
Scientists Fight Fungi With Fungi to Protect
Tomato PlantsBy Jan Suszkiw August 7, 1997 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
plants roots but dont 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 USDAs
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
Marylands 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: http://www.ars.usda.gov/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
bromides use in all domestic crops--not just tomatoes. The ban stems from
evidence that the fumigant depletes the Earths 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, [email protected]. U.S. Department of Agriculture | |