Plant Diseases and Insect Pests

The damage to plants caused by competition from weeds and by other pests including viruses, bacteria, fungi, and insects greatly impairs their productivity and in some instances can totally destroy a crop. Today, dependable crop yields are obtained by using disease-resistant varieties, biological control practices, and by applying pesticides to control plant diseases, insects, weeds, and other pests. In 1983, $1.3 billion was spent on pesticides—excluding herbicides—to protect and limit the damage to crops from plant diseases, nematodes, and insects. The potential crop losses in the absence of pesticide use greatly exceeds that value.

For about 100 years, breeding for disease resistance has been an important component of agricultural productivity worldwide. But the successes achieved by plant breeding are largely empirical and can be ephemeral. That is, because of a lack of basic information about the function of genes for resistance, studies are often random rather than specifically targeted explorations. In addition, any results can be short-lived because of the changing nature of pathogens and other pests as new genetic information is introduced into complex agroecological systems.

An excellent example of the effect of genetic change is the sterile pollen trait bred into most major corn varieties to aid in the production of hybrid seed. Plants containing Texas (T) cytoplasm transfer this male sterile trait via the cytoplasm; it is associated with a particular type of mitochondrion. Unknown to breeders, these mitochondria also carried vulnerability to a toxin produced by the pathogenic fungus Helminthosporium maydis. The result was the corn leaf blight epidemic in North America in the summer of 1970.

The methods used in the discovery of pesticide chemicals also have largely been empirical. With little or no prior information on mode of action, chemicals are tested to select those that kill the target insect, fungus, or weed but do not harm the crop plant or the environment.

Empirical approaches have produced enormous successes in controlling some pests, particularly weeds, fungal diseases, and insects, but the struggle is continuous, since genetic changes in these pests can often restore their virulence over a resistant plant variety or render the pest resistant to a pesticide. What is missing from this apparently endless cycle of susceptibility and resistance is a clear understanding of both the organisms and the plants they attack. As knowledge of pests—their genetics, biochemistry, and physiology, their hosts and the interactions between them—increases, better-directed and more effective pest control measures will be devised.

This chapter identifies several research approaches to a better understanding of the fundamental biological mechanisms that might be exploited to control plant pathogens and insects. Molecular biology offers new techniques for isolating and studying the action of genes. The existence of susceptible and resistant host plants and virulent and avirulent pathogens can be exploited to identify and isolate the genes that control the interactions between host and pathogen. Studies of the fine structure of these genes can lead to clues about the biochemical interactions that occur between the two organisms and to the regulation of these genes in the pathogen and in the tissues of the plant. It should be possible in the future to improve the methods and opportunities for the transfer of desirable traits for resistance into crop plants and, conversely, to create pathogens that will be virulent against selected weeds or arthropod pests. An increased understanding of insect neurobiology and the chemistry and action of modulating substances, such as the endocrine hormones that regulate metamorphosis, diapause, and reproduction, will open new avenues for controlling insect pests by disrupting their physiology and behavior at critical stages in the life cycle.

Post time: Apr-14-2021