The practice of monoculture in modern agriculture enables us to continue to provide foodstuffs for the world's ever increasing population. Monoculture is, however, an ecologically unnatural situation, that is inherently unstable and offers considerable opportunity for the development of diseases. Plant disease control has now therefore become heavily dependent on fungicides to combat the wide variety of fungal diseases that threaten agricultural crops. Even with intensive fungicide use, the destruction of crop plants by fungal pathogens is a serious problem worldwide that annually leads to losses of about 15% (Logemann and Schell 1993). The use of pesticides in general, has also resulted in significant costs to public health and the environment. Studies aimed at replacing pesticides with environmentally safer methods are currently being conducted at many research centers. In this context, control of plant pests by the application of biological agents holds great promise as an alternative to the use of chemicals. It is generally recognized that biological control agents are safer and sounder environmentally than is reliance on the use of high volumes of fungicides and other antimicrobial treatments. The heightened scientific interest in biological control of plant pathogens is a response, in part, to growing public concerns over chemical pesticides. However, there is an equally greater need for biological control of pathogens that presently go uncontrolled or only partially controlled by these "traditional" means (Cook 1993). Thus, biological control should and can be justified on its own merits, without giving it importance at the expense of chemical controls.
Biocontrol must be effective, reliable, consistent, and economical before it becomes an important component of plant disease management. To meet these criteria, we must increase our understanding of the biology of the biocontrol agent in question, which in most cases is extremely limited. Furthermore, superior strains, together with delivery systems that enhance biocontrol activity, must be developed (Harman et al. 1989). In this context, many biological control agents can be modified genetically to enhance their attributes. In addition, we can now think of microorganisms with inhibitory activity against plant pathogens as potential sources of genes for disease resistance.
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