Careful, accurate observation is essential. Isolation and identification of that microbe. Inoculation of healthy plants with the isolated microbe and the subsequent appearance of those symptoms observed on the original plant. Diseased plants are characterized by signs, which are structures of the pathogen, and symptoms, which result from deviation from the normal physiological or morphological development of a plant. Frequently, the sequence of symptom development characterizes a particular disease.
Signs are observable structures of the pathogen.