<b>HRAC Group 4</b> <font size='2'> (Legacy O) </font> resistant Carduus pycnocephalus from New Zealand

INTERNATIONAL HERBICIDE-RESISTANT WEED DATABASE

HRAC GROUP 4 (LEGACY O) RESISTANT ITALIAN THISTLE
(Carduus pycnocephalus)

Auxin Mimics HRAC Group 4 (Legacy O)

New Zealand

INTRODUCTION		ITALIAN THISTLE
Italian Thistle (Carduus pycnocephalus) is a dicot weed in the Asteraceae family. In New Zealand this weed first evolved resistance to Group 4 (Legacy O) herbicides in 1997 and infests Pastures. Group 4 (Legacy O) herbicides are known as Auxin Mimics (Auxin Mimics ). Research has shown that these particular biotypes are resistant to 2,4-D, MCPA, and MCPB and they may be cross-resistant to other Group 4 (Legacy O) 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 Dec 13, 2013 )
Common Name	Italian Thistle
Species	Carduus pycnocephalus
Group	Auxin Mimics HRAC Group 4 (Legacy O)
Herbicides	2,4-D, MCPA, and MCPB
Location	New Zealand
Year	1997
Situation(s)	Pastures
Contributors - (Alphabetically)	Kerry Harrington
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ACADEMIC ASPECTS

Confirmation Tests

Field, and Greenhouse trials comparing a known susceptible Italian Thistle biotype with this Italian Thistle 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 4 resistant Italian Thistle 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 4 (Legacy O) resistant Italian Thistle from New Zealand please update the database.

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CONTRIBUTING WEED SCIENTISTS

KERRY HARRINGTON

Associate Professor in Weed Science Massey University School of Agriculture Private Bag 11-222 Palmerston North, 4442, Manawatu New Zealand Email Kerry Harrington Web : Web Site Link

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

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Herbicide Resistant Italian Thistle Globally
(Carduus pycnocephalus)

Country

FirstYear

Situation

Active Ingredients

Site of Action

New Zealand

1997

Pastures

2,4-D, MCPA, and MCPB

Auxin Mimics ( HRAC Group 4 (Legacy O)

Literature about Similar Cases

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Harrington, K. C.. 1989. Distribution and cross-tolerance of MCPA-tolerant nodding thistle. : 39 - 42.

By using petri dish assays and collected seeds, at least 12 new sites were identified in Hawkes Bay and Waikato where nodding thistle (Carduus nutans) showed tolerance to MCPA at 1.1 kg/ha. Slender winged thistle (Carduus pycnocephalus) showed a similar level of tolerance at the one site at which it was studied. Tolerance by nodding thistle of other herbicides, especially MCPB at 2.4 kg and 2,4-D at 1.1 kg was also shown..

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Heap, I. The International Herbicide-Resistant Weed Database. Online. Friday, October 2, 2026. Available at www.weedscience.org

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