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A plant disease is a physiological or structural abnormality that is deletrious to the plant or to any of its parts or products, or reduces their economic value. It expresses itself in a definite series of symptoms. Plant disease may be temporary or permanent, depending on the speed of recovery. Further, a disease may be localised, involving only a limited part of a plant, or it may be systematic when its infection is generalized and spreads throughout the plant body. Plant disease may be caused by micro-organisms, including fungi, bacteria, viruses, mycoplasmas, etc. or may be incited by physiological causes including high or low temparatures, lack or excess of soil moisture and aeration, deficiency or excess of plant nutrients, soil acidity or alkalinity, etc. The micro-organisms obtain their food either by breaking down dead plant and animal remains (saprophytes) or by attacking living plants and animals (parasites). In order to obtain nutrients, the parasitic organisms excrete enzymes or toxins and kill the cells of the tissues of the host plant, as a result of which either the whole plant or a part of it is damaged or killed, or considerable disturbance takes place in its normal metabolic process. |
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