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Ten weeds we could live without.

Ten Weeds We Could Live Without

They're the bane of farmers and backyard gardeners, the scourge of the plant world. One word says it all: weeds.

In this unsavory kingdom, some are hated more than others. A recent survey of weed specialists across the country fingered some of the foulest of the leafy invaders nationwide. What follows are profiles from the specialists' "weeds hit list," and how scientists with USDA's Agricultural Research Service are fighting back.

One of the most notorious weeds is field bindweed, described by one researcher as "an out-of-control morningglory."

"Someone started digging field bindweed's roots one time and found they went down more than 6 1/2 feet," says Paul E. Boldt at ARS' Grassland, Soil, and Water Research Laboratory at Temple, Texas. "The seeds can lie in the soil for 30 years and still germinate."

Grain and forage yields are reduced by 20 to 80 percent because of competition from this creeping, crawling vine. Yield loss estimates for California alone run at least $25 million per year.

Hopes are pinned on a defender so tiny that it cannot be seen with the naked eye.

Discovered by ARS scientists in Greece, the microscopic Mediterranean mite Aceria malherbae attacks field bindweed, gnawing its leaves. Thousands of the mites released in 1989 at sites near Bushland, Texas, and in New Jersey have survived and reproduced at both locations, although their impact against bindweed has as yet been minimal.

Aceria malherbae alone probably cannot whip the bindweed monster. But it can be an important team player in a biological control strategy against this weed, Boldt says.

"The mite feeds on the leaves, and I think we're going to have to find something that attacks the roots as well," he notes. "If its crown, roots, and leaves all are attacked, it surely won't survive."

Happily, there are candidates for the other assignments. These insects - a flea beetle that attacks the crown and a second beetle whose larvae feed on bindweed's roots - are found in Europe.

"If we could get these three together, this would give us a good start on controlling field bindweed," Boldt says.

Annual morningglories, also on the weeds hit list, are notorious for their ability to spread. But for all its wandering, morningglory may find one of its worst threats at its very roots, in the soil. ARS microbiologist Robert J. Kremer has identified several strains of root-dwelling bacteria called rhizobacteria that will attack a variety of weeds, including morningglory.

Kremer's weed-fighters come primarily from the rhizobacteria genera Bacillus, Erwinia, Pseudomonas, and Flavobacterium. These bacteria break down the weed's root cell walls or deliver toxins to its leaves, disrupting production of needed chlorophyll.

Tests indicate these natural weapons could get an extra boost from commercial products such as the herbicide butylate and the insecticide carbofuran.

These commercial compounds stimulate the weeds to germinate, explains Kremer. In the process of germination, the weed seed releases nutrients that attract and nourish the attacking rhizobacteria. In addition, the germinating weed's roots begin developing and offering a resting place for the aggressive bacteria.

Kremer's microbes also work against another noxious weed, this one with the picturesque name of velvetleaf.

In greenhouse tests with this common pest of row crops, one rhizobacterium slashed top growth of 14-day-old velvetleaf seedlings by 88 percent, compared with uninfected plants.

Taproots of the microbe-infected seedlings were less than half the length of uninfected seedlings, and lateral root growth was also reduced. Shorter roots mean less nutrients and water and greater susceptibility to stresses such as herbicides.

If left unhindered, a single healthy velvetleaf plant can set up to 8,000 seeds, and those seeds are capable of remaining viable after half a century in the soil. But the seeds could also help lead to the weed's undoing, according to Kremer. That's because an insect that resembles the boxelder bug has a voracious appetite for velvetleaf seeds.

The insect, Niesthrea louisianica, uses its sharp sucking mouthparts to pierce the seed's coat and drain the contents. And there's more bad news for velvetleaf: Fungi that hitch rides on the bug's stomach, back, and legs then move in to infect the weakened seed. Only about 5 percent of seeds attacked can survive and germinate, Kremer notes.

The hitchhiking fungi are mostly from the genera Fusarium and Alternaria. Studies are now focusing on finding other fungi that might wreak even more damage than the ones already in place.

Also targeted by ARS researchers is purple nutsedge. Farmers spend millions of dollars annually combatting this weed. But thanks to ARS research, those amounts may someday be trimmed. Agency scientists have found that adding inorganic salts such as ammonium chloride to the herbicide boosted control by 30 to 50 percent.

The herbicides MSMA (monosodium methanearsenate) and glyphosate, currently used on the weed, are unreliable and expensive, according to Chester G. McWhorter, a plant physiologist at ARS' Southern Weed Science Laboratory at Stoneville, Mississippi.

Another weed that's unfortunately no stranger to soybean farmers is common cocklebur, an invader that has run rampant across the entire United States. But studying this plant's notable aggressiveness could lead to clues on hindering its progress, according to ARS scientist Edward W. Stoller.

