Diagnosis of plant virus-induced diseases can be difficult because many of the large number of viruses induce similar symptoms in the same host. Virus-induced inclusions have long been used in diagnosing animal virus infections with light microscopy. Most plant virus infections can be diagnosed at the group level and some at the specific or strain levels with cytological techniques. The large numbers of plant viruses confronting the diagnostician may appear to be overwhelming. Most cytological studies with light and electron microscopy devoted to diagnosis have attained identification at the group level. With the application of appropriate stains, inclusions of different types are easily differentiated from each other and from normal cell constituents in the light microscope. Examination of infected tissues in thin sections with the electron microscope has provided much information on the constituents and substructure of inclusions. K. S. Kim and R. Carr proposed that the nuclear inclusions induced by whitefly-transmitted geminiviruses were diagnostic, based on thin section studies.