Clove-smelling blossom thinner
A plant oil extract holds promise as a blossom thinner for organic tree fruit.
The left side of these John Boy peach trees at the ARS research station in Kearneysville that were treated with eugenol provides a stark contrast to the right, untreated side. The photo was taken 24 hours after application.
Since the ancient days of Rome, essential or fragrant oils of plants have been used as aromatherapy for bathing, soothing skin, healing, and even embalming. But it's not likely the Romans thought to try the essential oils as a blossom thinner for tree fruit.
Drs. Stephen Miller and Thomas Tworkoski, U.S. Department of Agriculture scientists at the Appalachian Fruit Research Station in Kearneysville, West Virginia, have been studying the essential oil eugenol for several years.
"Essential oils are not essential in the way we would think," said Miller, a research horticulturist. "Essential means that they are plant essences that have an odor."
The eugenol essential oil has a spicy clove or cinnamon odor that remains in the orchard for a day or two after spraying, he explained. "The trees have a unique odor after spraying. Some would call it a 'pleasant' fragrance. But in the spray tank, the fragrance stays there for a long time. It's a much nicer smell than some of the pesticides we use."
Tworkoski, a plant physiologist has been studying a variety of essential oils for their potential to control weeds and suppress diseases. It was during these studies that he observed the caustic properties of eugenol, Miller said.
"Many blossom thinners are basically caustic in their mode of action," Miller said, adding that Tworkoski shared with him information about eugenol's caustic characteristics. A few quick hand trials conducted about five years ago showed that the plant oil extract had potential as a chemical thinning agent. Miller and Tworkoski began collaborating on research, slowly expanding the studies to gain more knowledge about how the oil works. All the while, they found more promise in the plant oil as a thinner.
In the third year of the eugenol thinning study, airblast sprayers were used to apply the oil treatments to better reflect commercial settings and improve uniformity of coverage.
In 2008, the thinning trial will for the first time be conducted in different locations and environments with commercial cooperators, Miller said. Previous research focused on apple and peach trees at USDA's Appalachian research station, comparing eugenol with other thinning agents, including ammonium thiosulfate, liquid lime sulfur, and Wilthin (sulfcarbamide).
Concentration, timing, environment, and cultivar all have a bearing on the response of fruit growth regulators, he said.
At 6 percent and 8 percent concentration, the essential oil caused extensive phytotoxicity to flower and leaf tissue, a response that was dose-dependent and transitory. Miller said that within 3 to 4 weeks after treatment, the damage was no longer visible.
"The effects are pretty dramatic," he said. "When people first see the trees after treatment, they think 'Oh my gosh, you've killed the tree.' But the tree grows through it."
At rates of 2 percent to 6 percent, the essential oil treatment reduced crop load and improved fruit size in peach generally equal to that achieved by hand thinning at 40 to 60 days after full bloom. Tests over two years indicated that apple may be more responsive to an essential oil as a bloom thinner than peach, he stated.
The essential oil reduced apple crop loads far below the hand-thinned level. The crop was below a commercially acceptable level when an 8 percent concentration was applied, and in some tests, even at the 6 percent rate. It has been suggested that the greater thinning effect of the eugenol on apple may be the result of leaf phytotoxicity and reduced photosynthetic rates rather than a direct effect on the flower reproductive tissues, he said.
Miller notes that the treatments have had a positive impact on return bloom, which is particularly important in apples.
The USDA scientists are most interested in learning if the oil works as a thinner for peaches because there is more interest in finding a peach thinner than one for apples. "There's no real material labeled for peaches," Miller said, adding that there are several chemical thinners available for apples.
He has had many phone calls from organic growers looking for a chemical thinner that is approved for organic tree fruit production. An organically approved eugenol-based herbicide is already sold under the trade name Matran EC for broad-spectrum weed control.
A patent is pending for the eugenol essential oil.