Davis, California
September 6, 2007
Hairy fleabane, a common summer
annual in the south Central Valley, has joined the ranks of
weeds that are resistant to glyphosate, an herbicide sold under
the brand name Roundup. This is the first occurrence of
glyphosate-resistant hairy fleabane in the United States.
Researchers suspect that this resistant weed may be widespread
on Central Valley roadsides and in orchards and vineyards.
Glyphosate-resistant hairy fleabane has also been found in South
Africa, Spain, Brazil and Columbia.
A few years ago, some populations of rigid ryegrass and
horseweed were confirmed as glyphosate-resistant in California.
Worldwide, 13 weed species are resistant to the herbicide.
Hairy fleabane is a prolific producer of fluffy seeds that can
easily be spread by wind. Growers and land managers have been
having trouble controlling this species with labeled rates of
glyphosate, leading researchers to study this weed's resistance.
The resistance was first reported in 2005 in Fresno to UC
Statewide Integrated Pest Management Weed Ecologist Anil
Shrestha. Since then, Shrestha, USDA-ARS Scientist Brad Hanson,
UC Cooperative Extension Farm Advisor Kurt Hembree, and student
assistants Thomas Wang and Ivan Ramirez collected seeds of hairy
fleabane from several locations in the Central Valley and tested
them for resistance to glyphosate.
"After several tests, we found that the plants grown from seeds
collected from a roadside in Reedley, Calif., were more
resistant to glyphosate compared to the plants grown from seeds
collected from west Fresno and Davis," says Shrestha.
"We sprayed the herbicide at various growth stages of the plant
ranging from 8 to 11 leaves and 18 to 23 leaves. While most of
the plants collected from Davis and Fresno died with the labeled
rate of glyphosate, all the plants from Reedley showed
resistance to glyphosate. They generally survived after
applications of up to eight times the labeled rate. A few plants
from Reedley even survived a dose of 16 times the labeled rate
of glyphosate.
"Glyphosate is an herbicide that provides broad-spectrum weed
control," says Shrestha. "Appropriate strategies to prevent
herbicide-resistance must be taken to avoid losing this
effective herbicide."
Herbicide-resistant plants evolve when the same herbicide with
the same overall manner of affecting a plant is used on them
repeatedly. Several articles have been published on this
subject. An ANR article on this topic is available online at:
http://anrcatalog.ucdavis.edu/pdf/8012.pdf.
For more information about hairy
fleabane, visit
http://www.ipm.ucdavis.edu. |
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