By Tom Burfield

Herbicide-resistant weeds have taunted growers of agronomic crops, such as cotton and soybeans, for years. But now there’s concern that they also could threaten fruits and vegetables, if growers aren’t vigilant.

Soybean and cotton crops in Georgia have been especially hard hit, and vegetables that share their fields could suffer, too. “Resistant Palmer amaranth is devastating us,” says Stanley Culpepper, Extension agronomist at the University of Georgia in Tifton.

“It will cause us probably a $100 million loss in cotton alone this year” when all costs are considered.

“We’re hand-weeding 70-plus percent of our crop,” Culpepper says.

Since 2004, the issue has been strictly an agronomic one. But there have been two or three instances recently when tomatoes or bell peppers planted in fields that followed agronomic crops have fallen victim to Palmer amaranth, a pigweed species.

Fighting weeds

A couple of weeds also pose a problem in Florida, says Jay Ferrell, Extension weed specialist at the University of Florida in Gainesville. One is hydrilla, which appears in aquatic environments; another is Palmer amaranth, which he says, “is becoming a bigger problem every day,” especially with peanuts and cotton grown in the Florida Panhandle.

“There are populations of this weed that have been found to be resistant to the No. 1 most important herbicide that we have in peanuts, which is imazapic,” he says.

“When you lose a herbicide of that importance, you then have to put two or three different herbicides together in a program at much greater cost to even approach the same level of weed control.”

Generally, weed resistance in the state’s vegetable fields and groves is relatively rare, but there are two weeds in some Florida tomato fields—American black night shade and goosegrass—that are resistant to the herbicide paraquat.

Resistant Palmer amaranth also has been identified in two pecan orchards in New Mexico, says Jamshid Ashigh, Extension weed specialist at New Mexico State University in Las Cruces.

Fortunately, tests showed that the weeds only are resistant to glyphosate, marketed as Roundup, Touchdown and a host of other brands.

“If the growers, no matter what crop they have, use alternative herbicides that are effective on Palmer amaranth, they will be able to control them,” he says.

California threat

In California, Kurt Hembree, University of California Cooperative Extension farm adviser in Fresno County, says he’s found horseweed and fleabane in conservation tillage fields in western Fresno County, particularly where tomatoes and Roundupready cotton or corn are grown in rotation.

Onions and garlic are particularly vulnerable because they’re grown in areas of minimum cultivation and where herbicides don’t offer maximum protection, he says. Bell peppers are cultivated, so the threat is reduced, since tillage is an effective way to rid a field of weeds, he says.

Some problems with herbicide-resistant weeds have cropped up in California corn, alfalfa and cotton fields—all commodities that can be grown with a Roundup-ready system.

In areas where growers can cultivate their fields or use other effective products, Hembree says, “It’s generally not an issue.” Resistant weeds also threaten tree and vine crops, such as citrus, grapes and almonds, which are in the ground year after year and subjected to the same herbicide chemistries, he says.

Statewide surveys continue to show a high level of weed resistance to glyphosate.

The problem seems to be most pronounced in the state’s Central Valley.

Numerous tools are available to counter the weeds in orchards and vineyards, including pre-emergent products, such as Chateau, Matrix and Pindar GT, or postemergents Treevix and Rely.

Pigweed problem

Of all the weeds that can threaten a crop, pigweed is most problematic for Georgia Vegetable Co. Inc. in Tifton, Ga.

Once any regrowth pops up after the first cultivation, it’s very difficult to control, says Jamey Kilby, grandson of founder Robert Grist.

So far, pigweed is just a nuisance at the farm, which grows a variety of crops including snap beans, cucumbers, bell peppers and squash. Greens are the focus during the fall and winter.

But Kilby says the key to keeping the weed from getting out of hand is stopping it before it starts.

Weeds can be more difficult to control among vegetables than they are in cotton, he says.

“You’ve got to be very spray specific” in selecting an herbicide, Kilby says, or you could damage your crop along with the weeds.

A 2010 survey conducted for Greensboro, N.C.-based Syngenta Crop Protection indicated that 18 percent of acres in the corn and cotton belts harbored herbicide-resistant weeds, says Chuck Foresman, a technical brand manager for the company. In 2006, that figure was 1 percent.

Farm managers reported an average of a 5.5 percent yield loss per acre caused by glyphosate-resistant weeds and an increase of $16.90 per acre in herbicide costs.

“There is a direct cost to the grower when weeds become resistant, and they have to deploy practices to help manage the problem,” he says.

Veggie vigilance

Even though herbicide-resistant weeds pose their biggest threat to agronomic crops, fruit or vegetable growers should not let their guards down.

“This is kind of a wake-up call to ag production across the board,” Ferrell says. “We need to be thinking about what we’re doing, what we’re spraying, how we’re managing weeds.”

He called for a multi-faceted program approach to stay ahead of the evolution of herbicide-resistant weeds.

Tillage is an effective solution, but many tomato, pepper, eggplant and melon crops are grown on plasticulture, where tillage is not an option. Growers must then turn to labor-intensive hand weeding.

Ashigh advises growers to rotate their herbicidal modes of action, even if they don’t yet have herbicide-resistant weeds.

“If you use one herbicide this year, make sure you use another one next year,” he says.

Tankmixes or premixes that contain two or three different modes of action also are effective.

If a weed is resistant to one herbicide’s mode of action, the others might control it.

This process may be more expensive short term. But by delaying or preventing development of resistant weeds, you’ll come out ahead in the long run, Ashigh says.

Cultural practices, such as cover crops or mechanical cultivation, also can help reduce the total use of chemicals, and that’s a good thing.

“The more you apply chemicals, the more resistant [the weeds] are going to get,” he says.

For a list of herbicide-resistant weeds, visit http://www.weedscience.com.