For certain high-value crops like strawberries and tomatoes, many growers fumigate the soil to rid it of various soilborne root pathogens, harmful nematodes and weeds before planting their crops. In fact, approximately 80 percent of the methyl bromide produced in the United States is used to ready fields before planting. But with only four crop seasons left before the ban on methyl bromide is imposed in 2001, using alternatives to control these problems becomes increasingly more important.

"The most important pathogens to be controlled are those for which genetic resistance has not yet been identified, particularly those pathogens that can kill the plant," says Frank Martin. He is a plant pathologist at the ARS Crop Improvement and Protection Research Unit, Salinas, CA. "Two significant soilborne pathogens now controlled by methyl bromide and chloropicrin fumigation are verticillium wilt on California strawberries and fusarium wilt on Florida tomatoes."

Both these diseases, which can cause severe losses in a number of economically important crops, are caused by pathogenic fungi that infect a plant through the roots, move systemically through the plant, and ultimately kill it, Martin says.

While not as serious as the pathogens causing verticillium or fusarium wilt, damping-off and root-pruning pathogens cause types of diseases also controlled by fumigation. In damping-off, pathogens attack seeds soon after planting or attack roots of seedlings soon after transplanting. They rapidly take over the plant, causing death. Some of these same pathogens also can attack the roots of established plants, reducing root density which can stunt plant growth, vigor, and yield. The window of protection against plant death from damping-off can range from days to weeks; the sites needing protection are relatively small--the seed coat, the emerging root, or an early-developing root system. Because of the relatively restricted area needing treatment, it may be possible to add other microbes that inhibit disease to the seed coat or to the transplant plug before planting. Controlling pathogens with other microbes is referred to as "biological control."

Just what is biological control? "It is using microbes that are not detrimental to plants to protect them from microbes that are harmful," Martin says. "Some researchers expand this definition to include crop rotation because it causes changes in soil microflora that can be detrimental to specific soil pathogens."

An example of a biological control agent is SoilGard^TM, a biofungicide developed from a common soilborne fungus by ARS scientists at Beltsville, MD, and commercialized by Thermo Trilogy, Inc. SoilGard^TM protects vegetable and ornamental seedlings from soilborne pathogens.

Plant pathologists Robert Larkin and Deborah Fravel have found several nonpathogenic strains of Fusarium oxysporum that control fusarium wilt in tomatoes, watermelons, and muskmelons. Located at the ARS Biocontrol of Plant Diseases Laboratory in Beltsville, MD, they have conducted greenhouse studies and plan future field tests at several locations.

"We found some strains of the fungi that actually induced resistance to the wilt, and others that controlled it through competition," Larkin says. "Strains that induce resistance are effective at much lower concentration rates than strains that work through competition. We've tested these potential biocontrol agents under a variety of conditions and in different types of soil and found them effective in all but the heaviest clay soil."

"On tomatoes, we've reduced fusarium wilt up to 90 percent and in watermelon and muskmelon, up to 80 percent," Larkin reports. "Although not as effective as the fungal biocontrol agents, several isolates of bacteria were also effective." They tested strains of Burkholderia cepacia, Pseudomonas fluorescens, and Pseudomonas corrugata and are investigating specific strains of the fungal antagonists Trichoderma and Gliocladium.

Larkin and colleagues are working to develop and improve the level and effectiveness of control of wilt diseases. "We feel that fusarium wilt diseases can be controlled biologically. With more research, these potential agents could be viable alternatives to chemical control for several crops," he says. More information on ARS research on biocontrol of plant diseases is available on the Internet (http://www.barc.usda.gov/psi/bpdl/bpdl.html).

An Integrated Approach to Root Health

"For vegetable crops in the Midwest and cereal crops in the Pacific Northwest, methyl bromide has never been a viable option for root health management," says ARS plant pathologist R. James Cook. "Virtually all crops grown without adequate crop rotation respond more or less the same to soil fumigation simply because all crops are subject to damage from their soilborne plant pathogens." He is based at the ARS Root Disease and Biological Control Research Unit in Pullman, WA. Jennifer Parke, plant pathologist with the University of Wisconsin, made a joint presentation on this research with Cook at the November 1996 methyl bromide conference in Orlando, FL.

Plenty of healthy roots make a plant better able to absorb fertilizer and water from the soil, Cook says. To get and keep good root health, he says the following principles must be considered.

1. Reduce, even if only modestly, the power of soilborne pathogens to cause disease.

   This can be accomplished by starting the crop with pathogen-free planting material, avoiding fields known to be pathogen infested, allowing time for a natural decline of pathogens from crop rotation or by using soil solarization or short-term flooding.

2. Carefully manage the microenvironment of the soil where the pathogens are active to significantly reduce the severity of root diseases.

   This type of management includes instituting practices that improve soil drainage, adjust temperature or water potential, and raise or lower soil pH, whichever produces an environment less favorable for the pathogen than for its natural enemies.

3. Introduce or foster naturally occurring biocontrol agents in the soil that surrounds roots through cultural practices or by incorporation with planting material.

   Adding composts or other properly processed organic materials to the soil can make it inhospitable for some plant parasitic nematodes and some fungal pathogens, Cook says. Seeds and seedling transplants are ideal for introducing antagonists to biologically control specific pathogens because they allow the antagonist to be placed where it is most needed. And, the antagonist's growth can be supported by the plant it protects. Parke says that there are microbial inoculants on the market for use in the soil or on seeds and transplants to fight damping-off and root rots.

4. Target specific pathogens with narrow-spectrum fungicides applied with planting material or directly to the soil.

   "Applied singly or as mixtures, selected fungicides like metalaxyl, difenconozole, triadimefon, propiconazole, and imazalil can control soilborne pathogens," Cook says. "But they must be applied so that they are taken up by the roots or come in contact with the pathogen."

5. Genetically increase the trait already present in plants to tolerate or resist virtually any biological or environmental plant stress factor, including soilborne pathogens.

   Cook says that resistance has been found in plants to most, if not all, vascular pathogens such as Verticillium dahliae and Fusarium oxysporium, and to parasitic nematodes such as root knot and cyst.

   "We're also finding useful resistance to pathogens in wild relatives of crop plants and although building in genetic resistance is a long-term approach, we're making progress," Cook reports.