DNA Profiling: Guarding
Against a Plant Disease Epidemic
A healthy wheat head (left)
stands in contrast to one
inoculated with Fusarium
graminearum showing severe
symptoms of Fusarium head
blight disease (right).
|Curiosity compels some people to
research their family tree. But to ARS
scientists like microbiologist Kerry O'Donnell and molecular geneticist Corby
Kistler, reconstructing the genealogy of plant pathogenic fungi goes beyond
O'Donnell, at Peoria, Illinois, and Kistler, at St. Paul, Minnesota, along with
colleagues at North Dakota State University (NDSU), Fargo, are doing research
that may help keep grains from becoming contaminated with toxins that threaten
food safety and plant health.
Kistler and O'Donnell are among scientists researching toxin-producing
Fusarium fungi. These fungi are notorious for causing a disease called
scab, or Fusarium head blight, in grains such as wheat and barley, as well as
ear and stalk rot of corn. One of their goals is to develop technology for
identifying which species cause disease. They also hope to determine each
species' range of hosts, geographic distribution, and potential for producing
mycotoxins. This information is urgently needed by plant breeders and disease
control specialists worldwide to develop effective control measures to minimize
the threat of scab.
Although the pathogens in the
above cultures may look alike,
DNA data indicate they represent
distinctly different species within
the Fusarium graminearum
|One DiseaseBut Eight
At the National Center for Agricultural Utilization Research (NCAUR) in Peoria,
O'Donnell and molecular geneticist Todd J. Ward are using DNA evidence to
measure the genetic diversity of pathogenic fusaria that infect plant hosts
throughout the world.
In the past, all scab epidemics worldwide were thought to be caused by a single
pathogen. But new evidence proves that's not true. "To our surprise and
dismay, our DNA data so far reveal at least eight genetically distinct scab
pathogens, all of which are being called Fusarium graminearum,"
says O'Donnell. "We recognize them as distinct species based on
genealogical evidence obtained from six different genes."
Kistler, who works at the ARS Cereal Disease Laboratory in St. Paul, found that
all eight pathogens caused the devastating scab disease on wheat in greenhouse
experiments. Six of the eight species are known to infect corn.
In the Department of Veterinary and Microbiological Sciences at NDSU, mycotoxin
chemists Howard H. Casper and Beth K. Tacke found that all eight scab-causing
Fusarium species produce similar toxins on grain. They also found that
the scab species could not be distinguished from one another by the toxins they
Geneticist Corby Kistler looks for differences in wheat disease symptoms caused
by divergent strains of Fusarium to determine how the fungus spreads within the
| "We're confident that our
multidisciplinary genetic research, coupled with the toxin and pathogenicity
data, will help plant breeders develop varieties with broad-based resistance to
Fusarium and thereby avert scab epidemics," says Kistler. According
to O'Donnell, epidemics may also be averted if the research alerts USDA's
Animal and Plant Health Inspection Service officials to potential sources of
rogue fungi secretly inhabiting grain and other plant materials exported from
one country to another. Global Significance
Most wheat and barley varieties currently produced are susceptible to scab.
Because of yield losses and price discounts, the disease costs U.S. wheat
farmers at least $2.6 billion in the 1990s. Add to that losses from recent
outbreaks in Asia, Canada, Europe, and South America, notes Kistler, and it's
obvious that scab is truly a global problem. The disease has emerged as one of
the most important plant diseases worldwide within the past decade.
When wheat is infected, the grain typically becomes discolored and shriveled.
Often it's unsuitable for food or livestock feed because it may be contaminated
with trichothecenes and other chemicals that overstimulate estrogen production
in animals. Human exposure to these toxins results in a variety of symptoms
including acute dermatitis, diarrhea, and hemorrhaging.
Fusarium graminearum is responsible for most of the wheat scab and corn ear rot
in the United States. Above, geneticist Liane Rosewich examines one of the many
cultures of this species collected by the ARS Cereal Disease Laboratory.
Although several species of Fusarium have been isolated from scabby
grain in the Midwest, Kistler says the primary scab pathogen in the United
States is the "real" F. graminearum, not to be confused with
its closely related, look-alike "cousin" species that are part of the
F. graminearum family tree.
With the advent of molecular genetics tools, the DNA data gathered by O'Donnell
and Kistler show that only one of the eight species within the F.
graminearum family tree is responsible for recent scab outbreaks and
epidemics within the United States.
Based on similarities and differences in DNA sequence data, O'Donnell has
deduced the species' evolutionary histories and depicted their relatedness in a
genealogical chart called a phylogenetic tree. Ward recently joined O'Donnell's
laboratory to gain insights on how the scab pathogens developed their ability
to cause plant disease. In practical terms, that means he wants to use
molecular genetic data to more easily pinpoint the key genes contributing to
the pathogenicity of the various species. Another goal is to use information
about the global distribution of Fusarium species to gain insights into
their ability to cause disease on distantly related plants of economic
In Kistler's laboratory, postdoctoral geneticist Liane Rosewich has begun
research on the potential emergence and spread of new pathogenic
strainsan area of research known as population genetics. By observing DNA
patterns from scab-causing fungi collected throughout the Midwest, the
scientists can determine the presence of one or more genetically distinct
pathogens and whether hybridization may be occurring in farmers' fields.
