Herbicide-resistant weeds are not the exclusive domain of biotech, herbicide-resistant row crops.

University of California farm advisors and California pest control advisors say that herbicide-resistant and tolerant weeds in California's tree and vine crops should be a wake-up call for growers to use proven management practices that stop or delay development of resistance in these weeds.

California already has 18 herbicide-resistant weeds in crops ranging from almonds and asparagus to barley, onions and wheat, as well as non-crop areas of railways and roadsides. The first, atrazine-resistant groundsel, was found in asparagus in 1981, the most recent, 2002's find of quinclorac-resistant smooth crabgrass in rice. Glyphosate-resistant rigid ryegrass in almonds earned a place on the list in 1998.

And that's just the plant pests that meet the California weed scientists' definition of herbicide-resistant: “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that would normally be lethal to the wild type.”

Then there are the herbicide-tolerant weeds that don't meet the resistance test, but are becoming tougher to control.

Weed shifts

“We are certainly seeing increasing Roundup tolerance from certain weed species,” states Fresno County Farm Advisor Kurt Hembree. “Growers are finding that it now takes higher rates of Roundup or a tank mix of Roundup and Goal to get good control of some tough weeds.”

Scott Severson, a PCA with Hughson Chemical in Hughson, Calif., delivers a similar report. “We really started to see a weed shift in recent years,” he says.

Marestail or horseweed and hairy fleabane are the most-frequently cited tougher-to-control weeds shifting in orchards and vineyards. Neither weed has been confirmed as herbicide-resistant in California, but other states report marestail showing resistance to several herbicides including glyphosate, paraquat and atrazine. And, marestail poses a dual threat: besides competing in vineyards for nutrients, water and light, it can host the glassy-winged sharpshooter.

Herbicide-resistant and tolerant weeds develop because of continued use of herbicides that work on a single mode of action. Relying on any single chemistry kills susceptible weeds but allows weeds that are naturally tolerant or resistant to the herbicide's mode of action to create a resistant weed population within five or six years. So, management that attacks weeds with multiple tools can stop or delay resistance and tolerance development.

The first step is to understand the potential consequences of using only one type of herbicide and to know what weeds you're facing, notes Hembree. “A lot of guys don't know the weeds they have. If you start seeing a change in the weed flora, consider changing your herbicides to control them.”

Hembree also advises against the common practice of waiting to kill the most, and frequently the biggest, weeds, especially with reduced rates of herbicides. “You need to get out there on time to knock out weeds. And, use the highest rate necessary to kill weeds. Don't cut the rate just to save a couple of bucks an acre.”

Adds Chris Morgner, a PCA and owner of Agri-Valley Consulting of Merced, Calif., “If a weed is not as susceptible or not at the right stage for control, it's going to go to seed.”

Another option is to use residual, pre-emergence products in your weed control program. “We don't have a lot of choices for residual herbicides, but it's worth consideration,” Morgner says.

Then there's herbicide rotation, within the season as well as from season-to-season. “We've known for years that resistance and tolerance occur, but it was too easy to use glyphosate,” Morgner says. “Now we're forced to change. Everyone's recognizing weed problems and people are willing to rotate.”

Do homework

Alternating chemistry means reading product labels and knowing differences among crop protection products. Products with different brand names can still have the same chemistry. “There are a number of glyphosate formulations. But glyphosate that's not Roundup brand doesn't mean it's not glyphosate,” points out Ron Vargas, Madera County farm advisor.

One rotation choice is Rely herbicide, glufosinate-ammonium. This is a non-selective contact material whose active ingredient, glufosinate, was originally isolated from a naturally occurring soil bacteria.

“Rely is a different mode of action than glyphosate and does a better job on horseweed and hairy fleabane,” Vargas says.

Hembree says, “Rely is especially effective on those tough weeds that Roundup misses, including filaree, malva, fleabane and nettles.”

He has nearly 10 years of field trials showing that Rely performed as well as the standard Roundup/Goal tank mix or Roundup-only program. In fact, Rely knocks down dozens of tough grasses and broadleaves, including fleabane, marestail, clover, nightshade, lambsquarters, nettle, nutsedge, and bindweed in almonds, grapes, and apples.

Morgner adds, “Rely works really well. We're held back economically but you have to look at its effectiveness as well as economics.”

In those situations where cost is vital, spot spraying or new sprayer technology to pinpoint application of effective herbicides to control weed escapes are alternatives.

Besides rotating single herbicides, consider rotating tank mixes of products with different chemistries. For fleabane or marestail in almonds, Morgner cites a tank mix of 2, 4-D plus Gramoxone or glyphosate. “If we can't use 2, 4-D, then we'll use maximum rates of Gramoxone with a high volume of water or, we'll substitute Goal for the 2, 4-D,” he says.

“We like that Rely is a non-restricted material,” states Severson. “It makes it much easier to use and apply.”

Weed experts conclude that while herbicide resistant and tolerant weeds threaten California tree and vines, proven management practices such as these will delay development and preserve the effectiveness of current control tools.