Rick Boydston is part of team working to educate orchardists and vineyardists how to prevent and manage glyphosate-resistant weeds.

Rick Boydston is part of team working to educate orchardists and vineyardists how to prevent and manage glyphosate-resistant weeds.

For decades, glyphosate has been the cornerstone of weed programs in perennial crops like tree fruit and grapes. However, several cases of glyphosate-resistant weeds have been reported recently in the western United States, highlighting the need to step up resistance management efforts before weed resistance becomes widespread. Fortunately for grape growers and orchardists, help is coming.

A team of western state researchers, pooling their ­talents, is in the midst of developing an extension publication on glyphosate stewardship and optimizing glyphosate performance for integrated pest management in vineyards and orchards. Additionally, the team is developing materials for educational workshops to help prevent and manage glyphosate-resistant weeds.

The work is on a fast track—July is the target date for the extension publication, which will be available free online, said Dr. Kassim Al-Khatib, project director and director of the University of California Statewide IPM Program that’s headquartered in Davis. Workshops will be held by the year’s end to “train the trainer,” not only educating extension agents, farm advisors, and pesticide applicators about monitoring, preventing, and managing glyphosate-resistant weeds, but also providing educational materials and a set of presentations that can be used to reach grower audiences.

Team members include Al-Khatib; Rick Boydston, U.S. Department of Agriculture, Prosser, Washington; Tim Miller, Washington State University, Mount Vernon; Ed Peachey of Oregon State University, Corvallis; and Brad Hanson, UC-Davis.

The project is funded by the Western Region IPM ­Center at UC-Davis.


“Glyphosate is heavily used in orchards and vineyards,” said Al-Khatib. In just California alone, wine grape growers used 410,000 pounds of material, and peach and nectarine growers used 116,500 pounds in 2008. “Because of the industry’s intense use, and the development of glyphosate-resistant weeds in other states and in ­California, we were encouraged to put together this ­program.”

Researchers believe that when glyphosate costs declined in recent years as the patented Roundup became generic and was produced by various companies, many growers stopped using other weed management practices and instead shifted to using only glyphosate for weed control. As a result of this increased selection pressure, several cases of glyphosate-resistant weeds have been reported.

“Overreliance on one pest management tool is counter to the goals of an effective IPM program and can decrease the profitability and sustainability of western vineyards and orchards,” stated Al-Khatib. Glyphosate is a key ­herbicide in IPM programs because no alternatives are currently available to replace it.

Currently, 20 known weed species worldwide have evolved resistance to glyphosate, according to Dr. Ian Heap, organizer of an international survey of herbicide-resistant weeds conducted by the Weed Science Society of America. In California, rigid ryegrass, hairy fleabane, and horseweed have been reported as glyphosate resistant, while in Oregon, Italian ryegrass has been confirmed.
No glyphosate-resistant weeds have been reported in ­Washington State.

USDA agronomist Rick Boydston said that although Washington doesn’t have confirmation of any glyphosate-resistant weeds, no one has really surveyed the state. “I’ve had growers share their difficulty in controlling certain weeds when using glyphosate, and occasionally I’ll get calls from growers wondering why a specific weed survived the spray, but we haven’t had any documented cases of glyphosate resistance.
“Maybe we’re ahead of the game right now,” Boydston said, but added that weeds are very mobile, and seeds can move through the air, by birds and animals, and equipment. “With the potential of Roundup Ready crops like corn, alfalfa, and sugar beets being grown in the state, resistant weeds could easily move into orchards and ­vineyards.”

Suspecting resistance

Boydston said that growers should be suspicious when herbicides fail to kill weeds and the following takes place:

  • Other causes of herbicide failure have been ruled out.
  • The same herbicide or herbicides with the same mode of action have been used year after year.
  • A single weed species normally controlled is not, but other weed species are.
  • Healthy weeds are mixed with controlled weeds of the same species.
  • A patch of uncontrolled weed is spreading.

Preventing resistance

He encourages growers to take proactive steps to prevent resistance. Just as rotation of classes of material is important in fungicide resistance management programs, rotating and tank mixing herbicides with different modes of action are also key preventative measures for herbicide resistance management.

Other resistance management steps include using cultivation as another weed control tactic, and scouting and monitoring fields to look for weeds that escaped control. If weeds have escaped control, be sure to prevent their seed production.

He also recommends using shorter residual herbicides to reduce the selection pressure and to use the full ­recommended herbicide rate and proper timing.

Sanitation—using clean tillage and harvesting equipment and keeping field boundaries weedfree—is also a way to avoid resistance.