Apple dimple fruit viroid: Fulfillment of Koch's Postulates and Symptom Characteristics

ABSTRACT The previously reported properties of a small circular RNA isolated from plants of cultivar Starking Delicious with symptoms similar to those of the dapple apple disease induced by Apple scar skin viroid (ASSVd), strongly suggested that it is a distinct viroid species of genus Apscaviroid to which ASSVd also belongs. Nevertheless, the autonomous replication of this novel RNA, termed Apple dimple fruit viroid (ADFVd), and the nature of the symptoms incited when inoculated free of other pathogens that might be present in the original source, were not determined. We addressed these questions by mechanically inoculating purified ADFVd into young apple seedlings (Golden). Dot blot hybridization of leaf extracts collected ten months after inoculation showed the presence of ADFVd in most of the inoculated seedlings, thus proving the ability of this RNA to replicate independently. Material from these ADFVd-infected seedlings grafted onto the ASSVd indicator Starkrimson incited the appearance on the fruit...