ARS Update: Controlling Weeds and Soilborne PestsFt.
Pierce, Florida
As detailed in the
April 1998
issue of Methyl Bromide Alternatives (pp. 57), ARS has increased
research efforts on finding methyl bromide alternatives at the new Fort Pierce,
Florida, location. A new position of soil microbiologist established there was
recently filled by Richard Shelby, who came from Auburn University. In April,
weed scientist Erin Rosskopf replaced David Patterson, who retired from ARS in
March 1998. Rosskopf discovered a new species of fungi that controls pigweeds,
which are considered one of the world's worst weeds for many crops.
Roy McDonald, ARS research leader based at Orlando, Florida, is responsible
for the methyl bromide work at Fort Pierce. "We needed someone to look into
problems dealing with fungi, bacteria, nematodes, insects, and weeds. Shelby
will characterize soil microbes that benefit plant health by restricting the
ability of soilborne pests to affect crops. He'll also help develop ecologically
sound, biologically based integrated disease and pest management strategies for
tomatoes, peppers, and other vegetables. Rosskopf will deal with weeds."
Addressing Soilborne Problems
Specifically, Shelby was hired to identify soil microbes tht affect both the
survival and the ability of soilborne pathogens to cause disease in vegetable
and small fruit production systems.
"Since my background is mainly in mycology, I'll probably focus on
fungi endemic in the area, like Fusarium wilt, Fusarium crown rot, and southern
blight," Shelby says. "Several biologically based methods have been
suggested that might control these pathogens. For instance, nonpathogenic
strains of Fusarium appear to elicit a systemic acquired resistance to
strains that are pathogenic."
He plans to search for these nonpathogenic strains and others that produce
resistance or that are antagonistic to plant pathogens. "I plan to develop
molecular methods to identify and trace the movement of these organisms after
they've been introduced to plants."
As a broad spectrum fungicide, methyl bromide controls many diseases and
pests. However, Shelby's approach will target specific soil pests with specific
biological pesticides.
Soil amendments have been effective in reducing the incidence of some
soilborne plant pathogens. Shelby also expects to explore these, using waste
products local to Fort Pierce, such as paper mill and citrus wastes.
Controlling Weeds
Weed scientist Erin Rosskopf came to ARS-Fort Pierce from the
University of
Florida. She brought with her a biological agent that controls pigweeds and
amaranths. Rosskopf is co-owner of a patent for the agent, which is a new
species of fungi belonging to the genus Phomopsis Bubák. She will
test the new agentP. amaranthicolaagainst weeds that will be
significant when methyl bromide is banned in 2001.
"We identified this fungal species as the causal agent of a leaf and
stem blight that occurs on Amaranthus in Florida," Rosskopf says. "Pigweeds
and amaranths belong to this genus. They're broadleaf plants that are primarily
herbaceous annuals, although there are a few woody species."
According to Rosskopf, these types of plants thrive in climates with high
temperatures, intense sunlight, and dry conditions, although they're also
commonly found anywhere that the soil has been disturbed and is not excessively
wet.
"In Florida, pigweeds are included in the ten most commonly found weeds
in tobacco, soybeans, cotton, and peanuts," Rosskopf says. "Not only
do these tenacious weeds compete with crops, but they can also poison livestock.
In areas where herbicide use is limited by choice, such as in organic production
areas, pigweeds can render the land virtually unusable."
Pigweeds, she says, have caused corn yield losses as high as 40 percent and
may also dramatically reduce cotton yields.
"In a potato field, a single redroot pigweed plant per meter of row has
caused up to 32 percent loss of marketable tubers," Rosskopf reports.
Smooth pigweedAmaranthus hybridusis a principal problem
in crops like peas, sugar beets, sugarcane, potatoes, wheat, and soybeans. It is
an agronomic pest in 27 countries, including the United States, Brazil,
Argentina, New Zealand, and Mexico.
Spiny amaranth or Amaranthus spinosus L., affects several valuable
crops such as tobacco, cotton, cassava, upland rice, mangoes, sorghum, sweet
potatoes, and papayas. This weed is most troublesome in tropical and subtropical
regions and causes problems in more than 40 countries, including the United
States, Brazil, Taiwan, and Thailand.
Redroot pigweed (A. retroflexus) is also a serious weed problem in
many countries, as are other species. Various types of pigweed constitute the
worst of all weed problems in Puerto Rico where they affect many fruit crops,
beans, and garlic.
"Traditionally, triazine herbicides have been extremely effective in
controlling pigweeds, but the herbicides' persistence in the soil and highly
specific mode of action have resulted in many resistant populations,"
Rosskopf reports. "And although there are registered herbicides that
control pigweeds on major crops, there are problems associated with their use.
These problems include nontarget effects and toxicity."
Rosskopf has a cooperative project with the University of Florida (UF) to
evaluate the fungus Dactylaria higginsii for controlling purple and
yellow nutsedge. Her collaborator on this project is R. Charudattan of UF's
Institute of Food and Agricultural Science. Purple nutsedge is considered the
world's worst weed. It is extremely troublesome to Florida's raised-bed, plastic
mulch pepper and tomato production systems.
"This fungus reduces the competitive ability of nutsedges,"
Rosskopf says. "Some plastic mulches and soil solarization suppress this
weed somewhat. We want to combine D. higginsii with these practices to
see if we can get better control."
Rosskopf and Charudattan are studying the conditions necessary for
large-scale application, as well as what form of the organism would be
compatible with current cropping practices. They plan large-scale field testing
this fall.
Another weed that Rosskopf will study is Portulacca oleraceae, which
is also a problem in most crops produced in Florida using raised-bed plastic
mulch production systems. "We're looking at the fungus Dichotomophthora
portulacae as a potential biocontrol agent. Lab and greenhouse trials are
under way to find the conditions necessary to produce the fungus and those
needed for high field efficacy," she says. "We'll do field tests with
our next tomato and pepper crops."
Rosskopf is testing all of these organisms for compatibility with chemical
pesticides, both for ease of application and to determine possible synergistic
effects.
"Part of my mission is to test some of the alternatives developed by
other scientists for weed control. Also, there is a need to characterize
pathogens associated with weeds. For example, root-knot nematodes are often
found on nutsedge tubers. How will this affect the crop if methyl bromide is not
around to control nutsedge?"
With the proposed ban on methyl bromide only a couple of crop seasons away,
more data are urgently needed on weeds. A future area of research will focus on
fungal plant pathogens associated with weeds that may also affect crop health.
Weeds are often thought to harbor crop pests. "Beginning with the next
cropping period, I'll characterize the fungal pathogens found on weeds growing
in tomato and pepper fields," Rosskopf says. "Controlling weeds must
play a vital role in our efforts to find alternatives to methyl bromide."
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