A plant physiologist at ARS' Crop Protection Research Unit at Urbana, Illinois, Stoller says that the 4-foot-tall weed towers over soybeans, pushing its leaves both into and above the soybean leaf canopy. The weed competes with the crop for light; Stoller and fellow scientists have shown this shading can cut bean yields by more than 15 percent.

The researchers' expanded knowledge about the weed's biology will lead to improved means of control, according to Stoller. "By understanding how the weed grows, we may be able to reduce herbicide use and also find alternative methods to fight it," he says.

Another all-too-familiar weed in row crop fields is pigweed, an annual broadleaf that has harassed U.S. corn and soybean farmers for years. The good news is that for all its ability to spread and thrive, pigweed appears vulnerable to natural chemicals put out from the roots of such common plants as sunflowers and sorghum. Laboratory tests by ARS scientists have shown, for example, that populations of pigweed decline sharply when planted alongside germinating sorghum.

The secret, says ARS scientist Donald D. Bills, is allelopathic chemicals released by the nonweed plants. Further research on these allelopathic substances, which include a broad class of compounds called phenolics, could lead to breeding other crops with built-in weed resistance.

Researchers might also successfully synthesize these compounds to produce herbicides that are easy on the environment, says Bills.

Such natural weapons can pop up in surprising places. For example, researchers at ARS' U.S. Vegetable Laboratory at Charleston, South Carolina, tested extracts from the skin of a sweetpotato bred to resist insects. The sweetpotato extracts proved very effective at blocking germination not only of pigweed, but velvetleaf and morningglory as well.

Another notorious name on the "weeds hit list" is common lambsquarters. The sight of its distinctive purplish leaves pushing up means crops face a tough struggle against this highly competitive weed.

Fortunately, the scales are being tipped in favor of the crops by ARS scientists in Washington and Illinois. They've found that slow release of herbicides may boost weed kill of lambsquarters.

In lab tests, a starch-encapsulated herbicide that kills lambsquarters and a number of other weeds remained in the top half-inch to inch of soil when water was added to simulate irrigation. Non-encapsulated herbicides leached downward 5 to 6 inches, the scientists found.

When herbicides are held closer to the soil surface, they're more likely to come in contact with - and work against - weed roots and seeds, according to ARS research chemist Robert E. Wing, who worked on the study at the National Center for Agricultural Utilization Research at Peoria, Illinois.

Herbicide sensitivity in wheat poses a problem in combating wild oats, another of the worst weeds in America. Battle plans have been complicated by the fact that a herbicide widely used to control the weed also injured some wheat varieties.

But ARS scientists have discovered that tolerance to the herbicide, called difenzoquat, is inherited and can easily be bred into new wheat varieties.

Difenzoquat won't reduce yields of tolerant wheat, says Robert H. Busch, a geneticist in ARS Plant Science Research at St. Paul, Minnesota. "Breeding wheat for herbicidal tolerance could give farmers more choices in fighting wild oats," Busch says.

Northern Plains farmers face still another "worst weed" in Canada thistle. In North Dakota alone, this weed reduces yields of spring wheat by million of bushels each year.

But there's new hope in the form of a sequence of herbicide treatments developed by ARS scientists.

In a 3-year study, the scientists applied glyphosate to thick stands of thistles in the fall, followed by bromoxynil plus MCPA in the spring. By the third fall, reports ARS agronomist William W. Donald, thistle numbers were down from about 30 per square yard to only 1. All three herbicides are approved by the Environmental Protection Agency, Donald notes.

ARS scientists are still considering strategies to counter common ragweed, a native of North America. By comparison, it's a relative newcomer to Europe and the Soviet Union. But it's settled into the Soviet Union to such a degree that last year Oleg Kovalev, an insect hunter from Leningrad, came to this country to look for insects to devour it. Kovalev works at the Zoological Institute of the USSR's Academy of Sciences.

Kovalev was joined on one leg of his 5-week, 10-state search by ARS entomologist Stephen D. Hight. Hight is based at ARS' Insect Biocontrol Laboratory at Beltsville, Maryland. Kovalev's bug hunt was historic as the first by a Soviet scientist under some five-year research agreements reached in 1989 with ARS.

While some scientists are sharing beneficial insects globally, other have begun to think regionally, says Hight.

"There are different species of insects in different parts of the United States that feed on ragweed," he notes. "The thought is that you might bring the eastern insects to the West and the western insects to the East. But first, of course, we'd need considerable studies to ensure the insects won't harm nontarget plants. This would be a small-scale version of what Kovalev's done." - By Sandy Miller Hays, ARS

PHOTO : A costly crop pest, tall morningglory winds around these corn plants competing for sunlight and moisture.

PHOTO : Pigweed

PHOTO : Lambsquaters

PHOTO : Wild oats.

PHOTO : Canada thistle

PHOTO : Common ragweed
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Author:Hays, Sandy Miller
Publication:Agricultural Research
Date:Jun 1, 1991
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