Multiple resistant Papaver rhoeas from Morocco

International Survey of Herbicide-Resistant Weeds

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Wednesday, August 20, 2025

MULTIPLE RESISTANT CORN POPPY
(Papaver rhoeas)

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Morocco, Northern Morocco

INTRODUCTION		CORN POPPY
Corn Poppy (Papaver rhoeas) is a dicot weed in the Papaveraceae family. In Morocco this weed first evolved multiple resistance (to 2 herbicide sites of action) in 2025 and infests Durum wheat, and Wheat. Multiple resistance has evolved to herbicides in the Groups B/2, and O/4. These particular biotypes are known to have resistance to 2,4-D, florasulam, and tribenuron-methyl and they may be cross-resistant to other herbicides in the Groups B/2, and O/4. 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 Aug 19, 2025 )
Common Name	Corn Poppy
Species	Papaver rhoeas
Group	Inhibition of Acetolactate Synthase (B/2) Auxin Mimics (O/4)
Herbicides	2,4-D, florasulam, and tribenuron-methyl
Location	Morocco, Northern Morocco
Year	2025
Situation(s)	Durum wheat, and Wheat
Contributors - (Alphabetically)	Zakia El-Mastouri, Katerina Hamouzova, Pavlína Košnarová, and Josef Soukup
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NOTES ABOUT THIS BIOTYPE

GENERAL

Zakia El-Mastouri

Corn poppy (Papaver rhoeas L.) resistance to ALS inhibiting and 2,4-d herbicides in Moroccan and Tunisian rainfed wheat fields Zakia El-Mastouri¹, Pavlína Košnarová¹, Kateřina Hamouzová¹*, Ezzedine Alimi², Josef Soukup¹

¹Department of Agroecology and Crop Production, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague, Prague, Czech Republic ²Maghreb Phytotest, Tunis, Tunisia *Corresponding author: hamouzova@af.czu.cz

Citation: El-Mastouri Z., Košnarová P., Hamouzová K., Alimi E., Soukup J. (2025): Corn poppy (Papaver rhoeas L.) resistance to ALS inhibiting and 2,4-d herbicides in Moroccan and Tunisian rainfed wheat fields. Plant Soil Environ., 71.

Abstract: Corn poppy (Papaver rhoeas L.) is one of the most problematic weed species, mainly in rainfed Moroccan and Tunisian cereal crops. The overuse of acetolactate synthase (ALS) inhibiting and/or auxinic herbicides led to the spread of corn poppies resistant to both chemical families in this region. In order to identify and understand the selection drivers of resistance, appropriate characterisation of the resistance profile is necessary. Two experiments were carried out: biological sensitivity tests with ALS inhibiting herbicides (tribenuron-methyl and florasulam) and auxinic herbicides (2,4-d) were carried out with populations sampled in the field where the herbicide failure was observed. Bioassay tests confirmed resistance in all studied populations with an average frequency of 75.13, 30.81, 33.17 and 11.52% with tribenuron, florasulam, 2,4-d and florasulam + 2,4-d, respectively. Corn poppy sampled from both countries exhibited similar frequencies within populations for each tested herbicide. The molecular analysis was conducted with next-generation sequencing (Illumina), allowing massive, precise and rapid sequencing regions of the ALS gene carrying resistance codons. Using this technology, ALS mutant alleles were found in all populations at frequencies ranging from 1.4% to 63.3%, with an average of 16.7%. This study highlights the need to elucidate resistance mechanisms to understand herbicide responses and develop effective strategies for managing resistant corn poppy in rainfed cereals as an essential step to maintain the effectiveness of these molecules as long as possible.

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ACADEMIC ASPECTS

Confirmation Tests

Greenhouse, and Laboratory trials comparing a known susceptible Corn Poppy biotype with this Corn Poppy 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 Group B, O/2, 4 resistant Corn Poppy 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 multiple resistant Corn Poppy from Morocco indicate that resistance is due to unknown, and 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 multiple resistant Corn Poppy from Morocco please update the database.

