Mycoplasma infections of plants.

Plants can be infected by two types of wall-less procaryotes, spiroplasmas and mycoplasma-like organisms (MLO), both located intracellularly in the phloem tissues of affected plants. Spiroplasmas have been cultured, characterized and shown to be true members of the class Mollicutes. MLO have not yet been cultured or characterized; they are thought to be mycoplasma-like on the basis of their ultrastructure as seen in situ, their sensitivity to tetracycline and resistance to penicillin. Mycoplasmas can also be found on the surface of plants. These extracellularly located organisms are members of the following genera: Spiroplasma. Mycoplasma and Acholeplasma. The presence of such surface mycoplasmas must not be overlooked when attempts to culture MLO from affected plants are undertaken. Sensitive serological techniques such as the enzyme-linked immunosorbent assay (ELISA) can successfully be used to compare the MLO located in the phloem of affected plants with those eventually cultured from the same plants. In California and Morocco periwinkles naturally infected with both Spiroplasma citri and MLO have been reported. With such doubly infected plants, the symptom expression has been that characteristic of the MLO disease (phyllody or stolbur), not that given by S. citri. Only S. citri can be cultured from such plants, but this does not indicate that S. citri is the causal agent of the disease expressed by the plant. In California many nonrutaceous plants have been found to be infected with S. citri. Stubborn affected citrus trees represent an important reservoir of S. citri, and Circulifer tenellus is an active leafhopper vector of S. citri. Hence, it is not surprising that in California MLO-infected fruit trees could also become infected with S. citri but it would not mean that S. citri is the causal agent of the disease. Criteria are discussed that are helpful in distinguishing between MLO infections and S. citri infections.