Biological control of disease employs natural enemies of pests or pathogens to eradicate or control their population. This can involve the introduction of exotic species, or it can be a matter of harnessing whatever form of biological control exists naturally in the ecosystem in question. The induction of plant resistance using non-pathogenic or incompatible micro-organisms is also a form of biological control. Some diseases that can be successfully controlled using biological agents are pathogens of pruning wounds and other cut surfaces, crown gall, diseases of leaves and flowers, such as powdery mildew, diseases of fruits and vegetables, such as Botrytis, and fungal pathogens in the soil (disease suppressive soils).
All biological communities are complex ecosystems in which the abundance of any organism is dependent on its food supply, its environment and other organisms. If a pathogen is kept in check by the microbial community around it, then biological control has been achieved. Biological control appears to take place on the plant surface by the activity of epiphytic microflora. This is then an important consideration when applying chemicals to plants, since there is a risk of killing natural antagonists of pathogens other than the one being treated.
The most common mechanisms for microbial antagonism of plant pathogens are parasitism, predation, competition, induced resistance and the production of antimicrobial substances. Often, several mechanisms act together.
Competition exists between organisms that require the same resource for growth and survival. Use of the resource by one organism reduces its availability for the other organism. Competition for space or nutrients usually takes place between closely related species. Therefore, it can be effective to treat plants or seeds with a non-pathogenic strain of a related species that can out-compete the pathogenic organism. In some cases, the treating species need not be closely related to the pathogen, as long as it uses the same resources. For example, bacteria and yeasts can reduce fungal spore germination by competing with the spores for nutrients on the surface of leaves.
While micro-organisms can produce secondary metabolites that have anti-microbial properties when grown in culture, these chemicals are rarely detected in natural environments. Therefore, antibiotics would need to be produced in culture and then applied. However, antibiotics are easily lost to the atmosphere and are commonly broken down by organisms that are insensitive to them, and so they are not ideal biological agents against plant pathogens.
Parasitism of one fungus by another (hyperparasitism or mycoparasitism) is well documented and is affected by environmental factors, including nutrient availability. Formulations of some parasitic species of fungi are available commercially for the control of fungal plant pathogens in the soil and on the plant surface. The hyphae of parasitic fungi penetrate their victim, sometimes with the aid of wall-degrading enzymes. Bacteria on the plant surface and in the soil are also known to parasitise plant pathogens, such as other bacteria and fungal spores. Predation of plant pathogens by invertebrates can also contribute to general biological control. Bacterial feeding nematodes consume large numbers of bacteria in the soil and some amoebae are known to attack yeasts, small spores and fungal hyphae, although these organisms are generally non-specific predators and their relative importance in biological control is not well understood.
Induced resistance and cross-protection are two mechanisms of plant 'immunity' against a pathogen. In the case of cross-protection, an organism present on the plant can protect it from a pathogen that comes into contact with the plant later. For example, symptomless strains of tobacco mosaic virus can protect tomatoes from virulent strains of the same virus, rather like immunisation in animals. Induced resistance is a form of cross-protection, where the plant is inoculated with inactive pathogens, low doses of pathogens, pathogen-derived chemicals or with non-pathogen species to stimulate an immune response. This prepares the plant for an attack by pathogens, and its defence mechanisms are already activated when infection occurs. It provides protection against a wide rage of pathogens across many plant species.
Commercial application and grower acceptance of biological control has been slow to develop, mainly due to the variation in efficacy under the range of environmental conditions likely to occur in the field. This problem can only be overcome by better understanding the environmental parameters that limit biological control. In addition to this problem, there has been relatively little investment in the development of commercially viable products for biological control, partly due to the cost of developing, testing efficacy and risk, registering and marketing such a product. The most successful product would be one that can be applied using existing machinery or methods. Biological control agents are therefore generally formulated as wettable powders, dusts, granules and aqueous or oil-based liquid products, with various additives to attain all the desirable attributes.
While it is unlikely that biological control will completely replace chemical pesticides in the foreseeable future, we can expect that there will be some decline in the use of chemicals, particularly in developed countries. Thus far, most approaches have involved the single antagonist concept, although a biological systems approach, where disease is suppressed from several angles, might provide a better alternative. Similarly, the use of biological control agents could be used as one component of an integrated management program to achieve the best possible results.