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CONTRIBUTING WEED SCIENTISTS

ZAKIA EL-MASTOURI

PhD student Food and Natural Resources Czech University of Life Sciences Prague Department of Agroecology and Crop Production Faculty of Agrobiology Kamýcká 129 Prague, 165 00, Prague Czech Republic Email Zakia El-Mastouri

KATERINA HAMOUZOVA

Weed Research Scientist Czech University Of Agriculture In Prague Depart. Of Agroecology And Biometeorology Kamycka 129 Prague, 6 - Suchdol, 16521, Prague Czech Republic Email Katerina Hamouzova Web : Web Site Link

PAVLÍNA KOŠNAROVÁ

Researcher Czech University of Life Sciences Agroecology and Plant Production Kamycka 129 Prague , 16500, Czech Republic Czech Republic Email Pavlína Košnarová Web : Web Site Link

JOSEF SOUKUP

Professor Czech University of Life Sciences Prague Agroecology and Biometeorology Kamycka 129 Prague 6 - Suchdol, 16521, Czech Republic Czech Republic Email Josef Soukup Web : Web Site Link

ACKNOWLEDGEMENTS
The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Morocco have been instrumental in providing you this information. Particular thanks is given to Zakia El-Mastouri, Katerina Hamouzova, Pavlína Košnarová, and Josef Soukup for providing detailed information.

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Herbicide Resistant Corn Poppy Globally
(Papaver rhoeas)

Country

FirstYear

Situation

Active Ingredients

Site of Action

Belgium

2014

Wheat

florasulam, and metsulfuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Denmark

2003

Wheat

florasulam, iodosulfuron-methyl-Na, and tribenuron-methyl

Inhibition of Acetolactate Synthase (B/2)

France

2007

Wheat

iodosulfuron-methyl-Na, mesosulfuron-methyl, and metsulfuron-methyl

Inhibition of Acetolactate Synthase (B/2)

France

2015

Wheat

2,4-D, and aminopyralid

Auxin Mimics (O/4)

France

2016

Cereals

2,4-D, aminopyralid, iodosulfuron-methyl-Na, MCPA, mesosulfuron-methyl, and metsulfuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Germany

2012

Cereals, and Rapeseed

florasulam, and imazamox

Inhibition of Acetolactate Synthase (B/2)

Greece

1998

Winter wheat

chlorsulfuron, florasulam, imazamox, pyrithiobac-sodium, thifensulfuron-methyl, triasulfuron, and tribenuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Greece

2002

Wheat

2,4-D, iodosulfuron-methyl-Na, and mesosulfuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Italy

1998

Durum wheat

florasulam, iodosulfuron-methyl-Na, and tribenuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Italy

1998

Wheat

2,4-D, iodosulfuron-methyl-Na, and tribenuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Italy

1998

Wheat

2,4-D

Auxin Mimics (O/4)

Morocco

2025

Durum wheat, and Wheat

2,4-D, florasulam, and tribenuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Poland

2014

Winter wheat

tribenuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Portugal

2018

Wheat

2,4-D, and tribenuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Portugal

2018

Wheat

tribenuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Spain

1993

Cereals, and Wheat

2,4-D, and tribenuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

Sweden

2011

Winter wheat

amidosulfuron, iodosulfuron-methyl-Na, and propoxycarbazone-Na

Inhibition of Acetolactate Synthase (B/2)

Tunisia

2025

Durum wheat, and Wheat

2,4-D, florasulam, and tribenuron-methyl

Multiple Resistance: 2 Sites of Action
Inhibition of Acetolactate Synthase (B/2)
Auxin Mimics (O/4)

United Kingdom

2001

Cereals

metsulfuron-methyl

Inhibition of Acetolactate Synthase (B/2)

Literature about Similar Cases

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Joel Torra, Aritz Royo-Esnal, Jordi Rey-Caballero, Jordi Recasens, Marisa Salas. 2018.
Weed Science 66 : 764 - 772.

