โ Scribed by L. Bos
No coin nor oath required. For personal study only.
Over 50 plant diseases previously ascribed to viruses have now been associated with mycoplasma-like organisms (MLO). Their detection as plant pathogens in 1967 has started a new chapter in the history of plant pathology (Bos, 1970b).
Most of the diseases concerned are typical leafhopper-borne witches' broom (yellows-type) diseases characterized by hormonal disorders (witches' broom growth, virescence, phyllody, intensified negative geotropy and disturbed periodicity; e.g. t/os, 1970a). Phloem degeneration may also occur and lead to atypical growth reduction, discolorations, wilting and premature death. Some other diseases, like pear decline (psylla-transmitted) and elm phloem necroses, more recently ascribed to MLO, are exclusively characterized by deviations resulting from phloem degeneration. Symptomatologically the latter diseases are similar to those caused by certain phloem-limited viruses.
The diseases were long known to be caused by pathogens which could neither be transmitted mechanically, like many viruses, nor be cultivated in vitro like most microorganisms. Kunkel (1926) proved that aster yellows is transmissible artificially by grafting and that it is naturally spread by leaf hoppers. Black (1941) clearly demonstrated multiplication of the aster yellows agent in its leaf hopper vector. In this respect it does not differ from persistent insect-borne plant viruses. Although the leafhopper-transmitted pathogens met the then prevailing definition of viruses, virus particles could not be isolated.
Japanese researchers (Doi et al., 1967) first found wall-less pleomorphic MLO in ultrathin sections of phloem cells of witches' broom-diseased plants, whereas the organisms were absent in healthy plants. These observations were soon confirmed by several others for various systemic witches' broom diseases, and the MLO were also found in insect-vector tissue. Their presence as well as disease symptoms were suppressed by tetracyclines (Ishiie et al., 1967). In 1971 and 1972 French workers (Giannotti and collaborators, cf. Giannotti, 1972) first provided convincing evidence of plant mycoplasma cultivation in vitro from plant and insect tissue, reinfection of plants, and fulfillment of Koch's postulates.
Recently American and French workers have demonstrated the MLO of corn stunt (Davis et al., 1972, see Davis andWorley, 1973) and citrus stubborn (Cole et al., 1973) to be motile and of spiral shape (spiroplasma).
Other American plant virologists showed some other diseases of suspected virus etiology to be associated with Rickettsia-like organisms, clearly differing from mycoplasma in having a rigid cell wall and a spherical or slightly elongated form. They were found in phloem of clover with club leaf (Windsor and Black, 1972) and in xylem of grapevine with Pierce's disease and lucerne with alfalfa dwarf (Goheen, Nyland and Lowe, 1973).
Thus, within the group of non-bacterial plant-pathogenic organisms (MLO, sometimes referred to as PPLO or PLT) there is a beginning differentiation (see also Davis and Whitcomb, 1971). Mycoplasmas (Mollicutes) are considered the smallest living wall-less organisms, still having DNA and RNA. Viruses definitily differ in having only DNA or RNA, and in that they can be characterized physico-chemically. However, in their relation to plants (systemic infec-
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