Plant disease can be simply defined as any deviation from normal vegetative health and growth; and more scientifically as an injurious physiological process caused by the continued irritation of a primary causal factor, exhibited through abnormal cellular activity and expressed in characteristic pathological conditions called symptoms.
Some diseases can be attributed to inanimate and nonparasitic factors such as adverse environmental conditions. Physical and mechanical damage brought about by violent storms, improper cultivation practices, etc, besides being disastrous in themselves, may prepare the way for widespread infection by other disease agents. Living agents such as insects not only interfere directly with plant metabolism, but also frequently carry other disease agents from plant. A number of economically important disease-causing factors are plants themselves: bacteria and fungi. Algae are responsible for some relatively unimportant tropical plant diseases. Among flowering plants only a few forms are of occasional importance. Several slime moulds, numerous viruses and nematodes are also important agents of plant disease.
The symptoms of plant diseases may be death (necrosis) of all or any part of the plant, loss of turgor (wilt), overgrowths, stunting, or various other changes in the structure of the plant. A rapid death of foliage is often called blight, whereas localized necrosis results in leaf spots. Necrosis of stems or bark results in cankers. Overgrowths composed primarily of undifferentiated cells are called galls, or, less commonly, tumours. Chlorosis (lack of chlorophyll in varying degree) is the most common nonstructural evidence of disease. In leaves it may occur in stripes or in irregular spots (mosaic). These symptoms grade into one another and overlap. Often two or more bacteria, fungi, viruses or a combination of them together attack a plant to produce much greater damage than that resulting from a single agent alone. Furthermore, a plant which is already suffering from a deficient environment is more susceptible to attack by such agents.
Plant diseases have always caused substantial reductions in the yield of many economic crops and are a continuing threat to all of them. For example, in 1958, 70% of the production costs of a large fruit company in Central America were expended in controlling banana diseases. In addition to the actual losses in production caused by plant diseases, there also have been violent fluctuations in crop yields. When crops were in short supply, prices went up; when large plantings were made to allow for losses that never occurred, there was a superabundance of crop with a corresponding fall in prices and disturbance in economic balance. Worldwide losses from plant diseases have not been estimated but in the USA alone the cost of crop losses due to disease as long ago as 1951-60 has been estimated to be more than 3,250 million dollars annually, or 7% of the potential production.
Plant diseases pre-date agriculture and occur in habitats untouched by man, but their spread is favoured by the growing of a crop. Most pathogens are specialized parasites and can infect only a limited range of plants. The flora in most natural habitats is varied, so adjacent plants are unlikely to be hosts to the same pathogens, but in a crop, individual plants uniformly susceptible to the same parasites are crowded together, creating ideal conditions for rapid spread. With the elimination of variability in crops and the extensive deployment of monocultures, the potential for widespread plant disease is high. The possibility exists and is sometimes occurs that a new uncontrollable pathogen may seriously affect a major food plant. As it is, even without devastating important crops, plant diseases take a heavy annual toll in lost production.