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.