The pathogenicity of mycoplasmas for plants

Many plant diseases belonging to the yellows group are believed to be caused by wall-free prokaryotes resembling mycoplasmas, which are spread by leafhopper vectors. Im most cases the evidence for mycoplasma aetiology rests upon the finding by electron microscopy of mycoplasma-like bodies in phloem tissue of diseased plants, coupled in some cases with symptom remission following treatment of plants with tetracyclines. The only plant-pathogenic mycoplasmas which have so far been cultured are the spiroplasmas (motile, helical, filamentous mycoplasmas) which cause citrus stubborn, corn stunt and probably a small number of other plant diseases. Spiroplasma citri (the citrus stubborn agent) can infect members of many plant families, and disease symptoms suggest that the organisms produce toxins. Phytotoxic substances have been detected in, and partially purified from spiroplasma cultures. The corn stunt spiroplasma does not produce toxins and probably affects plants by interfering with hormone metabolism.