A plant is diseased when it is continuously affected by some factor which interferes with its "normal" structure or activities. The word "continuously" excludes such things as insect injury. Plant diseases usually show some outward signs and/or symptoms of disease.
Disease is a term usually reserved for those problems caused by parasites: fungi, bacteria, nematodes, viruses, mycoplasma-like organisms and parasitic seed plants. Some disorders caused by such things as unbalanced fertility, toxic chemicals and air pollution are often confused with diseases.
For example, certain types of spray burn look much like some leaf spot diseases. In many instances it is difficult to determine the cause without several days of laboratory work.
Symptoms and signs of a disease are the basic keys to plant disease identification. A symptom of a disease is the reaction of the host plant to invasion by the parasite or injury from the non-parasitic factor, i.e., spots on the leaves or wilting. A "sign" is physical evidence of the pathogen--visible growth of fungus spores, for example. All disease symptoms and signs are caused by an interaction between the host plant, the pathogen (parasitic causal agent) and the environment.
There are over 80,000 different parasitic diseases of plants. Of course, no crop has this many diseases. Some have only one or two common diseases; others have many more. Tobacco has over 70 different described diseases. Only a few of these commonly occur and cause economic problems, but the fact that the less common diseases do occasionally occur complicates accurate disease diagnosis. Quite frequently, a plant may have two or more diseases at one time.
Accurate identification and diagnosis is an art as well as a science and experience is important. This outline is not intended to make anyone an expert in identifying diseases. In an outline of this type, it is impossible to cover even the major disease problems of individual crops. This discussion is to acquaint you with the general symptoms that occur as a result of disease.