Corn poppy (Papaver rhoeas L.) is the most widespread broadleaf weed species infesting winter cereals in Europe. Biotypes that are resistant to both 2,4-D and tribenuron-methyl, an acetolactate synthase (ALS) inhibitor, have evolved in recent decades, thus narrowing the options for effective chemical control. Though the effectiveness of several integrated weed management (IWM) strategies have been confirmed, none of these strategies have been tested to manage multiple herbicide–resistant P. rhoeas under no-till planting. With the expansion of no-till systems, it is important to prove the effectiveness of such strategies. In this study, a field experiment over three consecutive seasons was conducted to evaluate and compare the effects of different weed management strategies, under either direct drilling (i.e., no-till) or intensive tillage, on a multiple herbicide–resistant P. rhoeas population. Moreover, evaluations were carried out as to whether the proportions of ALS inhibitor–resistant individuals were affected by the tillage systems for each IWM strategy at the end of the 3-yr period. The IWM strategies tested in this research included crop rotation, delayed sowing, and different herbicide programs such as PRE plus POST or POST. All IWM strategies greatly reduced the initial density of P. rhoeas each season (≥ 95%) under either direct drilling or intensive tillage. After 3 yr, the IWM strategies were very effective in both tillage systems, though the effects were stronger under direct drilling (~95%) compared with intensive tillage (~86%). At the end of the study, the proportion of ALS inhibitor–resistant plants was not different between the IWM strategies in both tillage systems (94% on average). Therefore, crop rotation (with sunflower [Helianthus annuus L.]), delayed sowing, or a variation in the herbicide application timing are effective under direct drilling to manage herbicide-resistant P. rhoeas. Adoption of IWM strategies is necessary to mitigate the evolution of resistance in both conventional and no-till systems..

Jordi Rey-Caballero, Julio Menéndez, Maria D Osuna, Marisa Salas, Joel Torra. 2017. Target-site and non-target-site resistance mechanisms to ALS inhibiting herbicides in Papaver rhoeas. Pesticide Biochemistry and Physiology 138 : 57 - 65.

Target-site and non-target-site resistance mechanisms to ALS inhibitors were investigated in multiple resistant
(tribenuron-methyl and 2,4-D) and only 2,4-D resistant, Spanish corn poppy populations. Six amino-acid replacements at the Pro197 position (Ala197, Arg197, His197, Leu197, Thr197 and Ser197) were found in three
multiple resistant populations. These replacements were responsible for the high tribenuron-methyl resistance
response, and some of them, especially Thr197 and Ser197, elucidated the cross-resistant pattern for imazamox and florasulam, respectively. Mutations outside of the conserved regions of the ALS gene (Gly427 and Leu648) were identified, but not related to resistance response. Higher mobility of labeled tribenuron-methyl in plants with multiple resistance was, however, similar to plants with only 2,4-D resistance, indicating the presence of non-target-site resistance mechanisms (NTSR). Metabolism studies confirmed the presence of a hydroxy imazamox metabolite in one of the populations. Lack of correlation between phenotype and genotype in plants treatedwith florasulamor imazamox, non-mutated plants surviving imazamox, tribenuron-methyl translocation patterns and the presence of enhanced metabolism revealed signs of the presence of NTSR mechanisms to ALS inhibitors in this species. On this basis, selection pressurewith ALS non-SU inhibitors bears the risk of promoting the evolution of NTSR mechanisms in corn poppy..

Joel Torra, Antonia M. Rojano-Delgado, Jordi Rey-Caballero, Aritz Royo-Esnal, Maria L. Salas, Rafael De Prado. 2017. Enhanced 2,4-D Metabolism in Two Resistant Papaver rhoeas Populations from Spain. Frontiers in Plant Science 8 : 1584 - .

