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Micro-pests inflict macro-headaches; but fungi and pheromones can spell relief.

A mutant fungus and some sexual confusion made life difficult for soybean cyst nematodes last summer.

It should happen again this year, in expanded tests by the Agricultural Research Service. The payoff, 5 to 10 years from now, could be new, safe alternatives to conventional pesticides for one of the soybean farmer's worst enemies.

Recent figures aren't available, but in 1981, scientists estimated that the soybean cyst nematode, Heterodera glycines, cost growers $420 million a year in crop losses.

The nematodes-transparent, microscopic, eel-shaped worms-drain nutrients from soybean roots. One hundred grams of soil may contain hundreds of nematodes.

Controlling the pests with chemical nematicides can be expensive, says plant pathologist Susan L.F. Meyer, who is with the ARS Nematology Laboratory in Beltsville, Maryland. And some nematicides, she adds, have been taken off the market because of concern that they may pollute groundwater.

Today's soybean farmers have two other options. "They can rotate soybeans with a crop, such as corn, that is not a host for the nematode," she says. "Or they can plant a resistant soybean variety. But in many cases, nematodes that aren't affected by the plant's resistance rebuild to damaging levels after several years."

The Nematology lab-the only one in ARS devoted exclusively to nematode research-is searching for longer lasting, more effective alternatives.

In early June, at sites in Delaware and Maryland, scientists will begin the second outdoor test of two potential altematives.

One is the female nematode's pheromone, or sex attractant--and some related compounds. The other is a fungus genetically altered by Meyer in the lab.

Former ARS microbiologist Robin L. Huettel and coworkers isolated and identified the pheromone. The related compounds were devised by chemist Albert B. DeMilo at Beltsville's Insect Chemical Ecology Laboratory.

Huettel, DeMilo, and Meyer have filed patent applications on their discoveries. The research trials are being run under a cooperative research and development agreement with Crop Genetics International, a company in Hanover, Maryland.

Huettel, the Nematology lab's research leader since 1986, recently joined another USDA agency, the Animal and Plant Health Inspection Service (APHIS) in Hyattsville, Maryland. There, she serves as chief operations officer for planning and designing APHIS programs for eradicating plant pests, including nematodes, diseases, and insects.

Huettel will continue to keep a close eye on the tests, though.

"If the fungus and pheromone prove themselves in large-scale tests," she says, "farmers could use them singly or in tandem. The best control might be to use both in conjunction with resistant crop varieties. That way, nematodes should take much longer to rebuild to damaging numbers."

Double-Whammy Ends an Underground Lifestyle

When immature nematodes leave their eggs, they wriggle through the soil in search of a young, tender soybean root. Using a needlelike mouthpart called a stylet, they penetrate the root and begin robbing nutrients from ceils.

When a male nematode matures, he leaves the root to seek a mate, drawn by the female's pheromone. As the pheromone grows in potency, it "tells" him he's nearing a female. He then begins a dancelike rite-coiling and uncoiling-in an effort to mate.

But if-as in the tests last summerthe male senses pheromone or a similar compound in all directions, he dances alone or not at all. Instead, he wanders, unable to home in on a female amid all the chemical "noise."

In 1989, Huettel and former ARS chemist Howard Jarfee identified the pheromone as vanillic acid. Isolating and identifying the pheromone was painstaking work that took about 6 years. Each chemical analysis required extracts from 40,000 female nematodes.

Huettel says the female nematode appears to make the pheromone by digesting lignin, the main component of the walls of the root ceils that make up her diet.

If the male nematode is fortunate and finds a female, he coils around her lemon-shaped body sac, deposits sperm, and soon dies.

The fertilized female lays about 300 to 500 eggs inside the sac and within a gelatinous mass just outside it. She dies shortly thereafter, and her body ttu'ns brown and only then is properly called a cyst. It serves as a protective incubator for the eggs inside it.

All this reproductive effort is for nothing, however, if Verticillium lecanii fungi prevent the eggs from hatching.

Meyer notes that this fungus had earlier been described as a parasite of the soybean cyst nematode. But the natural, or wild type, V. lecanii has a major weakness as a potential biological control agent. "The strains tested were not very effective when applied at rates we consider practical for commercial uses," she says.

To try to develop more effective, hardier strains, Meyer designed a series of lab and greenhouse studies. She bombarded about 39,000 V. lecanii spores with ultraviolet rays to induce potentially useful genetic mutations. She also exposed the spores to benomyl, a chemical registered for use on soybeans to control fungus diseases.

In earlier studies, she explains, scientists at Beltsville's Biocontrol of Plant Diseases Laboratory had found that some benomyl-resistant strains of other biocontrol fungi were more effective against their disease targets. "We wanted to see if that approach would work with V. lecanii. If so, we could improve its biocontrol potential while increasing its resistance to a fungicide that a farmer might be using."

Apparently, it did work. Meyer's studies yielded four promising strains of benomyl-resistant V. lecanii mutants. Meyer tested one strain outdoors for the first time last summer in Ingleside, Maryland. Some of the plots also had Huettel's nematode pheromone-or a similar compound.

Huettel ran additional, separate tests of the pherornone compounds in field plots in Delaware.

At Ingleside, nematode populations were 70 percent less on soybean plots protected by the fungus than on plots where nernaticide was mixed into the soil. The pheromone did nearly as well, cutting populations by 66 percent.

"If the reductions hold up in further tests, either treatment would constitute an effective control," Huettel says.

When both the fungus and a compound related to the pheromone were used, reductions were 86 percent. "That's a dramatic reduction," she says, "but we need to repeat the tests on a larger area."

This test compound, syringic acid, was one of 29 compounds provided by DeMilo. Most have yet to be screened for activity against the soybean cyst nematode.

At the test site in Laurel, Delaware, soybean yields were about 30 percent higher on plots protected by syringic acid. Another compound increased yields 57 percent. Huettel cautions that repeated large-scale testing is essential to get a good idea of how the pheromone compounds or the fungi will benefit yield.

The fungus and pheromone are carried by alginate pellets originally formulated by scientists at the Biocontrol of Plant Diseases Laboratory in Beltsville and the ARS Southern Regional Research Center in New Orleans. The pellets consisted of a cartier made of pyrax, a finely ground material containing quartz, for the pheromone. Huettel and Meyer used a bran carrier for the fungus and fungus/pheromone combinations.

In the tests, pellets were placed atop each soybean seed at planting time and gradually released their active ingredients. This year, the researchers will place the pellets a centimeter or so away from the seed. "When we determine the best placement, it will be one that farmers can easily do by adjusting their conventional planters," Huettel says.

For now, the scientists are focusing on making the fungus/pheromone duo work well on soybean cyst nematodes. But they say the approach might also work on other parasitic nematodes of field and orchard crops in the United States.

If so, farmers will have new, environmentally safe weapons against tiny worms that cause big problems.By Jim De Quattro, ARS.

Susan L.F. Meyer is with the USDAARS Nematology Laboratory, Room 152, Bldg. 011A. Phone (301) 5045660. Albert B. DeMilo is with the USDA-ARS Insect Chemical Ecology Laborator, Room 009, Bldg. 010. Phone (301) 504-6138. Both labs are at the Beltsville Agricultural Research Center, 10300 Baltimore Ave., Beltsville, MD 20705-2350.
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Author:De Quattro, Jim
Publication:Agricultural Research
Date:May 1, 1992
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