International Survey of Herbicide-Resistant Weeds

Countries Weeds Herbicides Mutations Graphs References Researchers
(Hydrilla verticillata)

Phytoene Desaturase inhibitorsHRAC Group 12 (Legacy F1)

United States, Florida
Hydrilla (Hydrilla verticillata) is a monocot weed in the Hydrocharitaceae family.  In Florida this weed first evolved resistance to Group 12 (Legacy F1) herbicides in 2002 and infests Aquatic.   Group 12 (Legacy F1) herbicides are known as Phytoene Desaturase inhibitors (Inhibition of Phytoene Desaturase ).  Research has shown that these particular biotypes are resistant to fluridone and they may be cross-resistant to other Group 12 (Legacy F1) 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.

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 Apr 29, 2009 )

Common NameHydrilla
SpeciesHydrilla verticillata
GroupPhytoene Desaturase inhibitors HRAC Group 12 (Legacy F1)
LocationUnited States, Florida
Contributors - (Alphabetically)Franck Dayan 
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Confirmation Tests

Field, Greenhouse, and Laboratory trials comparing a known susceptible Hydrilla biotype with this Hydrilla biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetic studies on HRAC Group 12 resistant Hydrilla indicate that the inheritance is determined by a one gene, nuclear trait.  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 Group 12 (Legacy F1) resistant Hydrilla from Florida indicate that resistance is due to 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 Group 12 (Legacy F1) resistant Hydrilla from Florida please update the database.
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Plant Physiologist
Natural Products Utilization Research Unit
P.O. Box 8048
University, 38677-8048, Mississippi
United States
Email Franck Dayan
Web   : Web Site Link


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Florida have been instrumental in providing you this information. Particular thanks is given to Franck Dayan for providing detailed information.
Herbicide Resistant Hydrilla Globally
(Hydrilla verticillata)
Herbicide Resistant Hydrilla Globally
(Hydrilla verticillata)
Drag a column header and drop it here to group by that column
Herbicide Resistant Hydrilla Globally
(Hydrilla verticillata)
#CountryFirstYearSituationActive IngredientsSite of Action
1 United States (Florida) United StatesFlorida2002 Aquatic fluridone 45 Phytoene Desaturase inhibitors (F1/12)
217Hydrilla verticillataHydrilla5166
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