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Neem oil locks out spores.

Oil extracted from neem seeds covers plant leaves like a raincoat, stopping fungi that cause diseases such as powdery mildew and rust from infecting plants.

Fungal spores are spread by wind and splashing raindrops. "If the spores can't adhere to a leaf, germinate, and penetrate the leaf cells, they can't cause disease," says Jim Locke, an ARS research plant pathologist.

Neem, or margosa, trees are native to India and Burma. They are related to mahogany, require a frost-free climate, and will grow in West Africa, the Caribbean, Australia, southern Florida, several southwestern states, and Hawaii.

Almost all parts of the versatile plant contain extractable compounds that have been used for centuries in India in personal hygiene products like soap and toothpaste. Seed extract has been used to treat skin diseases, sores, and rheumatism.

Locke says that in numerous tests, a spray of 1-percent neem oil in water "stopped 95 to 100 percent of the powdery mildew on hydrangeas, lilacs, and phlox."

A single spray application was sufficient to protect these ornamentals from infection. Repeated applications at 7- to 14-day intervals as the plants grew provided disease protection without any plant damage.

On plants where mildew had begun to develop, "it was arrested," he says, "providing control comparable to each of three chemical fungicides."

Powdery mildew, which also attacks crepe myrtles and roses, causes leaves to turn white. Preliminary results indicate the oil will arrest and control the fungus that plagues these popular ornamentals, especially in humid areas.

Locke says the oil is the first botanical product to exhibit fungicidal properties. He has been field-testing it for the past 4 years on several greenhouse and nursery crops.

"We're working now to discover how the neem oil protects the plant from infection," says Locke. Two of the possibilities are that the spores fail to germinate or are unable to penetrate the leaf.

One study involves numerous laboratory tests of roses by Locke's group in the Floral and Nursery Plants Research Unit at the agency's U.S. National Arboretum, Washington, D.C. He says the oil "seems to delay infection by black spot--the number-one disease of roses. As a result, rose bushes lose fewer leaves, compared to untreated, diseased plants."

Locke says this research, begun in cooperation with former ARS entomologist Hiram Larew, also demonstrated that neem oil can reduce damage caused by various pests, including spider mites.

"In preliminary tests, a 2-percent spray of neem seed oil applied directly to spider mite eggs resulted in an 87-percent mortality," he says.

Research at USDA on plant-derived natural pesticides, such as nicotine, dates back to the 1920's. Beginning in 1975, extraction products from neem seeds were evaluated for their insect-killing properties.

In 1987, ARS researchers demonstrated the systemic activity of a neem seed extract containing azadirachtin against leafmining flies. Larvae that fed on plants grown in azadirachtin-treated soil rarely survived to adulthood. Azadirachtin-based insecticides became the first neem product to be approved by the U.S. Environmental Protection Agency.

Locke says the botanical insecticides, Margosan-O and BioNeem, contain azadirachtin. Unlike neem oil, they have no fungicidal activity.

Under a cooperative research and development agreement with W.R. Grace and Company, Columbia, Maryland, Locke is testing the oil as a fungicide that may be available commercially later this year.

James C. Locke is in the USDA-ARS Floral and Nursery Plants Research Unit, U.S. National Arboretum, Bldg. 004, 10300 Baltimore Avenue, Beltsville, MD 20705-2350; phone (301) 504-6413, fax (301) 504-5096.
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Author:Becker, Hank
Publication:Agricultural Research
Date:Jun 1, 1994
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