Leaf samples of black bryony(Tamus communisL.) from Devon, UK, showing various virus‐like symptoms contained potyvirus‐like particles (normal lengthc.790 nm) and cytoplasmic cylindrical (pinwheel) inclusions. In immunoelectron microscopy, particles of most samples reacted with antiserum to dioscorea greenbanding mosaic virus, a potyvirus isolate fromDioscorea rotundatain Togo which is related to yam mosaic virus from the Ivory Coast. Potyvirus particles were not transmitted by sap or aphids(Myzuspersicae)from infected black bryony to black bryony seedlings orNicotiana benthamiana.One sample from a symptomless plant of black bryony contained a potexvirus which formed massed virion aggregates in the cytoplasm of cells of black bryony,Nicotiana benthamianaandN. megalosiphon.Virions of the potexvirus (normal length 553 nm) contained a coat protein with an apparent molecular weight of 27.7 kd. The potexvirus differed from an Italian potexvirus isolate from black bryony by a serological differentiation index of 4 and gave only weak or no reactions with 23 other antisera to potexviruses, including dioscorea latent virus. The potexvirus caused systemic symptoms in only a few host plants and could be transmitted back to black bryony in which it caused no symptoms. It is provisionally named tamus latent vi
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