Pathogens and Antagonists of Plant Disease and Post-Harvest Decay

Pathogens

Insects and mites, like plants, humans, and other animals, can be infected by disease-causing organisms such as bacteria, viruses, and fungi. Under some conditions, such as high humidity or high pest abundance, these naturally occurring organisms may multiply to cause disease outbreaks or epizootics that can decimate an insect population. Diseases can be important natural controls of some insect pests.

Some pathogens have been mass produced and are available in commercial formulations for use in standard spray equipment. These products are frequently referred to as microbial insecticides, biorational, or bio-insecticides. Some of these microbial insecticides are still experimental, others have been available for many years. Formulations of the bacterium, Bacillus thuringiensis or Bt, for example, are widely used by gardeners and commercial growers.

Most insect pathogens are relatively specific to certain groups of insects and certain life stages. The microbial products do not directly affect beneficial insects and none are toxic to wildlife or humans. Specificity, ironically, can be a disadvantage to the commercialization of these products because their small market may limit profitability.

Unlike chemical insecticides, microbial insecticides can take longer to kill or debilitate the target pest. This may limit their use to crops that can sustain some insect damage. To be effective, most microbial insecticides must be applied to the correct life stage of the pest, and some understanding of the target pest's life cycle is required. Some microbial insecticides must be eaten by the insect to be effective. Good spray coverage is therefore important.

Major characteristics of insect pathogens:

• they kill, reduce reproduction, slow growth, or shorten the life of pests
  
• they usually are specific to target species or to specific life stages
  
• their effectiveness may depend on environmental conditions or host abundance
  
• the degree of control by naturally occurring pathogens may be unpredictable
  
• they are relatively slow acting; they may take several days or longer to provide adequate control
  
• they may cause epizootics
  
• Microbial insecticides are compatible with the use of predators and parasitoids, which may help to spread some pathogens through the pest population. Beneficial insects are not usually affected directly because of the specificity of a microbial product, but some parasitoids may be affected indirectly if parasitized hosts are killed. Insecticide applicators should note that although microbials are non-toxic to humans in the conventional sense, safety precautions should be followed to minimize exposure.

Taken from:

Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.

Left: Healthy cleistothecium of the powdery mildew Uncinula necator splitting open and releasing sacs containing ascospores.
Right: Parasitized cleistothecium of U. necator which has been ruptured and is exuding conidia of A. quisqualis. D.Gadoury (both).

Antagonists

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.

Worldwide, diseases of crop plants cause losses estimated to be 12%, and post harvest losses due to food spoilage have been estimated to be between 10% and 50%. In the United States, these figures are estimated to be 12% and 9%, respectively. 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 (Antagonists)

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.

Pimentel, D. (1991) Diversification of biological control strategies in agriculture. Crop Protection, 10:243-253.

Pimentel, D. and Pimentel, M. (1978) Dimensions of the world food problem and losses to pests. In World Food, Pest Losses, and the Enviroment. D. Pimentel. Ed. Boulder, CO: Westview Press.

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.