfuron, imazapyr, metribuzin, pyrithiobac-sodium, and sulfometuron-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)

Italy

2011

Orchards

glyphosate

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

Japan

1980

Grapes, Orchards, Railways, and Roadsides

paraquat

PS I Electron Diversion ( HRAC Group 22 (Legacy D)

Japan

2014

Railways

glyphosate

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

Poland

1983

Orchards

simazine

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

Poland

2000

Railways

imazapyr

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

Poland

2010

Railways

glyphosate

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

Portugal

2011

Olive

glyphosate

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

South Korea

2017

Orchards

glyphosate

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

Spain

1987

Corn (maize), Orchards, and Roadsides

atrazine, and simazine

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

Spain

2006

Orchards

glyphosate

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

Switzerland

1982

Corn (maize), Nurseries, Orchards, Pastures, Railways, and Roadsides

atrazine, and simazine

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

Switzerland

2021

Grapes

glyphosate

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

United Kingdom

1982

Orchards

simazine

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

United States (Mississippi)

1994

Soybean

paraquat

PS I Electron Diversion ( HRAC Group 22 (Legacy D)

United States (Delaware)

2000

Soybean

glyphosate

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

United States (Tennessee)

2001

Cotton, and Soybean

glyphosate

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

United States (Ohio)

2001

Soybean

chlorimuron-ethyl, and cloransulam-methyl

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

United States (Kentucky)

2001

Soybean

glyphosate

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

United States (Ohio)

2002

Soybean

glyphosate

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

United States (Indiana)

2002

Soybean

glyphosate

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

United States (Maryland)

2002

Soybean

glyphosate

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

United States (New Jersey)

2002

Soybean

glyphosate

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

United States (Michigan)

2002

Corn (maize), and Soybean

chlorimuron-ethyl, and cloransulam-methyl

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

United States (Michigan)

2002

Blueberries

atrazine, diuron, and simazine

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

United States (Michigan)

2002

Blueberries

diuron

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

United States (Missouri)

2002

Corn (maize), Cotton, and Soybean

glyphosate

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

United States (Ohio)

2003

Soybean

chlorimuron-ethyl, cloransulam-methyl, and glyphosate

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)

United States (Arkansas)

2003

Cotton

glyphosate

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

United States (Mississippi)

2003

Corn (maize), Cotton, Rice, and Soybean

glyphosate

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

United States (North Carolina)

2003

Cotton

glyphosate

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

United States (Delaware)

2003

Soybean

paraquat

PS I Electron Diversion ( HRAC Group 22 (Legacy D)

United States (Pennsylvania)

2003

Soybean

glyphosate

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

United States (California)

2005

Corn (maize), Fencelines, Grapes, Orchards, Roadsides, and Wheat

glyphosate

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

United States (Illinois)

2005

Soybean

glyphosate

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

United States (Kansas)

2005

Cotton, and Soybean

glyphosate

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

United States (Virginia)

2005

Corn (maize), and Soybean

glyphosate

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

United States (Indiana)

2005

Soybean

chlorimuron-ethyl, and cloransulam-methyl

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

United States (Nebraska)

2006

Soybean

glyphosate

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

United States (Michigan)

2007

Nurseries

glyphosate

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

United States (Mississippi)

2007

Soybean

glyphosate, and paraquat

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 (West Virginia)

2007

Corn (maize), and Soybean

glyphosate

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

United States (Oklahoma)

2009

Soybean

glyphosate

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

United States (South Dakota)

2010

Corn (maize), and Soybean

glyphosate

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

United States (Delaware)

2010

Soybean, and Wheat

glyphosate, thifensulfuron-methyl, and tribenuron-methyl

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)

United States (Iowa)

2011

Corn (maize), and Soybean

glyphosate

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

United States (Kansas)

2011

Corn (maize), Cotton, Soybean, and Wheat

chlorsulfuron, iodosulfuron-methyl-Na, metsulfuron-methyl, rimsulfuron, thiencarbazone-methyl, thifensulfuron-methyl, and tribenuron-methyl

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

United States (Alabama)

2013

Corn (maize), Cotton, and Soybean

glyphosate

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

United States (Wisconsin)

2013

Soybean

glyphosate

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

United States (Illinois)

2013

Soybean

cloransulam-methyl

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

United States (California)

2014

Almonds

glyphosate, and paraquat

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 (Montana)

2015

Roadsides

glyphosate

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

United States (Oklahoma)

2018

Cotton, Soybean, and Wheat

chlorsulfuron, and metsulfuron-methyl

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

United States (Oregon)

2023

Fallow, and Wheat

glyphosate

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

Literature about Similar Cases

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Page, E., C. Granger, F. Tardif, I. Rajcan, M. Laforest, and R.E. Nurse. 2017. A chrono-geographical assessment of Canadian populations of Conyza canadensis. WSSA Proceedings 57 : 259 - .

