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These large crabgrass seedlings show the effect of preemergence application of manuka oil. Control is on the right. Concentrations of manuka oil increase toward the left. Large crabgrass was completely controlled with 1% of oil in water in the leftmost pot. Bleaching to the foliage in that pot is due to the mechanism of action of leptospermone that blocks the synthesis of carotenoids in plants

These large crabgrass seedlings show the effect of preemergence application of manuka oil. Control is on the right. Concentrations of manuka oil increase toward the left. Large crabgrass was completely controlled with 1% of oil in water in the leftmost pot. Bleaching to the foliage in that pot is due to the mechanism of action of leptospermone that blocks the synthesis of carotenoids in plants

Photo by J’Lynn Howell, USDA

Scientists of the U.S. Department of Agriculture have found a natural product that might in the future provide growers with an important tool that’s been missing from the organic weed management toolbox—an effective organic herbicide.

In his search for natural herbicides, USDA research scientist Dr. Franck Dayan has found interesting results from manuka oil, a distilled oil from the manuka tree, a shrub-like tree native to New Zealand and southeast Australia. The manuka tree—its bark, leaves, oil, and gum—is reported to have been used for centuries by New Zealand’s native Maori people to treat a variety of ailments ranging from fevers and head colds to stiff muscles and burns, and is still sold over the counter today.

In greenhouse and small-scale field studies, Dayan found both pre- and post­emergent activity from manuka oil on some broadleaf and grass weeds.

While manuka oil exhibited good postemergent activity when applied with GreenMatch EX, a commercial lemongrass oil-based organic herbicide, it also demonstrated preemergent activity, controlling crabgrass seedlings, he said in a phone interview with Good Fruit Grower.

Dayan is based at the Agricultural Research Service’s Natural Products ­Utilization Research Center at the University of Mississippi. The center works to develop natural products for uses in agriculture, looking for natural chemical ­compounds that will serve as pest management tools that are toxicologically benign or improve the nutriceutical value of crops. Dayan, with a background in ­herbicide physiology, weed science, and herbicide modes of action, has been searching for natural weed management tools that are ­selective and have residual ­activity. Weed management has been one of the toughest aspects of horticulture for organic growers because there are few good materials available. Most organic herbicide products, like lemongrass oil, pine oil, citrus oil, or acetic acid, are used after weeds emerge to “burn down” the undesired plants, but they are nonselective and typically require multiple applications at high rates because of their low efficacy. The process is expensive both in cost of material and costs of applications.

Serendipitous discovery

The main active ingredient in manuka oil is leptospermone, which is a beta-triketone. Beta-triketones target the same plant enzymes as some commercial synthetic herbicides. By focusing on synthetic compounds that have the same mode of action as the beta-triketones, Dayan said he took a “reverse engineering” approach, starting with ­synthetic compounds to work backwards to find which plant species may produce sufficient amounts of structurally related natural ­products.

“Manuka oil is different from other oils by being much more active, by having soil activity, and by inhibiting a specific enzyme associated with carotenoid synthesis,” he stated in his research report. In his studies, combining small amounts of manuka oil with lemongrass oil achieved better results than with the lemongrass oil alone. The oil combination made the lemongrass oil more potent in postemergent applications, reducing crabgrass growth, as measured by dry weight, as much as 94 ­percent.

The discovery of manuka oil’s preemergent activity was by chance. Though he did not originally intend to study preemergent activity, a few crabgrass seeds germinated after the postemergent application. He noticed that the emerged seedlings were completely bleached. This unexpected observation prompted Dayan to look further at manuka oil’s preemergent activity, and he found large crabgrass growth was reduced between 50 to 90 percent from preemergent applications, depending on the dosage of manuka oil.

“Manuka oil’s preemergent activity makes it an ­attractive option for a new organic herbicide, “ Dayan said.

Manuka oil and its main ingredient leptospermone were stable in the soil for up to seven days, with half-lives of 18 and 15 days, respectively. “The systemic activity of manuka oil addresses many of the current limitations associated with natural products,” he stated. “Additionally, its soil persistence opens up a multitude of new ­possibilities and may be a potential bridge between traditional and organic herbicides.”

Future work

Dayan has more studies planned, including work on weed species selectivity, activity in different soil types and conditions, and how to optimize formulations. Already, he’s found dramatic differences in the oil’s activity based on soil type. “In low pH soils, like some of Mississippi’s that are 4.5, we found no activity. We need to know what’s going on there and if oil compounds are getting bound to the soil.”

Testing of rain fastness is also needed, as well as better understanding of how the oil works in the plant. He is putting a tag in the material so that he can follow how it moves within the plant once applied.

All this work will require a couple more years of study, he said, but the USDA is collaborating with Marrone Bio Innovations on this project. Working with collaborators is a key part of USDA’s research because it speeds up the commercialization process once research is completed.

Dayan explained that USDA does this type of basic research as a public service because few companies, especially those involved with organic products, have the money for basic research.