Plants have evolved a battery of defense mechanisms that in aggregate provide protection against a wide range of potential viral, bacterial, fungal, and other pathogens encountered throughout the plant life cycle. However, in the artificial setting of agriculture, disease, although the exception, can be costly and even devastating. Crop diseases have played significant roles in human history, exemplified by the widespread starvation and mass emigration triggered by the failure of European potato crops in the mid-nineteenth century as a result of late blight. Today, the use of pesticides, breeding for resistance, and integrated pest management provide important tools for reducing crop losses to pre-and postharvest diseases. However, agrichemicals are expensive, prohibitively so for many fanners in developing countries, and there are increasing concerns about environmental load from their intensive application. Likewise, major disease resistance (R) genes are in many cases not durable, resistance breaking down within one or two seasons as a result of selection pressure on the pathogen population, and most breeding efforts now rely on combinations of minor resistance genes, each giving partial protection. For a number of important diseases, such as take-all of wheat, there is no effective genetic resistance. Population growth, migration to cities, desertification, and climate change all now contribute to an urgent need to secure diversified food production against disease losses. In this chapter I discuss the prospects that genetic engineering of disease-resistance mechanisms can contribute to durable, broad protection and hence underpin enhanced crop productivity. Plants have a number of performed physical and chemical defensive mechanisms that help protect against the myriad potential pathogens to which plants arc exposed (Osbourn, 1996). However, superimposed upon this preexisting protective armory, plants respond to the perception of pathogen attack by activation of inducible defense mechanisms (Lamb et al., 1989; Staskawicz et al., 1995). Many of the most important crop diseases involve specialized interactions between pathogen and host. Interactions between specific plant cultivars and defined physiological races or strains of potential pathogens are described as compatible (host susceptible, pathogen virulent) or incompatible (host resistant, pathogen avirulent).
Molecular Approaches to Disease Resistance in Fragaria SPP.
