Genetic Resistance a Key To Controlling
Ask Ron Henning how important the aflatoxin problem is to the peanut
industry and you get this response: "On a scale of 1 to 10, it's an
Henning, a scientific consultant to the National Peanut Council, calls
aflatoxin the Achilles heel of the peanut marketing system. Produced by certain
strains of Aspergillus fungi, this toxin canunder drought
conditionscontaminate peanuts in storage or in the field.
Aflatoxin is carcinogenic. Peanuts with more than 20 parts per billion
cannot be sold for human consumption but must be processed into lower value
products. That, of course, means losses for peanut growers and shelling
companies that buy peanutsmeaning a potential loss of jobs and income for
Peanut losses from aflatoxin are hard to pin down. They vary from year to
year based on weather and other factors, according to Henning and peanut
council president Kim Cutchins. In drought years, aflatoxin is usually worse,
because the peanuts are weakened and more susceptible to attack from the fungi.
Also, the Aspergillus fungi take over to a greater extent in the
soil, out competing other fungi and producing more toxin.
According to USDA estimates, U.S. growers in 1995 produced about 3.5 billion
pounds of peanuts, which not only were sold fresh but wound up in everything
from candy bars to peanut butter. Peanuts have become a staple food item in
many households, providing a valuable source of protein and other nutrients.
So it's no surprise, Cutchins and Henning say, that aflatoxin remains the
industry's number-one food safety priority.
"We're working with all the resources we have to reduce aflatoxin at
every level," says Henning, noting that the industry has a goal to
eliminate aflatoxin from peanuts by the turn of the century.
A key to reaching that goal, he says, is finding peanuts that have genetic
resistance to the fungi that produce the toxin. Two U.S. Department of
Agriculture scientists at Tifton, Georgia, have discovered peanut germplasm
that resists both aflatoxin-producing fungi and insects that can damage peanut
Robert E. Lynch, an entomologist with the Agricultural Research Service at
Tifton, has been studying the connection between aflatoxin contamination and
insect damage since the late 1980s, when he traveled to Burkina Faso, in
western Africa, to study peanut damage caused by termites.
These notorious wood feeders are also fond of the cellulose fibers in peanut
pods--the outside shells that many a baseball fan has cracked. When termites
feed on the pod, they often weaken or pierce itcreating an entrance for
aflatoxin-producing fungi, Lynch says.
It turns out that the lesser cornstalk borer also feeds on peanuts in
Georgia and other peanut-producing areas of the United States, damaging peanuts
in much the same way that termites do.
Lynch and cooperators in Africa have found that two accessions, NCAC 343
from North Carolina and RMP 12 from Africa, were highly resistant to termites
when grown under drought conditions in Africa. "Only about 5 percent of
those resistant peanuts suffered pod damage, compared to 40 percent damage for
susceptible peanuts," he says.
Now Lynch and cooperators are testing those varieties at Tifton and other
locations to see if they have the same genetic ability to ward off the
cornstalk borer. He says that appears to be the case in preliminary laboratory
and field studies.
Meanwhile, Corley C. Holbrook, Jr., an agency plant geneticist also based at
Tifton, has analyzed data for the 7,000 accessions in the peanut germplasm
collection to develop what is called a core collectiona more manageable
group of 831 accessions representing the genetic diversity of the entire
By screening the core collection, Holbrook says he's found peanut germplasm
that, in field studies, had 70 percent less aflatoxin than Florunner, a common
variety planted in Georgia and other areas.
"We have about 60 accessions that show promise," Holbrook says.
"The goal is to identify the best 10 to 20 that can be used to breed
Holbrook says it is important to find peanut germplasm that resists drought,
aflatoxin-producing fungi, and feeding insects. Such germplasm would go a long
way toward minimizing the problem, he says, if it could be bred with Florunner
and other varieties that produce high yields and top-quality peanuts.
Florunner, released by the University of Florida in the early 1970s,
still accounts for about 50 percent of peanut acreage but is not aflatoxin
resistant. Right now, we dont have any aflatoxin-resistant peanut
varieties, Holbrook says.
At the National Peanut Laboratory in Dawson, Georgia, researchers are taking
a slightly different approach. They are working with private companies to
develop ways to mass-produce Aspergillus strains that do not produce
The Dawson scientists found that spreading the harmless fungi in the soil
crowds out the aflatoxin-producing strainsserving as a biological control
against contamination. ARS and outside cooperators plan field tests to develop
ways to mass-produce and deliver the harmless fungi as a commercial product
peanut growers can use.
And ARS researchers at New Orleans, Louisiana, are looking at how the fungi
form aflatoxinwith an eye toward finding ways to block the fungi's toxin
Henning says all these approaches will ultimately be needed to effectively
control aflatoxin in peanuts. "I don't think there's a silver
bullet," he says. "We have to approach this from a lot of different
angles." By Sean Adams, ARS.
Holbrook, Jr., is at the USDA-ARS Crop Genetics and Breeding Research Unit,
Tifton, GA 31793; phone (229) 386-3176.
"Genetic Resistance a Key To Controlling Aflatoxin" was
published in the July
1996 issue of Agricultural Research magazine.