A non-mechanically transmissible isometric virus associated with asteroid mosaic of the grapevine

Introduction: Asteroid mosaic, a long-known viruslike disease of the grapevine (Vitis vinifera), was originally described from Napa Valley in Ca1ifornia (HEWITT 1954 ), where it remained localized without apparent spreading and is now disappearing (A. C. UOHEEN, personal communication). Naturally infected vines exhibit star-shaped chlorotic spots, sometimes with necrotic centre, irregularly distributed over the leaf blades which may be asymmetric, twisted and puckered along the veins. Foliar symptoms are often accompanied by stunting and unfruitfulness. The disease is caused by an unknown graft-transmissible agent which induces highly diagnostic localized clearing of the main and secondary veins in V. rupestris (HEWITT et al. 1962). Asteroid mosaic-like symptoms have been repeatedly observed in V. vinifera in Europe and South Africa (REFATTI 1970) but no published records exist of their graft transmissibility to V. rupestris . Recent successful graft-transmission trials carried out in Greece, constitute, therefore, the first experimental evidence of the occurrence of asteroid mosaic outside USA (K YRIAKOPouwu 1991 ). In the assumption that the agent of asteroid mosaic could be a virus, investigations were initiated using an authentic diseased Californian source which was maintained and propagated in isolation in V. rupestris under glasshouse conditions. This paper reports the preliminary results of these investigations.