Mycoplasma : SUSPECTED ETIOLOGIC AGENT OF CORN STUNT

Corn stunt, a disease affecting corn (Zea mays L.) and teosinte (Euchlaenia mexicana L.), which causes an economically important disease in Latin American countries and occurs in several southern states of the United States, was reported for the first time in 1945.1 It has been generally accepted that the disease is caused by a virus. This assumption was based on circumstantial evidence. Plant symptoms, such as interrupted streaks, chlorosis, stunting, and the production of adventitious shoots, were considered "typical" symptoms of plant virus infection. Several species of leafhoppers are known to transmit corn stunt.2 This transmission and particularly a protracted incubation period of the etiologic agent in leafhoppers, as well as in plants,3 further supported viral etiology. Finally, mechanical transmission of the corn stunt agent to leafhoppers by needle inoculation4 and interference of strains of corn stunt in plants and in insect vectors5 were considered characteristic for leafhopper-borne viruses. They paralleled interactions found to occur between well-characterized viruses of rice dwarf and wound tumor and their insect vectors.6 However, repeated efforts to characterize the suspected corn stunt virus morphologically remained unsuccessful. We have made an electron-microscopy study and found that Mycoplasma-like bodies are associated with diseased corn plants and with corn stunt-transmitting leafhopper vectors but are absent from healthy corn plants and from stock insects used as controls. Materials and ilethods.-Stock Dalbulus elimatus (Ball) insects were reared on corn plants, and infective vectors were obtained by confining leafhoppers to stunted corn for 30 davs. Afterwards, the vectors were individually tested for 1 week on healthy corn seedlings to ascertain their infectivity. Brains, ventral ganglia, and intestinal tracts were excised and processed for electron microscopy examination.7 Chlorotic, adventitious shoots were excised from diseased corn plants and prepared for thin sectioning as described previouslvy8 Control specimens from healthy plants were lerepared and examined in the same manner as the diseased material. Results. In 4 out of 11 corn stunt-transmitting leafhoppers, M1ycoplasma-like bodies, 80-800 mA in diameter, were observed in the ventral ganglia cells (Fig. 1A) and the filter chamber region of the intestine (Fig. 1B). These pleomorphic bodies possessed a variety of internal structures surrounded by a unit membrane. One type had central regions of netlike strands assumed to be nuclear areas. This type contained ribosomelike granules, about 10-15 myl in size, in a ground substance of intermediate density. A second type contained ribosomelike granules in a dark substance but no central areas of netlike strands. Some bodies contained other combinations of the internal structures described. No similar bodies were observed in ten healthy stock leafhoppers that had no access to stunted corn. In thin sections of diseased plant tissue, Mycoplasma-like bodies similar to those found in infective insect vectors were observed in the