176 Sinclair et al: Tree and Shrub MLO Diseases MYCOPLASMALIKE ORGANISMS AS CAUSES OF SLOW GROWTH AND DECLINE OF TREES AND SHRUBS

Abstract. Mycoplasmalike organisms are obligate parasites of plants and insects. In recent years, advances have been made in detecting and identifying them, distinguishing among those associated with various plant diseases, assigning them to groups based upon homology of DNA base sequences, and determining their roles in decline of trees and shrubs. These advances are reviewed with special reference to ash yellows and elm yellows. Known and probable relationships among MLOs associated with various plant diseases are tabulated. Mycoplasmalike organisms (MLOs) have been recognized as a distinct class of plant pathogens since 1967(25), but their obligately parasitic habit has hindered research into their biology, ecology, and relationships to other organisms. Therefore, most diseases caused by MLOs remain poorly known, and those affecting trees and shrubs are generally uncontrolled. The rise of biotechnology has fostered advances in understanding of my-coplasmal plant pathogens. The purposes of this review are to highlight some of these advances and point out their significance for the eventual control of MLOs that affect trees and shrubs. Ash yellows and elm yellows will receive emphasis, because they are significant diseases of shade trees and subjects of the authors ' research