ABSTRACT
Plant diseases are recognized by the symptoms (external or internal) produced by them or by sick appearance of the plant. The term plant disease signifies the condition of the plant due to disease or cause of the disease. Plant disease is mainly defined in terms of the damage caused to the plant or to its organ. The other definitions for the term disease are:
Disease is a malfunctioning process that is caused by continuous irritation, which results in some suffering producing symptoms. This definition is accepted by both American Phytopathological Society and British Mycological Society.
Disease is an alteration in one or more of the ordered sequential series of physiological processes culminating in a loss of coordination of energy utilization in a plant as a result of the continuous irritation from the presence or absence of some factor or agent.
A plant is said to be ‘diseased’ when there is a harmful deviation from normal functioning of physiological process (Federation of British Plant Pathologists, 1973).
The disease can also be defined as ‘any disturbance brought about by a living entity or non-living agents or environmental factors which interfere with manufacture, translocation or utilization of food, mineral nutrients and water in such a way that the affected plant changes in appearance with or without much loss in yield than that of a normal healthy plant of the same variety.
Symptomatology, which deals with (a) the presence of readily apparent morphological abnormalities of the host in response to a causal agent (symptoms) and (b) an indication of disease other than a reaction of the host, such as the mycelium or fruiting body of a causal fungus (sign).