Corn poppy (Papaver rhoeas), the most problematic broadleaf weed in winter cereals in Southern Europe, has developed resistance to the widely-used herbicide, 2,4-D. The first reported resistance mechanism in this species to 2,4-D was reduced translocation from treated leaves to the rest of the plant. However, the presence of other non-target site resistance (NTSR) mechanisms has not been investigated up to date. Therefore, the main objective of this research was to reveal if enhanced 2,4-D metabolism is also present in two Spanish resistant (R) populations to synthetic auxins. With this aim, HPLC experiments at two 2,4-D rates (600 and 2,400 g ai ha−1) were conducted to identify and quantify the metabolites produced and evaluate possible differences in 2,4-D degradation between resistant (R) and susceptible (S) plants. Secondarily, to determine the role of cytochrome P450 in the resistance response, dose-response experiments were performed using malathion as its inhibitor. Three populations were used: S, only 2,4-D R (R-703) and multiple R to 2,4-D and ALS inhibitors (R-213). HPLC studies indicated the presence of two hydroxy metabolites in these R populations in shoots and roots, which were not detected in S plants, at both rates. Therefore, enhanced metabolism becomes a new NTSR mechanism in these two P. rhoeas populations from Spain. Results from the dose-response experiments also showed that pre-treatment of
R plants with the cytochrome P450 (P450) inhibitor malathion reversed the phenotype to 2,4-D from resistant to susceptible in both R populations. Therefore, it could be hypothesized that a malathion inhibited P450 is responsible of the formation of the hydroxy metabolites detected in the metabolism studies. This and previous research indicate that two resistant mechanisms to 2,4-D could be present in populations R-703 and R-213: reduced translocation and enhanced metabolism. Future experiments are required to confirm these hypotheses, understand the role of P450, and the relationship between both NTSR mechanisms. On this basis, selection pressure with synthetic auxins bears the risk of promoting the evolution enhanced metabolism in Papaver rhoeas..

Jordi Rey-Caballero, Aritz Royo-Esnal, Jordi Recasens, Ignacio González, and Joel Torra. 2017. Management Options for Multiple Herbicide–Resistant Corn Poppy (Papaver rhoeas) in Spain. Weed Science 65 : 295 - 304.

Corn poppy is the most widespread broadleaf weed infesting winter cereals in Europe. Biotypes that are resistant (R) to both 2,4-D and tribenuron-methyl have evolved in recent decades, thus complicating their chemical control. In this study, field experiments at two locations over three seasons were conducted to evaluate the effects of different weed management strategies on corn poppy resistant to 2,4-D and tribenuron-methyl, including crop rotations, delayed sowing and different herbicide programs. After 3 yr, all integrated weed management (IWM) strategies reduced the initial density of corn poppy, although the most successful strategies were those which either included a suitable crop rotation (sunflower or field peas), or had a variation in the herbicide application timing (early POST or combining PRE or early POST and POST). The efficacy of IWM strategies differed between both locations, possibly due to different population dynamics and the genetic basis of herbicide resistance. Integrated management of multiple herbicide–resistant corn poppy is necessary in order to reduce selection pressure by herbicides, mitigate the evolution of new R biotypes, and reduce the weed density in highly infested fields..

Jordi Rey-Caballero, Julio Menéndez, Jordi Giné-Bordonaba, Marisa Salas, Ricardo Alcántara, Joel Torra. 2016. Unravelling the resistance mechanisms to 2,4-D (2,4-dichlorophenoxyacetic acid) in corn poppy (Papaver rhoeas). Pesticide Biochemistry and Physiology 133 : 67 - 72.

In southern Europe, the intensive use of 2,4-D (2,4-dichlorophenoxyacetic acid) and tribenuron-methyl in cereal crop systems has resulted in the evolution of resistant (R) corn poppy (Papaver rhoeas L.) biotypes. Experiments were conducted to elucidate (1) the resistance response to these two herbicides, (2) the cross-resistant pattern to other synthetic auxins and (3) the physiological basis of the auxin resistance in two R (F-R213 and D-R703) populations. R plants were resistant to both 2,4-D and tribenuron-methyl (F-R213) or just to 2,4-D (D-R703) and both R populations were also resistant to dicamba and aminopyralid. Results from absorption and translocation experiment revealed that R plants translocated less [14C]-2,4-D than S plants at all evaluation times. There was between four and eight-fold greater ethylene production in S plants treated with 2,4-D, than in R plants. Overall, these results suggest that reduced 2,4-D translocation is the resistance mechanism in synthetic auxins R corn poppy populations and this likely leads to less ethylene production and greater survival in R plants..

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