|Spiroplasma--discovery and phylogeny|
For decades following the original descriptions of the disease (Kunkel, 1946,1948), corn stunt, as well as other so-called plant "yellows" diseases, was presumed to be caused by a virus. In 1968, pleomorphic cell wall-less microbes (mycoplasmalike organisms, MLOs) were reported in diseased corn (Zea mays L.) plants and in insect vectors capable of transmitting the disease (Granados, 1969; Maramorosch et al. 1968).
In 1972, the cell wall-less microorganism associated with corn stunt disease was found to be helical and motile and to represent an entirely new group of pathogens, for which the term "spiroplasma" was coined (Davis and Worley, 1973; Davis et al. 1972).
(Image- Helical cells of Spiroplasma kunkelii in sieve cell in phloem tissue of a corn stunt diseased corn plant. Photo from Davis and Worley. 1971. Phytopathology 63:403-408.)
In 1975, cultivation of the corn stunt spiroplasma in artificial media in vitro permitted conclusive evidence that it was the cause of corn stunt disease (Chen and Liao, 1975; Williamson and Whitcomb, 1975) and description of the spiroplasma as a new species, Spiroplasma kunkelii (Whitcomb et al. 1986).
Spiroplasma species are members of the class Mollicutes, which also contains Mycoplasma and Acholeplasma species, "Candidatus Phytoplasma species", and other cell wall-less bacterial species.