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Morphology of Filamentous Forms of a Mycoplasmalike Organism Associated with Hydrangea Virescence. S. S. Hearon, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705; R. H. Lawson(2), F. F. Smith(3), J. T. McKenzie(4), and J. Rosen(5). (2)(3)(4)(5)Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705. Phytopathology 66:608-616. Accepted for publication 22 December 1975. DOI: 10.1094/Phyto-66-608.

The hydrangea virescence agent was graft-transmitted but not mechanically transmitted in Hydrangea macrophylla, the florists’ hydrangea. Transmission of the virescence agent was not achieved with the leafhopper, Macrosteles fascifrons, or dodder, Cuscuta campestris. Mechanical inoculations of healthy hydrangea seedlings with hydrangea ringspot virus failed to induce virescence symptoms. A mycoplasmalike organism (MLO) was observed in ultrathin sections of florists’ hydrangea showing flower proliferation and virescence. Spherical and polymorphic mycoplasmalike bodies of varying size and cell content were present in mature sieve cells. The MLO also commonly occurred as branched filaments, beaded filaments, and spherical bodies connected in long chains by a continuous membrane structure. The filamentous forms resembled those observed by Freundt in the culture of the animal pathogen, Mycoplasma mycoides. “Octopus-like” structures, consisting of numerous filaments attached to a central body that was devoid of protoplasm, were common in rapidly declining plants.

Additional keywords: electron microscopy, hydrangea phyllody, Hydrangea macrophylla ‘Strafford’.