ARS Update: Controlling Weeds and Soilborne Pests—Ft. Pierce, Florida

As detailed in the April 1998 issue of Methyl Bromide Alternatives (pp. 5–7), 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 agent—P. amaranthicola—against 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 pigweed—Amaranthus hybridus—is 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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Last Updated: October 6, 1998