Using Natural Plant Volatiles: A Joint US/Israeli/South African Venture

The challenge of finding viable alternatives to methyl bromide has brought about a unique consortium of scientists from the United States, Israel, and South Africa. These scientists are pooling their knowledge and energies to develop natural plant volatiles that could serve as alternatives to methyl bromide for fumigating fruit, soil, and grain storage facilities.

"For years methyl bromide has been vital in fumigating soil for growing row crops and nursery seedlings," says Charles L. Wilson, a plant pathologist with USDA's Agricultural Research Service at Kearneysville, WV. "It is also used extensively as an export quarantine treatment for fruit and vegetables and in granaries to protect grain from pests. Natural plant compounds have been used effectively in all these areas and could be potential replacements for methyl bromide, since it will no longer be available to growers after January 1, 2001."

Wilson and Eli Shaaya of the Volcani Center in Bet Dagan, Israel, have identified natural plant compounds that could serve as alternatives to methyl bromide as a soil fumigant. They've also developed a way to determine the effect these compounds have on soilborne pathogens, fruit, and grain storage facilities. South African plant pathologist Johan Combrink and colleagues are studying a number of South African plants for compounds that could possibly replace methyl bromide. Combrink is with the INFRUTEC Center for Fruit Technology of the Fruit, Vine, and Wine Research Institute, Agricultural Research Council, in Stellenbosch, South Africa.

United States

As coordinator of the consortium, Wilson keeps in close communication with his counterparts in Israel and South Africa.

"We have collaborative research going on in each of the three countries," Wilson says. "One of the first problems we tackled was the difficulty in evaluating the effectiveness of fumigants applied to planting beds or greenhouse containers to control pathogens, insects, and weed seeds. This type of research requires large volumes of soil to be fumigated and elaborate evaluation procedures devised to test a fumigant's effectiveness. Along with Deborah Fravel, a plant pathologist with ARS at Beltsville, Maryland, we built an apparatus that quickly and easily evaluates the effectiveness of a fumigant to control soil pathogens." The research is conducted cooperatively at the ARS Appalachian Fruit Research Station in Kearneysville, West Virginia, and at the ARS Biocontrol of Plant Diseases Laboratory in Beltsville, Maryland.

The soil fumigation apparatus is simple to use. A nitrogen tank is hooked to a stainless steel, gas-tight compartment that holds a plastic container filled with charcoal which absorbs the fumigant. This compartment is connected by a hose to a stainless manifold to which six, cigar-shaped soil containers are attached, with a gas-flow regulator at the top of each container. Each container has an outlet nipple at the bottom with a flow meter attached.

"This equipment allows the soil to retain uniform amounts of the fumigant for a definite period. Once the soil is fumigated, the containers are opened and the soil can be tested for fungal activity," Wilson explains. "The apparatus will speed up the process of testing natural compounds as soil fumigants."

Wilson has already successfully used this system to evaluate several naturally occurring plant compounds.

"We found that benzaldehyde plus nitrogen controlled four major soil pathogens: Fusarium oxysporum, Rhizoctonia solani, Phythium aphanidermatum, and Sclerotinia minor," Wilson reports. "Benzaldehyde would be a desirable alternative to methyl bromide since it is inexpensive, easily biodegradable, and breaks down into products that aren't harmful to humans, animals, or the environment."

Wilson found that although soil fumigated with benzaldehyde initially had significantly lower soil pH values, within 2 weeks the pH returned to previous nonfumigated levels. Therefore, the changes in soil pH are readily reversed and should not interfere with crop production.

"In evaluating natural plant compounds as alternatives to methyl bromide, we need more research on the biocidal activity of these compounds against a wide range of pathogens and insects, as well as weeds," Wilson says. "We also need to look not only at the efficacy of natural fumigants in different soil types and different applications, but at delivery systems as well."

The Israeli Connection

At the Department of Stored Products at The Volcani Center in Israel, Eli Shaaya and colleagues have found several essential oils extracted from herb and spice plants that are effective as fumigants. They have successfully used these natural fumigants to control pests in grain and dry, stored food products and quarantine insects in cut flowers for export. (See Phyto-Oils Control Insects in Stored Products and Cut Flowers, Methyl Bromide Alternatives, January 1998, pp. 6–7.)

South aAfrica's Contribution

Wilson has been working with the Fort Hare University in the eastern Cape of South Africa to find natural compounds that might replace methyl bromide.

"We've got two small South African companies, Ulimocor and the Ciskei Agricultural Corporation, interested in this project," Wilson says. "And there is a pilot plant operating at Fort Hare University to extract essential oils from indigenous South African plants."

These oils are now being marketed as flavor and fragrance compounds, and Wilson thinks they provide a potentially rich source of new compounds that may be candidates for fumigating soil, agricultural commodities, and physical structures.

Wilson had also been collaborating with Johan Combrink of INFRUITEC at Stellenbosch, South Africa, on research to find natural plant compounds that fight Botrytis cinerea, Penicillium expansum, Mucor piriformis, and Rhizopus nigricans. These pathogens attack pome fruit and are now controlled with synthetic pesticides.

The South African group, consisting of researchers from INFRUITEC and the Plant Protection Research Institute (part of the South African Agricultural Research Council), will also address the following problems:

• Controlling weevils, the dried fruit moth and mites on dried fruit.
  
  
• Fighting the root-knot nematode (Meloidogyne javanica), a worldwide pest and one of the most significant nematode pests on a wide range of crops, including stone fruits and vegetables. Glasshouse colonies are available in South Africa.
  
  
• Containing the ring nematode (Criconemella xenoplax), a serious pest of stone fruit and peach orchards not only in South Africa, but in Georgia and South Carolina as well.
  
  
• Fumigating to control soilborne pathogens on specific crops such as Fusarium wilt on melons, root rot on strawberries, Phytophthora root rot on citrus, replant syndrome on apples, clubroot on cabbage, and damping-off on vegetable crops.
  
  

"We expect that this collaborative research effort will produce patentable products that can be commercialized," Wilson reports. "The Maktishim Chemical Company in Israel is interested in our work, and we've been talking with a couple of U.S. companies about our results."

Last Updated: April 22, 1998