HRAC GROUP 2 (LEGACY B) RESISTANT PASTO HONDURAS (MEXICANGRASS)
(Ixophorus unisetus)

Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B)

Costa Rica, Guanacaste

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INTRODUCTION PASTO HONDURAS (MEXICANGRASS) Pasto Honduras (Mexicangrass) (Ixophorus unisetus) is a monocot weed in the Poaceae family.  In Costa Rica this weed first evolved resistance to Group 2 (Legacy B) herbicides in 1988 and infests Roadsides.   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 imazapyr 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.

QUIK STATS (last updated Sep 15, 2000 ) Common Name Pasto Honduras (Mexicangrass) Species Ixophorus unisetus Group Inhibition of Acetolactate Synthase HRAC Group 2 (Legacy B) Herbicides imazapyr Location Costa Rica, Guanacaste Year 1988 Situation(s) Roadsides Contributors - (Alphabetically) Bernal Valverde  Edit this Case  |  Add New Case of Resistance  |  Add Note

NOTES ABOUT THIS BIOTYPE AREAS FOUND Bernal Valverde Northern Pacific area of Costa Rica only. Puntorenas and Guanacaste provinces. Edit this Note  |  Add New Note   GENERAL Bernal Valverde Proc. Brighton Crop Prot. Conference on Weeds. 1993. 8C-10. p. 1189-1194. Edit this Note  |  Add New Note

ACADEMIC ASPECTS Confirmation Tests Field, and Greenhouse trials comparing a known susceptible Pasto Honduras (Mexicangrass) biotype with this Pasto Honduras (Mexicangrass) 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 2 resistant Pasto Honduras (Mexicangrass) 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 2 (Legacy B) resistant Pasto Honduras (Mexicangrass) from Costa Rica please update the database.   Edit Case  |  Add Note  |  Add Article  |  Add New Case  |  Help

CONTRIBUTING WEED SCIENTISTS BERNAL VALVERDE Honorary Associate Professor The University of Copenhagen Department of Plant and Environmental Sciences Hojebakkegaard Allé 13 Taastrup, DK-2630, Hovedstaden Denmark Email Bernal Valverde

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

Herbicide Resistant Pasto Honduras (Mexicangrass) Globally (Ixophorus unisetus) Herbicide Resistant Pasto Honduras (Mexicangrass) Globally (Ixophorus unisetus) Drag a column header and drop it here to group by that column Herbicide Resistant Pasto Honduras (Mexicangrass) Globally (Ixophorus unisetus) # Country Country StateName FirstYear Situation Active Ingredients CountryID Site of Action WeedID Species CommonName ResistID 1 Costa Rica Costa Rica   1988 Roadsides imazapyr 11 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B) 100 Ixophorus unisetus Pasto Honduras (Mexicangrass) 74 2 Mexico Mexico   2014 Corn (maize) nicosulfuron 29 Inhibition of Acetolactate Synthase ( HRAC Group 2 (Legacy B) 100 Ixophorus unisetus Pasto Honduras (Mexicangrass) 9925

Literature about Similar Cases

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Chaves, L. ; Valverde, B. E. ; Garita, I.. 1997. Resistance of Ixophorus unisetus to acetolactate synthase inhibiting herbicides. Manejo Integrado de Plagas : 20 - 25.

Resistance to imazapyr was demonstrated in populations of Ixophorus unisetus growing along irrigation and drainage canals in parts of the North Pacific region of Costa Rica that had been treated repeatedly with the herbicide. Bioassays were conducted in a screenhouse with eight populations of I. unisetus. Plants were treated with increasing rates of six acetolactate synthase (ALS) inhibiting herbicides and fresh weights were recorded. Based on biomass reductions produced by the herbicides in relation to the untreated control, GR₅₀ (dose required to reduce plant growth by 50%) values were calculated. Resistance indices (ratio of GR₅₀ of tested population and that of a known susceptible population) indicated that two I. unisetus populations were 5 and 80 times more resistant to imazapyr compared to the susceptible population. Imazapyr-resistant populations exhibited cross resistance to other imidazolinones (imazaquin, imazethapyr) but not to the sulfonylurea herbicides included in the experiments..

Valverde, B. E. ; Chaves, L. ; Gonzales, J. ; Garita, I.. 1993. Field evolved imazapyr resistance in Ixophorus unisetus and Eleusine indica in Costa Rica. : 1189 - 1194.

The 1st confirmation of imazapyr-resistance in populations of I. unisetus growing along ditch banks in rice producing areas of Costa Rica is described. These populations were subjected to imazapyr at 0.5-0.7 kg/ha for >5 years before becoming resistant. Imazapyr resistance in E. indica biotypes, collected from a poultry farm in Costa Rica, is also discussed. These populations became the most prevalent weed after continuous overuse of imazapyr for >4 years. Screenhouse studies indicated that selected I. unisetus biotypes are 5- to 80-fold more resistant to imazapyr than the most susceptible biotype. The resistant E. indica biotype required 14-fold more imazapyr for a 50% growth reduction compared with the susceptible biotype. Some cross-resistance to other imidazolinone herbicides and some sulfonylurea herbicides was observed..

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