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Sugarbeets: controlling pests in the next century.

George Washington loved to experiment in his garden and fields at Mount Vernon. Some of the crops that struck him as novel and innovative in the 1790's are standard today on the American agricultural scene - alfalfa, for example, and cultivated pecans. Others, such as the forage legume called sainfoin, are still seeking a toehold in U.S. agriculture.

One current crop that would have been new to Washington's America is the sugarbeet, from which crystalline sugar is made. Introduced in the late 1700's, sugarbeets gradually gained acceptance until they now provide slightly over half of the sugar produced in this country.

In the 1930's. their rising significance caused U.S. Department of Agriculture plant geneticist George H. Coons to go plant exploring. During his travels through seven European countries, he looked for sugarbeets and their wild relatives that might resist a costly leaf spot disease caused by the fungus Cercospora beticola.

Coons' scientific harvest was, unfortunately, a lean one.

After carefully crossbreeding wild plants and cultivated sugarbeet varieties, he found little or no resistance in the hybrid offspring. Afterward, he wrote with regret that his "tests were given up, probably too soon." In this, he turned out to be right. Years later, colleagues found the resistance he had searched for in the materials he'd gathered.

William M. Bugbee, a plant pathologist with USDA's Agricultural Research Service, took the fight against sugarbeet diseases a few steps closer to victory when he isolated and identified certain bacteria from inside and around the roots of sugarbeets in the early 1970's. These bacteria did not cause disease, Bugbee noted.

But it wasn't until the late 1980's that ARS scientists at Fargo, North Dakota, began to consider a use for these seemingly meaningless bacteria - specifically, to serve as work-horses for biological control of sugarbeet pests.

The pest at the top of plant geneticist Garry A. Smith's "most wanted" list was the sugarbeet root maggot, which was emerging as a serious problem in North Dakota and Minnesota farms of the Red River Valley.

The maggot had shown itself capable of reducing yields by 10 to 50 percent by feeding on the surface of tap and lateral roots. The larvae consume a mixture of plant juices and bacteria and form slime tunnels along the roots.

But chemical insecticides seemed only a short-term solution, Smith noted, since soil was sometimes too dry for the insecticides to work well and they appeared to be losing their effectiveness.

So, with partial funding from the Beet Sugar Development Foundation and increased federal funding, the Fargo lab embarked on a renewed search for natural weapons against the sugarbeet root maggot.

Smith envisioned genetically engineering Bugbee's ubiquitous bacteria to produce a fatal Bacillus thuringiensis (Bt) toxin and then lying in wait for the voracious sugarbeet root maggot.

Derived from a common bacterium and used against crop pests for more than 30 years, Bt toxin is harmless to humans and livestock.

Smith says that another biocontrol route - genetically engineering a sugarbeet variety to repel or resist the pest - would take more time, which the sugarbeet industry could ill afford.

Alternative Biological Strategies

"In our long-range view, we embrace the idea of genetically engineering sugarbeet plants to resist pests," Smith says.

One method that might help with this strategy is if a strain of Agrobacterium tumefaciens can be found that infects sugarbeets. The idea is to use the microbe to ferry DNA material from Bt into the hereditary makeup of the sugarbeet plant. So far, molecular biologist Chris A. Wozniak has discovered several natural strains in the soil near sugarbeets, but none that actually infect sugarbeets.

The Fargo scientists are trying to identify the Bt strains that produce the most potent toxins against the maggot, with the hope of applying the bacterium directly to plants without first engineering the toxin gene. This would allow them to pit Bt directly against the maggots.

If that doesn't work, perhaps a bacterium such as Xanthomonas maltophilia (Xm) from the sugarbeet's microbe-rich ecosystem could be redesigned to make Bt toxin.

Wozniak and ARS plant physiologist John D. Eide have identified about 500 bacteria from among 2,000 bacteria samples collected from the sugarbeet root maggot's midgut and slime tunnels and from the surface of sugarbeet roots.

Among biological samples from the maggot-infested cropland in Minnesota, North Dakota, Montana, Wyoming, and Nebraska, the researchers found Xm everywhere, making the bacterium a likely candidate for their genetic engineering work.

The ubiquity of Xm makes the scientists suspect the microbe might be needed for the maggot to thrive. Indeed, after rearing some gern-free maggots from disinfected eggs, the researchers found none capable of larval development unless Xm was added to their diet.

By genetically engineering Xm to make Bt toxin, "we hope to create a situation in which the sugarbeet root maggot can't live with the microbe and can't live without it," says Wozniak.

Serendipitous Nematode Study

It came about in the fall of 1992, when Smith and ARS plant pathologist David T. Kaplan of Orlando, Florida, found themselves at the same location on temporary assignments.

As they sat at dinner one evening, they talked about the work they did back home. Their discussion led to a question: Could a microscopic nematode known to Kaplan that had shown promise against a citrus pest also be put to work in northern sugarbeet fields?

The prospect was appealing because the nematodes in question, from the enus Steinernema, kill only insects, leaving plants and warm-blooded animals untouched.

Steinernema nematodes are like a "gift that keeps on giving," says Wozniak. They enter natural openings in their host, then release bacteria from their intestines that get into the maggot's blood and produce deadly toxic waste products.

