Field ecology of Spiroplasma citri in western North America.

Spiroplasma citri naturally infects plants representing several dicotyledonous families in the southwestern U.S., where it is commonly found in Old World brassicaceous weeds and cole crop plants as well as in citrus. Of its leafhopper vectors, it is most frequently found in Circulifer tenellus (Baker). It is rarely found in field-collected Scaphytopius nitridus (DeLong), one of two species of Scaphytopius capable of transmission from plants in the laboratory. Spiroplasmas presumed to be S. citri have been isolated several times from field-collected Ollarianus strictus (Ball), a leafhopper that, like several other common California leafhoppers, can acquire S. citri from plants under laboratory conditions but apparently cannot transmit it. In California, several plants commonly are doubly infected with S. citri and either the aster yellows agent or a C. tenellus-transmitted agent that causes floral virescence. In the Southwest, S. citri has been found throughout the year in naturally infected perennial and annual plants and in C. tenellus. Its isolation from C. tenellus and from cultivated turnip plants in Washington, from C. tenellus in Utah, and from citrus and other plants in Arizona, indicates a wide geographical distribution in the western U.S.