COMPARATIVE ULTRASTRUCTURE OF DOUBLE MEMBRANE-BOUND PARTICLES AND INCLUSIONS ASSOCIATED WITH ERIOPHYID MITE-BORNE PLANT-DISEASES OF UNKNOWNETIOLOGY - A POTENTIALLY NEW GROUP OF PLANT-VIRUSES

Occurrence of unique virus-like particles and associated inclusions in five different plant diseases: yellow ringspot of redbud, rose rosett e, thistle mosaic, fig mosaic and high plains disease of corn and whea t, is described. Quasi-spherical, double membrane-bound particles (DMP s), 120-200 nm in diameter, were found in the cytoplasm of all cell ty pes in symptomatic leaves of infected plants. No DMPs were observed in symptomless plants. The DMPs in symptomatic thistles were associated with two types of inclusions, electron-dense amorphous material and tu bular aggregates. Similar amorphous inclusions were also found in corn and wheat with high plains disease, while tubular inclusions were obs erved in figs with mosaic symptoms. The particles and inclusions were similar in some aspects to immature particles associated with viroplas ms of animal and insect poxviruses and also to the double-enveloped pa rticles of tomato spotted wilt virus associated with viroplasms during early stages of infection, but were unique and unlike any known plant viruses. Based on cytopathology, symptomatology and mite and/or graft -transmissibility, the five diseases described in this paper are poten tially caused by virus(es) and the DMPs associated with these diseases may represent virus particles. If the DMPs are indeed viral in nature , they would comprise a new group of plant viruses.