Antagonists of Plant Disease and Post-Harvest Decay

Antagonists of plant disease and food spoilage microorganisms are not yet well understood. However, the research that has been done has yielded exciting and promising results, and the study of antagonists has become a rapidly expanding field in plant pathology.

Diseases of crop plants cause great losses in agriculture, and post harvest losses due to food spoilage are also great. Finding ways to prevent microorganisms from causing these losses would help ensure a stable food supply for the world's ever expanding population. Outside of agriculture, diseases can cause the destruction of entire stands of plants in marshes, forests, or other natural settings, and in other plant systems.

Knowledge of the interactions among microorganisms and ways to manipulate microbiota is growing as research in this field rapidly expands. Antagonists have been successfully used to suppress tomato mosaic, foot and butt rot of conifers, citrus tristeza disease, and crown gall of several crops. Seeds have been coated with antagonists that reduce infection by pathogens and also enhance plant growth. Brown rot of peaches in storage was controlled under simulated commercial conditions by incorporating the antagonist Bacillus subtilis into wax used in the packing process. Inoculation of hosts with antagonists has been used with good results against a common fungal pathogen of conifers and chestnut blight. The future also holds much promise for the suppression of plant-parasitic nematodes by microbiota.

Growers have applied antagonists to the above-ground parts of plants, to the soil (and roots), and to plant seeds. The above-ground environment is the least stable for antagonists, because of the extreme variability in moisture and nutrients. Soil is a more stable environment for microbiota, but soil in most fields is generally nutrient poor, pH may range from 4-8, and temperatures and moisture may vary widely. In contrast, greenhouse planting mixes can be managed more effectively to promote antagonist colonization. Finally, it is practical to treat seeds to favor microbial antagonists.

To be most effective, antagonists of plant disease and food spoilage should be:

• genetically stable
• effective at low concentrations
• easy to culture and amenable to growth on an inexpensive medium
• effective against a wide range of pathogens in a variety of systems
• prepared in an easily distributable form
• non-toxic to humans
• resistant to pesticides
• compatible with other treatments (physical and chemical)
• non-pathogenic against the host plant

Relative Effectiveness

Under ideals conditions, such as in the laboratory, antagonists can completely protect plants from pathogens. In the field, disease control is likely to be less successful.

Proper deployment of the antagonist appears to be crucial. Critical factors include moisture and nutrient availability and pH. If the deployment system can meet the needs of the antagonist, successful colonization is more likely. Careful selection of an aggressive strain of the antagonist is also important.

Acknowledgement

Thanks to David Gadoury, Department of Plant Pathology, NYSAES, Cornell University, Geneva, New York, for reviewing an earlier version of this section.

References

Fry, W.E. (1982) Principles of Plant Disease Management. Academic Press, New York. 378 pp.

Harman, G.E. (1990) Deployment tactics for biocontrol agents in plant pathology. New Directions in Biological Control: Alternatives for Suppressing Agricultural Pests and Diseases, Alan R. Liss, Inc., 779-792.

USDA ARS (1965) Handbook 291. 120 pp.

Wilson, C.L., and Wisniewski, M.E. (1989) Biological control of postharvest diseases of fruits and vegetables: an emerging technology. Ann Rev. Phytopath., 27: 425-441.

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