NEW APPROACHES TO THE DIAGNOSIS AND CONTROL OF PLANT YELLOWS DISEASES

A group of agents new to plant pathology and fundamentally different in nature from viruses appear to be the causal agents of yellows diseases of plants (Davis and Whitcomb, 1971; Hampton, 1972; Maramorosch et al., 1970; Whitcomb and Davis, 1970). Doi et al. (1967) and Ishiie et al. (1967) proposed that these agents are mycoplasma- or PLT (chlamydia)-like organisms. The principal lines of evidence developed by these workers and extended by others constitute the major support for this hypothesis. The final proof of pathogenicity and the determination of the nature of the causal agents depend upon isolation and purification or cultivation of the agents in vitro. The progress of research in this area has been discussed in several reviews (Davis and Whitcomb, 1971; Hampton, 1972; Maramorosch et al., 1970).

In most reports on yellows diseases, the evidence from thin section electron microscopy, antibiotic therapy, and cultivation in vitro has been interpreted to indicate involvement in disease production of organisms closely similar to "classical" mycoplasmas. Nevertheless, the nature of the yellows pathogens as mycoplasmas has not clearly been established. Some of the yellows agents possibly may resemble certain obligate intra-cellular parasites more closely than they resemble mycoplasmas (Davis and Whitcomb, 1971). Indeed, recent findings on corn stunt (Davis et al., 1972a, b); clover club leaf (Windsor and Black, 1972, 1973), and certain other diseases (Beakbane et al., 1971; Bové et al., 1973; Lafleche and Bové, 1970) suggest that the yellows disease pathogens may be of an important and previously unsuspected heterogeneity. Eventually it may prove appropriate to consider the yellows diseases as several separate groups of disorders that may best be delimited based on the nature of their causal agents. In the case of clover club leaf, a rickettsialike organism appears to be associated with disease (Windsor and Black, 1973). In the case of corn stunt and citrus stubborn, unusual helical microorganisms, termed "spiroplasmas" (Davis and Worley, 1973) are associated with disease. These spiroplasmas can readily be seen in juice expressed from infected corn and, in the case of citrus stubborn, can be isolated from infected tissues and cultivated in vitro. It seems reasonable to expect rickettsialike organisms or spiroplasma in other diseases, but organisms most closely resembling "classical" mycoplasmas eventually may prove to be associated with many of the remaining yellows diseases. Any difference in the nature of the yellows pathogens, of course, will necessitate a broadening of the range of methods considered for diagnosis and control.

At present, data available on the spiral organisms associated with