Phytoplasmas is a class of bacteria that is pathogenic to a wide variety of plant species, and was first identified in 1967. It is characterized by its small genome size, ranging from 0.3 to 1.2 micrometers, and polymorphic polymorphism. It is important for agriculture to identify the factors that contribute to its pathogenicity, as well as to identify effective measures for the control of its diseases. Despite its economic importance, Phytoplasm remains the least characterized plant pathogen, largely due to the lack of in vitro cultivation, gene transfer, or mutagenesis. It is capable of infecting the eggs of insect vectors, and has been known to cause disease in more than 1,000 plant species. As it is not possible to culture the bacterium, most of the information regarding its morphology can be derived from the analysis of serologically thin sections of infected plant sieve tubes under a dark field light microscope. Phytoplasmas with well-studied host plant ranges show a wide range of host species, with some infecting multiple crops or non-crop species.