(Image courtesy Billy E. Warrick, Texas Cooperative Extension.)
New Tests Screen Weed for Resistance to Major
By Jim Core
June 20, 2005
Two rapid, nondestructive tests have
been developed by Agricultural Research Service (ARS) scientists to screen a troublesome
weed for resistance to the world's most-used herbicide.
H. Koger III and
L. Shaner developed the test to determine if horseweed plants were
resistant or susceptible to the herbicide glyphosate.
Koger is an agronomist with the
Genetic and Production Research Unit at Stoneville, Miss., and Shaner is a
plant physiologist with the
Management Research Unit at Fort Collins, Colo. Koger was based in the ARS
Weed Science Research Unit at Stoneville when the research was done.
In 2000, horseweed (Conyza canadensis) became the first weed species
to develop resistance to glyphosate in cropland where glyphosate-resistant
soybeans were grown. Glyphosate-resistant biotypes of horseweed have now been
confirmed in 13 states east of the Mississippi River.
Glyphosate is effective at killing all plant types including grasses,
broadleaves and sedges, as well as perennial and woody plants. After emergence,
glyphosate-resistant crops are capable of tolerating multiple applications of
the herbicide, while weeds are killed. However, repeated use over many years
has left several weed species resistant to glyphosate.
The two tests can be used together. One method, which involves dipping a
whole leaf into a glyphosate-based mixture and looking for signs of injury, is
quick and easy to perform. To achieve double confirmation of the weed's status,
a second assay can be used. This method takes advantage of glyphosate's mode of
action, which involves inhibiting amino acid metabolism in what is known as the
shikimic acid pathway. Leaf tissue samples are removed, and amino acid levels
are measured with specialized laboratory equipment.
If glyphosate resistance is confirmed, the tests should help reduce the
spread of resistant horseweed populations because growers will use different
herbicides to manage the resistant weeds.
Koger and Shaner are testing both assays to see if they're useful for
screening other weed species for resistance to glyphosate.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.