Mycoplasmas, Spiroplasmas, and Rickettsia-Like Organisms as Plant Pathogens

Mycoplasmas, spiroplasmas, and rickettsia-like organisms are considered to be the causal agents of yellows diseases, formally thought to be of viral origin. They belong to various groups of procaryotic microorganisms that are morphologically similar to or identical with members of the Mycoplas­ matales and Rickettsiales (6). Their similarity to these two orders has given rise to the terms mycoplasma-like organisms (MLO) and rickettsia-like organisms (RLO). The genus Spiroplasma has been placed in the order Mycoplasmatales. Mycoplasma-like organisms have been found in more than 200 plant species and in a large number of arthropods. Pathogenicity has not been proved in that Koch's postulates have not been fulfilled. The main reason for the failure to prove pathogenicity is that most attempts to isolate and cultivate these organisms have failed. Spiroplasmas occur as internal parasites in plants, leaiboppers, Drosoph­ ila, honey bees, and ticks. They have also been recently isolated from the surface of several flowers (25). Spiroplasmas have been cultivated on artifi­ cial media and their pathogenicity to plants has been demonstrated. Spiro­ plasma citri is the only recognized species in the family Spiroplasmataceae; all other spiroplasmas lack binomial names. Although many isolates are