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Pitting fungus against fungus.

They're feuding in Georgia. No, it's not the Hatfields and McCoys, but two fungi that are closely related. And Agricultural Research Service scientists are using this rivalry as a promising food safety tool.

When fungi such as Aspergillus parasiticus and A. flavus infect peanuts, they can produce a natural toxin known as aflatoxin, which means financial losses to peanut growers.

Aflatoxin contamination costs the nation's peanut growers an estimated $25 million annually, according to the Peanut Advisory Board in Atlanta.

The peanut industry's goal is to eliminate this potent natural toxin by the year 2000.

Scientists at ARS' National Peanut Research Laboratory in Dawson, Georgia, have discovered an A. parasiticus strain that does not produce aflatoxin. They have applied for a patent on this non-toxin-producing strain for use in controlling the hamfful A. parasiticus strains in peanut fields.

The toxin-producing strains found naturally in soil would be replaced by the non-toxin-producing strain that is added to the soil, says Richard J. Cole, a co-inventor and research leader at the Dawson lab. Peanuts subjected to lateseason drought stress would be invaded predominantly by the competitive fungus, which doesn't produce aflatoxin.

"It's in those last days before harvest when peanuts under drought stress are most susceptible to aflatoxin contamination," Cole says.

The U.S. Food and Drug Administration requires that grain and finished products with 20 parts per billion or more of aflatoxin not be sold for human consumption or animal feed with certain exceptions. One part per billion is equivalent to less than 1 drop in 10,000 gallons.

Many states and export markets are considering stricter tolerance levels for aflatoxin, intensifying the peanut industry's eagerness to eliminate any chance of contamination.

"The only known methods for controlling preharvest aflatoxin contamination in peanuts are irrigation or early harvest," Cole says. "But irrigation is an expensive option not available to most peanut growers--and harvesting early reduces quality and yield."

Scientists at Dawson initially isolated the first non-toxin-producing fungus in 1980, Eventually, Cole and coinventors Joe W. Dorner and Paul D. Blankenship chose three strains as prospective aflatoxin prevention tools. But first, the three had to pass a few tests.

Researchers wanted to see if these strains could survive the hot temperatures and dry soils where their toxinproducing relatives thrive. And they needed to make sure the good strains always failed to produce toxin.

It was also important to know if these strains were rich producers of persistent-survival structures called sclerotia, which should enhance survival and competitiveness in the soil.

In a 3-year study that began in 1987, the three scientists saw dramatic reductions in aflatoxin contamination when beneficial fungi were applied to the soil during conditions that were ripe for aflatoxin formation.

Peanuts from treated soil contained 11, 1, and 40 parts per billion aflatoxin in crop years 1987, 1988, and 1989. Meanwhile, those from untreated soil during the same period had aflatoxin concentrations of 531, 96, and 241 ppb.

"Significantly, the greatest effect was seen in edible peanuts," Cole says. "Aflatoxin concentrations in edible, treated peanuts in 1988 were by far the lowest ever observed during 9 years of research using a specialized environmental test facility."

While the parent A. parasiticus strain doesn't make aflatoxin, it produces a compound known as Omethylsterigmatocystin (OMS) that is chemically related to aflatoxin. With this in mind, Cole and co-workers developed two altered forms, or mutants, of the parent fungus that don't produce OMS.

"It is important that any mutant used as a biocontrol not produce any related chemical that is toxic or carcinogenic," Cole says. "One mutant was discarded because it produced versicolorin A, which is a distant chemical relative of aflatoxin."

The scientists evaluated the performance of the parent strain and the mutant as biocompetitive fungi. They found that both fungi were very effective in controlling aflatoxin contamination in edible peanuts.

In 3 years of testing, neither the parent nor mutant strains ever developed the ability to produce aflatoxin, thus making them appropriate candidates for controls, Cole says. The parent and mutant strains also demonstrated that they were highly competitive in the soil, compared with native A. parasiticus strains that produce aflatoxin.

But Cole and colleagues observed an interesting shift between the funsal population of the non-toxin-producing strains and natural strains in treated and untreated soils.

"When the tests were complete, the overall funsal population was no higher in soils treated with the beneficial strains than in untreated soil," Cole says. "This is an important ecological consideration; one would not want to drastically alter the normal levels of funsal populations."

He says research is under way to determine the most effective concentration and proper timing for the beneficial A. parasiticus strains to be applied to preharvest peanut soils. Also, scientists are working on various formulations and methods for peanut growers to apply the nonchemical aflatoxin control.

Although aflatoxin contamination will probably not be completely eliminated with this approach, Cole says, coupling it with other available techniques could go a long way toward ensuring an aflatoxin-free supply of edible peanuts.-By Bruce Kinzel, ARS.

Richard J. Cole, Joe W. Dorner, and Paul D. Blankenship are with the USDA-ARS National Peanut Research Laborator, 1011 Forrester Drive, S.E., Dawson, GA 31 742. Phone (912) 995-4441.
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Title Annotation:using two rival fungi to prevent aflatoxin contamination of peanut crops
Author:Kinzel, Bruce
Publication:Agricultural Research
Date:May 1, 1992
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