These bacteria multiply in the maggot cadavers and serve as food to nourish the nematodes as they prepare to reproduce. If a nematode harbors a potent species of these bacteria, it typically produces thousands of offspring.

Smith, Kaplan, and their colleagues set up laboratory studies at Orlando and obtained permits to allow interstate transport of the insects and nematodes and to conduct field studies at Fargo. The U.S. Environment Protection Agency had earlier determined that Steinernema nematodes, with their associated bacteria, are exempt from registration requirements under the Federal Insecticide, Fungicide, and Rodenticide Act.

In preliminary North Dakota field tests, all of six Steinernema strains - representing the carpocapsae, feltiae, and glaseri species - proved capable of entering the sugarbeet root maggot and reproducing as their hosts are about to pupate.

Besides learning that sugarbeet root maggots are infected in the larval stage by Steinemema, researchers have discovered that the adult or fly stage can be infected within 2 hours by these nematodes and die within a day. This year, they are testing fly traps baited with an attractant containing nematodes, to determine the rate of adult infection in the field.

But researchers aren't yet certain how well the nematodes suppress the maggots. Even in plots without the beneficial nematodes, sugarbeets grew too well to visibly suffer much maggot damage.

So the studies have been refined to include more field sites and nematode applications at different stages of crop growth.

"Our outlook for success is good," Smith says. "Although nematodes may not become the complete answer, they could help fill a big control gap if available insecticides should ever be taken off the market."

Also, the nematodes might be relatively inexpensive - perhaps less than 10 cents per million, or about $30 for enough to treat an acre. A large ethanol-producing company is licensed to rear them in fermentation tanks. They are being marketed through garden supply firms.

"We don't know yet whether farmers would need to apply nematodes each year, or if sufficient numbers of infective juveniles would overwiter," Smith says.

Breeding in Plant Resistance

Can the sugarbeet root maggot problem also be curtailed by breeding beets with insect resistance? In research plots near St. Thomas, North Dakota, that are normally heavily infested, ARS plant geneticist Larry G. Campbell has developed breeding plant populations with resistance comparable to that achieved in plots treated with chemical insecticides.

But because the beets lack yield potential and other desired qualities, and because genes affecting resistance may be widely dispersed in the plants' chromosomes, Campbell says resistant sugarbeets won't soon be commercially developed.

Campbell is evaluating sugarbeet root maggot resistance in 17 wild sugarbeets initially screened by entomologist Albin "Andy" W. Anderson of North Dakota State University from about 190 accessions obtained from the USDA-ARS Plant Introduction Station in Ames, lowa.

Other conventional breeding work to develop beets resistant to Cercospora leaf spot is under way, says Smith, including some that plant explorer Coons had collected.

Cercospora now infects about half of all U.S. sugarbeet acreage, causing millions of dollars worth of losses each year.

Recently, Smith and Eide found high levels of the enzyme chitinase in sugarbeets that have a natural ability to withstand Cercospora. They are now preparing antibodies to chitinase that researchers may use in screening sugarbeet seedlings for resistance with a test known as ELISA (enzyme-linked immunosorbent assay).

It is expected that an ELISA test will greatly speed up the search for resistant sugarbeets.

Often, the value of genetic resources may not be recognized until decades after they are collected, says ARS plant geneticist Devon L. Doney. He has helped plan the Third Biennial Conference of the World Beta Network, an international forum of sugarbeet geneticists set for August 4-6, 1993, in Fargo.

Doney points to a devastating fungus-transmitted virus disease of sugarbeets, Rhizomania, first discovered in the United States in 1983.

From germplasm collected by Coons in his plant explorations more than 50 years ago, ARS plant geneticist Robert T. Lewellen and colleagues at Salinas, California, recently developed breeding lines that resist Rhizomania, along with virus yellows, powdery mildew, and other important sugarbeet diseases. Commercial sugarbeet varieties derived from these lines may be available in 6 to 8 years.

During the past 5 years, Doney and researchers with North Dakota and Texas agricultural experiment stations, Beet Sugar Development Foundation, and ARS laboratories in Fort Collins, Colorado, and Salinas, California, have evaluated about 400 accessions from the collection at Ames for traits that might benefit the sugarbeet industry.

Doney also breeds exotic germplasm to produce more sugar and impart hybrid vigor. He hopes to increase genetic diversity of sugarbeets used in advanced breeding programs.

Diversity of genes in present-day commercial hybrids is low, Doney says. Beet sugar-processing technology is less than 200 years old, and early selective breeding was narrowly focused on sugar yield. Before then, Beta species were cultivated as red garden beets, leafy vegetable beets, Swiss chard, and fodder beets.

In several overseas trips, Doney has explored native habitats for Beta germplasm, searching mostly near the Mediterranean Sea, but also in northern and eastern Europe.

His latest expedition was to Egypt in 1992, to find out whether many wild beets were left in Egypt's intensively farmed areas.

He reports, "We found lots of them because farmers have grown wild beets as a leafy vegetable for many centuries along Middle Egypt's Nile. And in the Nile Delta, where beets are regarded as weeds, seeds have been dispersed along many canals and ditches."
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Author:Hardin, Ben
Publication:Agricultural Research
Date:Aug 1, 1993
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