The causes of plant diseases: non-parasitic agents
There is no entirely satisfactory definition of disease. It has been defined as harmful deviation from normal functioning of physiological processes (Anon., 1950), a definition which has the merit of emphasizing the dynamic nature of disease as a reaction between the plant and the causal agent of the disease. It also makes the point that disease involves abnormal physiological changes which result in the visible changes by which we recognize the disease, and that diseases adversely affect the plant. These aspects are discussed by Stakman and Harrar (1957) who define a plant disease as ‘a physiological disorder or structural abnormality that is deleterious to the plant or to any of its parts or products, or that reduces their economic value’, that is including deterioration of harvested plant products such as timber. It is important to distinguish between the disease, for example potato blight, and the causal agent (Phytophthora infestans), and between injury (usually a sudden short-lived act) and disease which is a more prolonged process — transient as opposed to continuous irritation (Horsfall and Dimond, 1959). There is, however, growing evidence that the response of plants to physical or chemical injury resembles in some respects their response to at least some pathogens (p. 248) so that similar symptoms can and do result from dissimilar agents.
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