Botanical Extracts Reduce Populations of Soil Pathogens

Principal Investigators: James C. Locke and John H. Bowers, Research Plant Pathologists, Floral and Nursery Plants Research Unit, U.S. National Arboretum, USDA–ARS, Beltsville, MD 20705

There are a number of soilborne fungal pathogens that are of concern in the production of ornamental crops. To combat these pathogens, methyl bromide has been used to fumigate soil prior to planting many of these crops. One such example is Fusarium wilt of chrysanthemum, a widespread and destructive disease of this major horticultural crop. Symptoms of this disease may not become evident until near the time of flower production, although infection takes place early in the crop cycle.

The overall goal of this research project is to develop and evaluate new or existing, biologically based alternative control methods that can be integrated into cropping systems which currently use methyl bromide. Environmentally safe alternatives such as natural plant products, biological control agents, and cultural methods are being investigated. Major soilborne pathogens, Fusarium, Rhizoctonia, Verticillium, and Phytophthora are being used in bioassays designed to determine impact on soil populations and disease reduction in selected crops. This report focuses on the effect of several formulated plant extracts in reducing soil populations of Fusarium oxysporum f. sp. chrysanthemi (F.o.c.).

Artificially infested soil samples were treated with several rates of the botanical materials, incubated under standardized conditions, and assayed periodically to determine survival of the pathogen. The materials evaluated included the following formulations: 70 percent clove oil, 90 percent neem oil, pepper extract plus oil of mustard (4.94 percent capsaicin, 4.43 percent allyl isothiocyanate), cassia tree extract (Abion M), and the standard soil-drench fungicide Banrot 40W. Fusarium population determinations were made at 0, 1, 3, 7, 14, and 21 days after soil treatment. The natural product formulations were applied as 1, 5, and 10 percent aqueous emulsions at 5 ml per 150 cubic centimeters of soil.

Neither Banrot nor 1 percent applications of any of the botanical extracts reduced the population of F.o.c. compared to the untreated control. However, the 5 and 10 percent applications of the botanical extracts resulted in significant differences in the surviving F.o.c. populations at each assay date. Treatment with the neem oil material actually resulted in increased F.o.c. populations.

Soil populations of F.o.c. were lowest after 3 to 7 days of incubation with the pepper extract. In addition, populations of other soil microbes were also reduced to the greatest degree when treated with the pepper extract. The F.o.c. populations did rebound over time, showing a steady increase beginning 7 days after treatment. This increase could be the result of either breakdown of the pepper extract in soil or the lack of microbial competition which allows any surviving F.o.c. propagules to rapidly recolonize the treated soil. This may also occur with the clove oil and cassia extract but to a lesser extent. Further research will address these population dynamics questions.

The ability of these natural products to reduce soil populations of these pathogens coupled with their environmentally friendly composition makes these materials attractive candidates for use in biologically based management strategies. In the highly intense management of high value ornamental crops, these materials may be part of the answer to the quest for an alternative to methyl bromide fumigation. However, to fully develop an alternative cultural practice, these materials may need to be integrated with other practices such as the use of beneficial or biocontrol agents.

Additional research needs to be completed to scale-up these findings to actual production systems. Application rates and methods, pre-plant incubation periods, tolerance of various crop plants, and impact on soil microbial systems are being addressed. Whether this technology could be adapted for use with agronomic crops remains to be determined.