International Survey of Herbicide-Resistant Weeds

Countries Weeds Herbicides Mutations Graphs References Researchers
(Panicum capillare)

Photosystem II inhibitors (C1/5)

Canada, Ontario
Witchgrass (Panicum capillare) is a monocot weed in the Poaceae family.  In Ontario this weed first evolved resistance to Group C1/5 herbicides in 1981 and infests Corn (maize), and Cropland.   Group C1/5 herbicides are known as Photosystem II inhibitors (Inhibition of photosynthesis at photosystem II).  Research has shown that these particular biotypes are resistant to atrazine and they may be cross-resistant to other Group C1/5 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 Jun 06, 2000 )

Common NameWitchgrass
SpeciesPanicum capillare
GroupPhotosystem II inhibitors (C1/5)
LocationCanada, Ontario
Situation(s)Corn (maize), and Cropland
Contributors - (Alphabetically)Clarence Swanton 
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Clarence Swanton
Found in Waterloo, Grey, and York counties.

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Hugh Beckie
G. R. Stephenson, M. D. Dykstra, R. D. McLaren, and A. S. Hamill. 1990. Agronomic practices influencing triazine-resistant weed distribution in Ontario. Weed Technol. 4:199-207.

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Confirmation Tests

Field, Greenhouse, and Laboratory trials comparing a known susceptible Witchgrass biotype with this Witchgrass 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 Group C1/5 resistant Witchgrass 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

Triazine resistant weeds often exhibit a lower relative fitness when compared to susceptible biotypes.  The most common mutation conferring triazine resistance (Ser 264 to Gly mutation of the psbA gene) also causes a reduction in CO2 fixation, quantum yield, and seed and biomass production.  There is no record in this database referring specifically to fitness studies on Group C1/5 resistant Witchgrass from Ontario.
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University Of Guelph
Dept. Of Crop Science
Ontario Agricultural College
PO Box 249
Guelph, N1G 2W1, Ontario
Email Clarence Swanton


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Ontario have been instrumental in providing you this information. Particular thanks is given to Clarence Swanton for providing detailed information.
Herbicide Resistant Witchgrass Globally
(Panicum capillare)
Herbicide Resistant Witchgrass Globally
(Panicum capillare)
Drag a column header and drop it here to group by that column
Herbicide Resistant Witchgrass Globally
(Panicum capillare)
#CountryFirstYearSituationActive IngredientsSite of Action
1 Canada (Ontario) CanadaOntario1981 Corn (maize), and Cropland atrazine 7 Photosystem II inhibitors (C1/5)
121Panicum capillareWitchgrass63
Literature about Similar Cases

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