In the decade since the introduction of herbicide tolerant (HT) crops, an increasing number of weed species have been identified with resistance to the widely utilized herbicide, glyphosate. In the USA, there are now 14 weed species with confirmed resistance to the non-selective herbicides utilized in these systems. Similarly in Canada, there are 5 weed species that exhibit glyphosate resistance (GR); two of which were documented in the last year alone. Conyza canadensis has developed GR in both countries and its capacity for long-distance dispersal raises the potential of cross-border movement of herbicide resistant traits. The objective of this research was to examine the origins of GR C. canadensis populations from SW Ontario and explore their mechanism(s) of resistance. New and historic populations of C. canadensis collected throughout the counties comprising the most SW portion of the province of Ontario, as well as from bordering states of the USA. Eight microsatellite markers were used to characterize the relatedness of these populations and the target site gene for glyphosate (EPSPS2) was also sequenced. Dose-response studies were carried out to ascertain or confirm the resistance levels of the various populations. Results from the microsatellite and dose response studies indicate that the majority of Canadian GR populations are distinct from the American populations tested and exhibit notably higher resistance. Sequencing of EPSPS2 confirmed that, while GR in American populations is mediated by non-target site mechanisms, the predominate mechanism of resistance in Canadian populations of C. canadensis is target site mediated. These results represent the first report of target site mediate resistance to glyphosate in C. canadensis and clearly expose geographically distinct mechanisms of resistance to a given active ingredient on a regional scale within a single highly mobile species..

Matzrafi, M., T. W. Lazar, M. Sibony, and B. Rubin. 2015. Conyza species: distribution and evolution of multiple target-site herbicide resistances. Planta 242 : 259 - 267.

Conyza canadensis (CC) and Conyza bonariensis (CB) are troublesome weeds around the world. Extensive use of herbicides has led to the evolution of numerous Conyza spp. herbicide-resistant populations. Seeds of 91 CC and CB populations were collected across Israel. They were mostly found (86 %) in roadsides and urban habitats, two disturbed habitats that had been dramatically impacted by human activities, thus we classify these species as anthropogenic. Although pyrithiobac-sodium was only used in cotton fields, 90 % of Conyza spp. populations were identified as pyrithiobac-sodium resistant, suggesting possible natural resistance to pyrithiobac-sodium. CC21 and CC17 C. canadensis populations were highly resistant to all tested ALS inhibitors due to a substitution in the ALS gene from Trp574 to Leu. They were also atrazine resistant due to a substitution in the psbA gene from Ser264 to Gly. The high level of imazapyr and pyrithiobac-sodium resistance observed in the CC10 population was due to an Ala205 to Val substitution. However, high resistance to sulfometuron methyl and pyrithiobac-sodium in population CC6 was due to a point mutation at Pro197 to Ser. All resistant plants of CC21 population showed both psbA (Ser264 to Gly) and ALS (Trp574 to Leu) substitutions, leading us to the conclusion that the attempt to overcome resistance to one mode of action by overuse of another will most likely lead to multiple herbicide resistance. Furthermore, we concluded that only individuals that carry both mutations could survive the shift between the two modes of action and overcome the fitness cost associated with the PSII resistance..

Ford, L. ; Soltani, N. ; McFadden, A. ; Nurse, R. E. ; Robinson, D. E. ; Sikkema, P. H.. 2014. Control of Canada fleabane (Conyza canadensis) with glyphosate DMA/2,4-D choline applications in corn (Zea mays). Agricultural Sciences 5 : 77 - 83.

Glyphosate resistant (GR) Canada fleabane (horseweed) has quickly spread across southwestern Ontario and is a difficult weed to control in GR crops. Glyphosate dimethylamine (DMA)/2,4-D choline (Enlist Duo®^TM Dow AgroSciences LLC), a new herbicide premix developed by Dow Agro Sciences, provides control of GR and other problematic weeds. The objective of this study was to compare single and sequential applications of glyphosate DMA/2,4-D choline for the control of GR Canada fleabane in GR corn. Three single applications of glyphosate (DMA)/2,4-D choline (1720 g.ae.ha^-1) were evaluated: (1) preplant (PP) applied to Canada fleabane up to 10 cm diameter/height, (2) postplant 1 (POST 1) applied when Canada fleabane was up to 20 cm tall and (3) postplant 2 (POST 2) applied up to 30 cm tall Canada fleabane. Four sequential applications were also examined:(1) PP followed by (fb) POST 1, (2) PP fb POST 2, (3) POST 1 fb POST 2 and (4) PP fb POST 1 fb POST 2. The single applications provided 69%-86% control of the GR Canada fleabane while the sequential applications increased control to 92%-100%. Three applications did not provide an increase in control over a sequential two-pass application at 8 weeks after the application (WAA). Results from this research indicate that a sequential 2-pass application of glyphosate DMA/2,4-D choline provided acceptable control of GR Canada fleabane in corn..

