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Fruit growers are always looking for better, cheaper ways of controlling vegetation under the trees in their orchards, but, in general, weed control gets more expensive and more complicated. That’s partly because of the way they’ve managed their herbicide programs in the past.

Either the weed spectrum shifted or weeds moved in that were tolerant to the herbicides they were using. So, they have to find other herbicides—newer ones, patented ones, that cost more.

But it’s also because of weeds, like horseweed, Palmer amaranth, and fleabane, that developed herbicide resistance on row-crop farms and blew into their orchards on the wind. Dwight Lingenfelter, a weed control specialist at Pennsylvania State University, spoke to fruit growers about weed management during the Mid-Atlantic Fruit and Vegetable Convention.

He reminded growers that weeds that tolerate herbicides occur naturally. Some weeds just can’t be killed by a given herbicide. So, growers should monitor orchards to identify problem weeds and see how herbicides are working.

New weeds that appear may just be taking advantage of an open opportunity, meaning a different product needs to be added to the mix. Resistant weeds are different. These are mutated weeds that were once killed by a given chemical but can now survive.

“Resistance drives up the cost of weed control,” Lingenfelter said.

“We have to rely on currently available herbicides for the foreseeable future.”
—Dwight Lingenfelter

Worldwide, 162 species of weeds have developed resistance to one or more kinds of chemicals that once killed them. Of these, 27 are orchard weeds, he said, but not all of them are in orchards in the United States. Worldwide, 142 weed species have become resistant to the ALS (acetolactate ­synthase) inhibitors, like Sandea (halosulfuron-methyl) and Matrix (rimsulfuron).

Seventy-two species have become resistant to triazines and other photosynthesis inhibitors, like Princep (simazine), ­Karmex (diuron), and Sinbar (terbacil). Twenty-nine are resistant to paraquat, and—most disturbing to many—25 have become resistant to glyphosate, the world’s most widely used herbicide. Growers today rely on herbicides that fall into 11 groups, each characterized by its mode of action.

“No herbicides with new mechanisms of action are in advanced development trials,” Lingenfelter said. “The last new mechanism of action was introduced more than 20 years ago. Therefore, we have to rely on currently available herbicides for the foreseeable future. Strategies that successfully delay and mitigate the evolution of herbicide-resistant weeds must be in place to preserve and sustain herbicides as resources in weed management.”

Not easy

Dr. Brad Majek, a weed control specialist at Rutgers University in New Jersey, says that growers should think of controlling weeds starting in the fall, not the spring, and will probably have to make two applications a year. But think in terms of starting in the fall, he said, when weeds like horseweed and other winter annuals are in the rosette stage and are easier to kill.

Once trees are nearly dormant, herbicides like 2,4-D are effective—and cheap. Nearby grapes are not at a stage where they can be damaged. It’s also effective on some perennial weeds.

“In the fall, perennials are building root reserves, so the sugars are moving down, and the herbicides go with the flow,” he said. “In the spring, the weeds are getting tougher, and you’re working against the flow.”

Princep, another old chemical that is less expensive, is a good one to add to the tank mix anytime. For some reason, triazines always seem to make other herbicides work better, he said. Because of the growing presence of glyphosate-resistant weeds, especially horseweed, Majek thinks that glyphosate has been relegated to the role of spot treatments. It is still a very effective weed killer where resistance is not a factor.

A couple of new materials that have recently joined the ranks are Stinger (clopyralid) and Alion (indaziflam). Alion should be applied in the fall because rainfall activates, and reactivates, it, providing year-long weed control on annual grasses and annual broadleaves.

Stinger, which has been labeled for stone fruits for a few years, has been labeled for apples and is very effective against composite broadleaf weeds. It is also effective against clover.

“We’re no long enthusiastic about clover in orchards,” Majek said. “If you read insecticide labels, many say, ‘Do not use when bloom is present in the orchard.’ That means all bloom, not just fruit bloom.” Insecticides can kill bees and other pollinators whenever they are in the orchard, and bees are attracted to dandelions and clover after the fruit bloom season is over.

Alternate

No matter what specific chemicals the tank mix contains, Majek would like to see the problem weeds addressed by two chemicals with two different modes of action.

Alion, the new product, provides long residual control, but, he says, “Don’t use it alone. Use resistance management.” Majek and Lingenfelter recommend that growers become familiar with herbicides grouped by mechanism of action, just as they have with fungicides and insecticides.

The table shows the different groups and some of the herbicides contained in each group. Use of tank mixtures combining modes of action is a good step in resistance management, Majek said. Make sure your target weeds can be killed by that mode of action. Knowing the weeds you have, and what products are effective against them, is obviously the first step. •