Host plant resistance is the most important component of many plant disease management systems. The development of molecular biology research tools has resulted in identification and analysis of genes that control the interactions of plants and pathogens and is contributing to efforts to produce transgenic crop plants resistant to a variety of plant pests.
Although resistance is less central to management of arthropod pests than of diseases, it is a major component of certain control programs such as the wheathessian fly and small grains-greenbug systems. Such programs may become more important for all crops when research on genetic engineering of arthropod-resistant plants gains momentum. Research has focused on expressing toxic proteins in crop plants to kill pest species. Scientists are now limited to use of a few toxic proteins such as Bt toxins, but this may change based on (a) screening of novel microbes for active compounds and (b) research into physiology and biochemistry of metabolic and hormonal systems unique to certain taxa of arthropod pests. The recent discovery of an insecticidal cholesterol oxidase produced by a fungus offers the possibility that other such toxins will be discovered. The fact that the activity of Bt toxins and other protein-based toxins is limited to arthropods is encouraging. Industry is involved in discovering novel toxins, and academic research in arthropod physiology and biochemistry is likely to help identify arthropod-specific targets and novel approaches for engineering plants.
Plants can respond to infection by an active resistance response, termed induced resistance, which is maintained systemically throughout the plant for a period of time after the infection. This response can protect the plant from some subsequent infections by plant pathogens. Recent progress in understanding the mechanism of signaling the advent of such challenges between different parts of the plant offers hope that this phenomenon will soon be explained at the molecular level (Kessmann et al., 1994). An understanding of the process could lead to novel, biologically based means to control damage caused by some pests. Clearly, basic studies in plant biology can have important impacts in agriculture. Continued support of research in this area is required if the full potential of these and other approaches is to be realized (National Research Council, 1989b).
A founding principle of classical biological control of exotic pests is that natural enemies of the pest can be found in the geographic region where the pest evolved. Likewise, plant geneticists have found that the best place to find pest-resistant plant varieties is in the geographic region where the plant and pest coevolved. Collection and identification of germplasm from such regions has been a high priority for plant breeders. It is probable that every pest has at least one biological-control agent that could be identified by this approach.