Managing Microbial Populations As Agents Of Biological Control

One of the fastest growing areas in pesticide development is the use of microorganisms for the biological control of pests (biopesticides). The fundamental concept of biological control is the selection and introduction of specific organisms to control a particular pest. The organism(s) selected might be a predator, parasite, competitor, or pathogen of the targeted pest. Biological control occurs widely in nature, masking the fact that most pests never become serious problems because of the controls exerted by other members of the community. Biological control methods are commonly a part of an overall integrated pest management program to reduce the legal, environmental, and public safety hazards of chemicals. In addition, it may be a less expensive alternative to the use of some pesticides. Unlike most pesticides, biological control agents are often specific for a particular pest. Successful use of biological control requires a greater understanding of the biology of both the biocontrol agent and the targeted pest. Often, the outcome of using biological control is not as dramatic or quick as the use of pesticides. Most biocontrol agents attack only specific types of plant pathogens, nematodes, or insects unlike broad-spectrum pesticides, which kill a wide range of organisms.

There are three strategies by which organisms are managed to control pest populations. Classical biological control involves collecting organisms pathogenic to the pest from locations where a pest originated and releasing them in an infected area to control the pest. The natural increase in the biocontrol organisms is relied upon to control the pest without future intervention. Augmentation is a method of increasing the population of a natural enemy that attacks a pest. This can be done by mass producing inoculum in the laboratory and releasing it into the field at the proper time. Control results from and requires the increase of the antagonistic disease through many disease cycles to reach threshold levels that cause death or control of the target pest. The augmentation method relies upon continual human management. Conservation or enhancement of indigenous populations is an important part in any biological control effort. This involves identifying any factors that limit the effectiveness of a particular population and facilitate them to help the beneficial species. Conservation of natural enemies involves either reducing factors that interfere with the populations of natural enemies or providing needed resources that help them.

Many bacteria and fungal genera have been used extensively for the control of plant pathogens. Various nonpathogenic fungal strains of Rhizoctonia, Phialophora, Fusarium, and Trichoderma, as well as mycorrhizal fungi, have been used to reduce damage caused by related strains or other pathogenic fungi. As biocontrol agents, Trichoderma species dominate, most likely because of their ease of culture and wide host range. The most commonly targeted pathogens are Pythium, Fusarium, and Rhizoctonia species, reflecting their worldwide importance and their relative ease of control (Whipps, 2001). The most commercially successful bacterial-based biocontrol system is the use of the nonpathogenic Agrobacterium strains to control crown gall.

The primary modes of action of pathogen control or management of both bacteria and fungi are competition for C, N, and Fe and increased colonization of the rhizosphere by the nonpathogenic strains (Table 17.6). For more information on competition and food web interactions (see Chap. 8). Other mechanisms include antibiosis, induced resistance, and mycoparasitism. Competition for commonly used substrates and for their excretion sites on roots plays an important role in the root zone for controlling pathogens. Organisms capable of rapid growth rates with wide distribution and population numbers throughout the soil have a distinct advantage of colonization and substrate utilization that outcompetes many pathogens. Ectomycorrhizal fungi because of their physical sheathing morphology may occupy infection sites, thus excluding pathogens. In contrast, arbuscular mycorrhizas provide control of pathogens through induced resistance and improved plant growth rather than niche competition.

TABLE 17.6 Mechanisms of Biocontrol by Soil Bacterial and Fungal Agents

Method of control

Organism

Mechanism

Competition

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