Fungi Can Whack
In a field of kudzu,
technician Jimmy McAlpine (left)
takes a sample to check the
effectiveness of a bioherbicide.
Plant pathologist Douglas
Boyette looks on.
|American farmers and homeowners spend
millions combating weeds and other alien organisms introduced from foreign
countries. With the increase in international commerce and trade, the number of
alien species becoming established in this country is growing every year.
"Luckily, fungi provide a vast arsenal of ammunition to control noxious
weedsboth established and newly arrivedthat invade roadsides,
rangelands, and waterways and crowd out useful and native plants," says
mycologist Amy Y. Rossman. She heads the Agricultural Research Service's Systematic
Botany and Mycology Laboratory in Beltsville, Maryland.
Fungi are among the most biologically diverse organisms on Earth. Once
discovered and characterized, many previously unknown species can be put to
work. Thanks to research at Rossman's lab, ARS scientists at several U.S.
laboratories are testing the effectiveness of three new fungus species as
biocontrols for some of the United States' major invasive weeds: ragweed,
purple loosestrife, kudzu, and morningglory.
Mycologist David F. Farr is curator of ARS' U.S. National Fungus Collections,
maintained at Beltsville. A systematist, Farr probes the collection's 1 million
fungal specimens to discover, name, scientifically describe, and classify
agriculturally important fungi.
"Once these organisms are characterized, their weed-control potential can
be tested in field and lab experiments," says Farr. He recently discovered
several fungitwo new to sciencethat may offer nonchemical control
of these four weeds.
Flowering kudzu is a
fast-growing legume with
a grapelike odor.
|An Irritant Wherever It Grows
"Ragweed, Ambrosia artemisiifolia, is a noxious plant that infests
thousands of acres of arable land worldwide and causes allergic
reactionsoften seasonalin many people," says Farr. "Its
pollen causes irritated eyes, runny noses, and general discomfort for many
"Last year, scientists in Hungarywhere ragweed is even more of a
problemreported that they had found a fungus, possibly a Septoria,
that was pathogenic to ragweed. It causes leaves to die and kills some plants,
probably by entering through leaf pores."
After searching the literature and fungus collection, Farr determined that this
beneficial species of Septoria is also found in the United States,
though not previously described anywhere. Using molecular sequencing, he
characterized it, named it S. epambrosiae, and illustrated it. Then he
showed it to be distinct from three other related, known Septoria
"Scientists will use this information to communicate about the new fungus
in developing it as a biocontrol agent for ragweed," says Farr.
Mycologist Amy Rossman
examines giant ragweed.
| Garden Plant Gone Wild
One of the other fungi that Farr discovered was one that attacks purple
loosestrife, Lythrum salicaria.
"This perennial garden plant has become a noxious weed and is spreading
rapidly throughout North America," says Farr. "Native to Europe and
Asia, purple loosestrife grows and reproduces prolifically in wetlands and
other moist habitats. It's degrading the quality of thousands of acres of
wetlands, becoming the dominant vegetation by outcompeting native plants that
provide critical food, shelter, and breeding areas for wildlife."
After careful examination of form and structure and DNA sequencing, Farr was
able to determine the molecular fingerprint of the fungusalso new to
"We described, illustrated, and named it Harknessia lythrii,"
says Farr. "Many species of Harknessia are host specificbut not all
of themso scientists need to ensure that this fungus attacks only purple
Leaves of giant ragweed
may harbor fungi that
could be used to control
this noxious weed.
Mycologist David Farr
searches for signs of
Plant pathologist C. Douglas Boyette at ARS' Southern Weed Science Research
Unit, Stoneville, Mississippi, recently discovered that the sicklepod fungus
Myrothecium verrucaria is also an effective bioherbicide for controlling
kudzu. This fast-growing, nonnative weed covers more than 7 million acres of
"Kudzu resembles a giant beanstalk," says Boyette. "It spreads
at a rate of about 120,000 acres a year, reducing land productivity. Homeowners
have a hard time controlling this vine, which grows up the sides of buildings,
along fences, and on trees and telephone poles. Control costs increase by
nearly $6 million each year."
In greenhouse and small field plot studies, Boyette and ARS plant pathologist
Hamed K. Abbas found that the Myrothecium bioherbicide killed 100
percent of kudzu weeds treated at different growth stages and under varying
physical and environmental conditions. It should provide a good nonchemical
control alternative, since one spray treatment kills leaves and stems and
appears to invade the roots. This research was done in collaboration with
Louisiana Tech University-Ruston.
The Stoneville researchers are doing extensive toxicological studies on the
fungus and are looking for a company to license the patented kudzu-control
Quarantine and Morningglories
Farr recently collaborated with ARS plant pathologist Douglas G. Luster, who is
at the ARS Foreign Disease-Weed Science Research Unit in Frederick, Maryland,
on the systematics of a different strain of M. verrucaria. With four
microbial containment greenhouses, this unit is the nation's largest facility
for studying whole plants under quarantine conditions.
Scientists there have developed techniques to monitor biocontrol agents after
release into the environment. They use the polymerase chain reaction, amplified
fragment length polymorphism, DNA sequencing, molecular marking, and other
sensitive technologies to detect and identify a weed pathogen's unique genetic
fingerprint. This lets them differentiate strains of the same fungal species.
"DNA fingerprinting also helps scientists keep close tabs on spore growth
and spread, host range, and effectiveness of biocontrol pathogens like
Myrothecium once they've been released," Luster says. They've
fingerprinted several strains of this soil fungus that kill morningglories, a
weed that plagues sugarcane growers.
Luster and ARS plant pathologist Dana K. Berner are testing the M.
verrucaria fungus for broad-spectrum weed control. In field studies,
spraying redroot- and smallflower-morningglories with an oil-based carrier
containing Myrothecium spores proved as lethal to these weeds as the
herbicide atrazine. Berner conducted the study with ARS agronomist Rex W.
Millhollon at the Sugarcane Research Unit, Houma, Louisiana.
DNA fingerprinting offers genetic evidence linking a specific microbial release
to a specific disease seen in target weeds. It can also reveal the spread of
biocontrol microbes and demonstrate their effectiveness in reducing invasive
weed populations, the scientists say.By Hank Becker, formerly with
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Amy Y. Rossman and
David F. Farr are with the USDA-ARS
Systematic Botany and Mycology
Laboratory, 10300 Baltimore Ave., Bldg. 011A, Beltsville, MD 20705-2350;
phone (301) 504-5364, fax (301) 504-5810.
C. Douglas Boyette is in the
USDA-ARS Southern Weed
Science Research Unit, P.O. Box 350, Stoneville, MS 38776; phone (662)
686-5217, fax (662) 686-5422.
Douglas G. Luster is in the
USDA-ARS Foreign Disease-Weed
Research Unit, 1301 Ditto Ave., Fort Detrick, MD 21702; phone (301)
619-7344, fax (301) 619-2880.
"Fungi Can Whack Invasive Weeds" was
published in the November 2001
issue of Agricultural Research magazine.