Scientists Fight Fungi With Fungi to Protect Tomato Plants

BELTSVILLE, Md., Aug. 7--Tomato plants grown in experimental plots here have a new, natural ally against a fungus that causes wilt disease. The allies are actually relatives of the wilt fungus that crowd it from the plant’s roots but don’t harm the plant itself.

Wilt is caused by strains of Fusarium oxysporum fungi that attack tomato and other plants through their roots. But the researchers have found benign strains of Fusarium that actually protect the plants, said Deborah Fravel with USDA’s Agricultural Research Service. Fravel is a plant pathologist at the ARS Biocontrol of Plant Diseases Laboratory at Beltsville.

Many of the helpful fungi crowd their disease-causing cousins away from choice nutrients and space on or around the roots, Fravel said. A few of the good-guy Fusarium strains help jump-start the plants’ natural defense system, she added.

The helpful fungi have potential as a natural alternative to methyl bromide, a soil fumigant scheduled to be banned in 2001, said Fravel.

In May, she and ARS plant pathologist Bob Larkin began the first outdoor trials of the new approach on a one-acre plot at Beltsville. They planted tomato seedlings whose roots harbor colonies of the helpful fungi. The scientists began a second test on an experimental field near Salisbury on Maryland’s Eastern Shore.

In the lab, Fravel and Larkin used fermentation techniques to “brew” millions of spores of Fusarium. They applied the spores to the roots of potted tomato or watermelon seedlings. Within about a week, when the fungi germinated to form a living protective coat around the roots, the seedlings were ready to be transplanted outdoors.

In earlier studies in the greenhouse, up to 90 percent of treated seedlings grew into mature, wilt-free plants. “Our goal is to develop an effective biocontrol of fusarium wilt on tomatoes and some other crops of economic importance,” Fravel said in an article about the research in the August issue of Agricultural Research magazine. The magazine story can also be seen on the World Wide Web at:

/is/AR/archive/aug97/fusarium0897.htm

Unchecked, fusarium wilt disease can wipe out a susceptible tomato crop, said Larkin. “The younger the seedling, the more vulnerable it is to the pathogens,” he noted. “Most losses are in the seedling stage, those four to six weeks after they are transplanted into the field.”

In Florida, second only to California in producing domestic tomatoes, growers spend about $3,800 per acre applying methyl bromide to rid the soil of fusarium wilt fungi. This chemical fumigant also kills other soil-dwelling pests such as nematodes and weed seeds.

But starting Jan. 1, 2001, the U.S. Environment Protection Agency plans to impose a ban on methyl bromide’s use in all domestic crops--not just tomatoes. The ban stems from evidence that the fumigant depletes the Earth’s ozone layer.

In the lab, Fravel and Larkin isolated and screened several hundred bacteria, actinomycetes and other microbes with wilt-fighting potential. They narrowed the search to 10 strains of benign Fusarium.

Of particular interest were five that stimulate tomato seedlings’ to churn out chemical defense compounds, a phenomenon called “induced systemic resistance.” Larkin said the compounds apparently kill or block wilt-causing fungi that try to grow and spread in the plants’ vascular tissues, a network that carries water and nutrients.

Scientific contact: Deborah Fravel and Bob Larkin, plant pathologists, Biocontrol of Plant Diseases Laboratory, Agricultural Research Service, USDA, Bldg. 011A, Beltsville, Md. 20705. Phone (301) 504-5678, fax 504-5968, dfravel@asrr.arsusda.gov.