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Diseases, Pests, And Mechanical Injury .
Like all living things, plants continuously subjected to a wide variety of potential disease-causing agents and to the stresses caused by mechanical injury. Generally speaking, the causes of disease fall into two broad groups, parasitic and nonparasitic. An organism that lives in or on another individual for part or all of its life, securing its food directly from the tissues of the host organism but not benefiting the host in any way, is referred to as a parasite. Many but not all parasites are pathogenic, that is, disease-producing organ-isms. Disease refers to any disturbance in functioning and growth that causes a lower operating efficiency or a breakdown in the plant’s metabolism. Disease that afflict plants generally result from microbial infections. Invasions of the body by pathogenic viruses, bacteria, fungi, or other microorganisms. Disease also can result from infestations of insects and nematode worms. The reaction of the affected plant to the cause of the disturbance produce a spectrum of visible symptoms and consequence that are commonly recognized as the effects of disease, such as wilting, cell death and dieback, stunting, and discoloration. Many changed in cellular structure and metabolism occur when plants are infected by parasites. Some of these changes are thought to be closely related to the ability of the plant body to defend itself.
Vertebrate animals possess a sophisticated immune system that provides the body with a means of resisting infection. In the apparent absence of such a system, plants must rely in various other defense mechanisms. Among the multiple defenses that occur in plant cell and tissues in response to stress and invasion by pathogens and that are implicated in disease resistance and susceptibility are genetic and biochemical defenses; other defense mechanisms are structural in nature. Plants also can be subjected to mechanical injury by chewing animal or by forces in the physical environment that create open wounds. These structural changes often are accompanied by chemical alteration in the host tissue that are thought to be inhibitory or toxic to pathogens.
Not all animal and microbial invasions in plants are lethal. In some woody species the larvae of insect cambium miners injure the cambial initial so that irregular patches of abnormal parenchymatous tissue are produced and become embedded in the wood. Visible as a longitudinal streak on the surface.
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