Aromatherapy — for watermelons, tomatoes and strawberries? Research conducted at the University of Florida shows that essential oils — those strong-smelling liquids beloved by people seeking relief from stress — could be the basis of a new generation of environmentally friendly defenses against a variety of plant diseases.
The findings may be particularly noteworthy in light of a multi-national agreement reached recently that would ban the use of the prized pest control agent methyl bromide in the United States in less than two years.
“Many of these (essential) oils have been used in traditional medicine solutions for bacterial infections and other illnesses,” said Tim Momol, a professor of plant pathology with UF's Institute of Food and Agricultural Sciences. Now we're finding that the natural chemicals in some oils are effective against soil-borne diseases caused by bacteria or fungus.
Momol and a team of UF scientists have done several studies, including one published in the journal Plant Disease, that show essential oils are effective against a number of common plant pathogens. Other members of the team include Jeff Jones, a professor of plant pathology, David Mitchell, an emeritus professor of plant pathology, and Steve Olson, a professor of horticultural sciences.
The researchers are among hundreds of scientists around the country searching for an alternative to methyl bromide, a pest-control agent that is commonly used on high-value crops such as strawberries and tomatoes. Methyl bromide is a soil fumigant — a chemical that, when added to soil, produces a gas that permeates the soil and kills microorganisms, weeds and fungi. It is prized by farmers for its effectiveness against a broad variety of pests.
Methyl bromide has also been linked to depletion of the ozone layer, and under a United Nations agreement reached recently, it will be banned in the United States by Jan. 1, 2006, completing a phase-out that began in 1999.
In their pursuit of an effective substitute, Momol and his colleagues began calling aromatherapy companies and ordering essential oils — plant extracts that can fill a room with an odor in a matter of seconds.
Essential oils aren't typically regarded as pesticides, but the UF researchers saw an important similarity between the oils and most soil fumigants. Essential oils produce a powerful smell because they are highly volatile — meaning they vaporize quickly even at room temperature. Fumigants are also volatile, a property that allows them to permeate soil, kill pests and then quickly evaporate, leaving no residue.
The researchers began applying essential oils to the soil in potted greenhouse plants with some promising results. They found that the oils from thyme and palmarosa — both often recommended by aromatherapists as a treatment for bacterial infections — can kill Ralstonia solanacearum, a bacterium that causes wilt or rot in a wide range of plants, including major crops such as tomatoes, potatoes and bananas.
They found the oils were also effective in controlling some soil-borne fungi known to damage crop plants.
The team discovered that both oils contain naturally occurring chemicals already used in some pest-control products. The researchers are particularly interested in thymol, the active substance in thyme oil, which is already known to repel insects and kill some kinds of mold.
At low doses, thymol is safe for human consumption — so safe the U.S. Food and Drug Administration allows its use as a food additive. At high concentrations, the chemical is known to cause skin and lung irritations in humans, but the UF researchers say thymol can be applied in a way that would keep the user from receiving a strong dose of the material.
“It isn't harmful at these concentrations, but I have to admit it smells awful,” Olson said. “If you were applying this to a crop in the field, you'd probably want to wear a mask just because of the strong odor.”
While applying thyme or palmarosa oil to crops on a large scale wouldn't be cost-effective, thymol can be produced at low cost. Research at other institutions has identified other essential oils with pest-controlling properties, the UF researchers say.
Chemicals from those oils could be used along with thymol to create a fumigant that controls many or most of the pathogens killed by methyl bromide, without depleting the ozone layer, Momol said.
Growers, anxious about the future loss of methyl bromide, say they are eagerly awaiting the chance to give an essential-oil fumigant a try.
“I've seen the trials the researchers are doing, and so far it looks promising,” said Stewart Suber, who co-owns a 700-acre farm in Gadsden County, 50 acres of which are planted in tomatoes fumigated with methyl bromide.
“If they're able to get it on the market, I'm willing to try it,” he said.