Plant Diseases, Diagnosis of

the science of the methods of examining plants to diagnose their diseases.

Accurate and timely diagnosis of plant diseases is of value in correctly choosing and using effective methods of protecting plants. Many diseases (various kinds of rust, smut, rot, scab, downy and powdery mildew, and so forth) can be diagnosed from macroscopic symptoms using classification keys. If an accurate diagnosis cannot be made by this method, the diseased plants are analyzed microscopically, and the causative agents are investigated (shape; size; color; characteristics of the reproductive organs; morphological, physiological, and biochemical features; intracellular inclusions, and so forth).

In the case of causative agents that have not been much studied, the parasitic specialization and virulence for different plants are ascertained. The method of diagnosing phytopathogenic viruses and bacteria by infecting indicator plants with them is highly effective. Serodiagnosis is useful in identifying viruses and bacteria in plants with manifest and latent symptoms of disease. Immunofluorescence analysis, a new approach in serodiagnosis, helps to detect the causative agents directly in plant cells and tissues. Fluorescence analysis and electron-microscopic methods are also widely used to diagnose plant diseases. Noninfectious dystrophic diseases caused by disturbances of mineral nutrition are diagnosed mainly by visual and chemical methods.