USDA, ARS, USNA, Floral and Nursery Plants Research Unit, BARC-W, Beltsville, MD 20705, USA

Background and objectives
Traditional agricultural practices have utilized natural products for centuries. The advent of synthetic fertilizers and pesticides has lead to the intensification and specialization of agriculture. With concerns for environmental pollution, worker safety and consumer well being, researchers are revisiting the potential of natural products to be substitutes for the dependency on synthetic products in agricultural production systems. Our project objectives have been to evaluate natural products from plants for their ability to either directly reduce fungal pathogen populations or to protect the crop from infection and to develop formulations that are commercially acceptable as biopesticides.

Materials and methods
Aqueous and ethanolic extracts of a variety of herbs were evaluated for activity using the standard poison plate, agar well technique for direct toxicity to mycelial growth. A filter paper assay disc saturated with test extract, placed in petri plate lids above a plug transfer of the test organism, was used to evaluate for volatile activity. A water emulsiflable formulation of clove oil was evaluated as a pre- and post plant treatment in a zinnia seedling bioassay to determine activity against both Pythium ultimum and Rhizoctonia solani damping-off. In vitro mycelial and conidial inhibition bioassays were conducted with Botryosphaeria dothidea utilizing the formulated clove oil and aqueous herb extracts. Soil bioassays utilizing Fusarium oxysporum f.sp. chrysanthemi and Verticillium dahliae were conducted to determine the effect of formulated clove oil, cassia tree extract, and pepper extract on survival of these pathogens. Standard soil dilution plate methods using semi-selective media were used to follow pathogen populations over time.

Results and conclusions
The clove oil formulation was more active than any of the aqueous herb extracts evaluated in vitro [1]. Volatile activity was demonstrated by inhibition of mycelial growth and conidiai germination of Pythium ultimum, Botrytis cinerea and Fusarium roseum. In the zinnia seedling damping-off bioassay, formulated clove oil controlled both Pythium and Rhizoctonia as well as a standard fungicide drench when applied either as a preplant incorporation or post plant drench. Formulated clove oil and cassia extracts added as 10% aqueous emulsions and pepper extract added as a 5% aqueous emulsion significantly reduced the population density of F. oxysporum f.sp. chrysanthemi compared to that in untreated soil [2]. Similar results have been obtained with V. dahliae. Soil populations of F. oxysporum were lowest 3-7 days after treatment with the pepper extract, however, the population began increasing thereafter for the duration of the assay. This suggests the short residual life of the extract in soil. In addition to reducing F. oxysporum, the pepper extract also eliminated most of the other background microflora possibly creating a biological vacuum in the soil. The observed reduction of soilborne pathogens resulting from soil treatment with these natural plant extracts, combined with their environmentally-friendly rapid breakdown suggests that they could play an important role as a component in biologically-based management strategies. The possibility of using these materials either as a substitute for fungicide drench treatments or to 'knock down' initial pathogen populations followed by the introduction of beneficial or biocontrol agents are interesting possibilities to explore.

1. Jacobs KA, Carter MA, Locke JC, 1996. Biological and Cultural Tests for Control of Plant Diseases 11, 47.
2. Bowers JH, Locke JC, 1997. Proceedings Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions, pp. 21-24.