Fernando Machado dos Santos , Leandro Vargas , Pedro Jacob Christoffoleti , Franciele Mariani. 2013. Evaluation of horseweed (Conyza sumatrensis) resistance to glyphosate in Rio Grande do Sul – Brazil. Abstract from Doctorate Dissertation : .

(Conyza bonariensisandConyza canadensis) recorded in Rio Grande do Sul State, however resistance ofConyza sumatrensisto glyphosate is registered only in Paraná State. This report has the objective of stating the presence ofConyza sumatrensisresistant to glyphosate biotypes in Rio Grande do Sul State Brazil. To support this report, researches to evaluate the occurrence of glyphosate resistance in horseweed biotypes, in Rio Grande do Sul State, and for that, seed samples were collected in soybean areas with unsatisfactory control, 25 biotypes. These biotypes were preliminary evaluated for resistance to glyphosate, applying a dose of 720 g/ha. Seeds from this first test of two selected biotypes of horseweed, which were not controlled in the first trial, with contrasting degree of susceptibility, were chosen to develop the second trial. The selected biotypes were grown after herbicide application (first trial) to seed production. It was also made, from these biotypes, herbarium specimens, which were sent to the Department of Biology of the Federal University of Santa Maria, where it was cataloged and classified by Dr. Thais Scotti Canto Dorow, plant systematic specialist, asConyza sumatrensis(Retz.) E. Walker (TELES; BORGES; HEIDEN, 2013) and subsequently deposited in the herbarium, SMDM under numbers SMDM 13950, 13951, 13952 and 13953. The biotypes selected, derived from seeds produced were evaluated by dose-response curves, using eight herbicide doses (0, 45, 90, 180, 360, 720, 1,440 and 2,880 g/ha) applied at growth stage of 3 to 4 leaves and/or 0.5 to 1.0 inch height. The experiments were conducted in a greenhouse in a completely randomized arrangement of treatments with four replications. The results indicate that the susceptible biotype to glyphosate has reduced 50 % of dry matter when used dose of 34.3 g/ha, compared with the resistant biotype, that requires 143.6 g/ha for the same control. The dose that provides 50% control of the susceptible population was 172 g/ha, compared with the resistant biotype that needs 2,028 g/ha for the same control, showing a resistance factor of 11.8..

Pavlovic, D. ; Reinhardt, C. F. ; Bozic, D. ; Vrbnicanin, S.. 2013. Determination of Conyza canadensis levels of sensitivity to glyphosate trimesium sulphosate. International Journal of Agriculture and Biology 15 : 1091 - 1097.

Changes in leaf anatomy were examined in two S. African populations of Conyza canadensis of which one was presumed to be resistant (CCPR) and the other susceptible (CCS) to glyphosate. Glyphosate was applied to plants, which were grown from seed collected from these populations, at rates of 1, 2 and 4 kg a.e. ha^-1 of TOUCHDOWN® [active substance: glyphosate trimesium salt, 500 g L^-1] that are equivalent to 2, 4 and 8 L ha^-1 of the herbicide Touchdown. Leaf samples for the light microscope (LM) analysis were collected 3, 7 and 24 h after treatment (HAT). Changes in chlorophyll and shikimate content of leaf material were also examined. Changes in the palisade and pith tissue of leaves were not detected in the investigated populations at 3 and 7 HAT. However, at 24 HAT the different herbicide doses caused changes in leaf anatomy. These changes (injuries) were detected in the CCS at all tested doses, but in the CCPR population of C. canadensis the injuries were observed at only the two highest rates, 2 and 4 kg a.e. ha^-1. Chlorophyll and shikimate contents indicated significant differences between the treated and untreated plants of susceptible population only. Difference in glyphosate resistance between the CCS and CCPR populations was confirmed with an index of resistance of 1.58. This value of the index of resistance indicates that CCS population is 1.58 times more susceptible to glyphosate compared to CCPR population..

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