Microbiologist Kerry O'Donnell
stores genetically characterized
wheat scab pathogens in liquid
nitrogen for future studies.
Then, using satellite-based global positioning systems to mark the precise spot
where samples were taken, researchers can monitor the spread of pathogenic
strains from season to season to predict the outcome of disease spread in the
field. But more than that, their goal is to find ways to most effectively
manipulate cultural practices to shift the balance of the plant and fungus
interaction in favor of plant health.
Until now, most scientists throughout the world have had to use visual means to
identify Fusarium pathogens. But as curator of Fusarium in the
ARS Culture Collection housed at NCAUR, O'Donnell and his colleagues Ward and
technician Eileen Sullivan are developing a comprehensive DNA database for all
toxin-producing and disease-causing Fusarium species.
By comparing each pathogen's DNA with DNA collected from more than 3,000
strains, the team has discovered more than 100 new Fusarium species
within the ARS collection in the past 5 years, including the 8 researched at
St. Paul and Fargo. The ARS Culture Collection includes the most complete,
genetically characterized collection of fusaria in the world.
While developing molecular markers for fusaria, O'Donnell uncovered clues that
threats of scab could be lurking in some unlikely places. Although the name
graminearum means "of the grasses," O'Donnell says several of the
species are known to occur only on trees, shrubs, or vines in Africa and South
and Central America.
In research to control the spread of cereal pathogens, geneticist Todd Ward
uses DNA sequence data to track the global distribution of wheat scab.
| O'Donnell analyzed the DNA of one
species within the F. graminearum family tree that was isolated from
diseased tissue of a tropical ornamental called grape ivy or oakleaf ivy
(Cissus rhombifolia). The infected plant was believed to have been
imported into a garden center in Pennsylvania from South or Central America,
where it is known as Venezuelan tree-bine. Fortunately, the Fusarium
inhabiting this common horticultural houseplant is not known to cause scab in
U.S. grain. But this fungus is among the eight that Kistler found pathogenic to
wheat and that Casper and Tacke found also produced toxins.
New Threats From Hybridization and Evolution
O'Donnell and Kistler made a disturbing discovery when they examined strains
isolated from moldy corn in Nepal. One strain had a mixture of DNA from two
F. graminearum family tree species thought to be geographically
isolatedone from Africa and one from Asia.
"We theorize that the African species was recently introduced to Asia on
moldy corn exported from Africa," says O'Donnell.
The findings graphically illustrate that international horticultural and
agricultural trade in plants and plant products can bring together closely
related species that have evolved in isolation from one another.
Could previously isolated species hybridize and become a serious plant disease
threat? Natural hybridization of two fungal species, also called interspecific
hybridization, may be rare, O'Donnell says, but such incidences seem more
likely when a fungus is introduced into a new habitat occupied by a closely
One major lesson: Scientists and government officials involved with quarantine
regulations, plant disease prevention and control, and plant breeding need to
appreciate the tremendous genetic diversity and global distribution of
Fusarium species that might cause scab, says O'Donnell. For plant
breeding programs to be fully effective, for example, breeders need to take
into account all eight Fusarium species.
Ward, O'Donnell, and Sullivan have recently developed a DNA-based diagnostic
procedure that can be used to rapidly and accurately distinguish the eight
species within the F. graminearum complex. From the data, they can
reasonably deduce the pathogen's geographic origin. By learning which plants
from various parts of the world may be most likely to harbor the pathogens,
quarantine officials should become better able to prevent new scab epidemics
within the United States.
Of the eight scab pathogen species, the researchers found that three appear to
be native to Africa, two to South and Central America, one to New Zealand, and
one to Asia. "The other specieslike the common coldis
widespread," says O'Donnell, "although it is most common in the
Just as patterns of DNA sequences in humans suggest to researchers that human
origins are in Africa, O'Donnell and Kistler's DNA evidence implies a similar
genesis of the scab pathogens. Their data suggest that the pathogens appear to
have initially evolved in Africa and South America and then spread into the
Northern Hemisphere millions of years before the origin of agriculture, which
dates back only an estimated 10,000 years. Insofar as agriculture and world
trade developed relatively recently, the scientists believe the scab pathogens
evolved in an environment of geographic and host isolation.
Now that Kistler realizes that the eight scab species can infect wheat and
knows that at least six can infect corn, the scientists are concerned about the
possibility of new pathogens evolving through hybridization.By
Ben Hardin, Agricultural
Research Service Information Staff.
Kerry L. O'Donnell and
Todd J. Ward are at the
USDA-ARS National Center for
Agricultural Utilization Research, 1815 N. University St., Peoria, IL
61604; phone (309) 681-6383, fax (309) 681-6672.
H. Corby Kistler and
U. Liane Rosewich are at the USDA-ARS
Cereal Disease Laboratory, 1551 Lindig
St., St. Paul, MN 55108; phone (612) 625-9774, fax (651) 649-5054.
The research is part of three ARS National Programs: Plant, Microbial, and
Insect Genetic Resources, Genomics, and Genetic Improvement (#301) and Crop
Protection and Quarantine (#304), described on the World Wide Web at
and Food Safety (#108), described at
"DNA Profiling: Guarding Against a Plant Disease
Epidemic" was published in the
issue of Agricultural Research magazine.