Homology of the Agent Associated with Dapple Apple Disease to Apple Scar Skin Viroid and Molecular Detection of These Viroids

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Abstract
Gel electrophoresis coupled with molecular hybridization analyses using 32P-labeled SP6-generated apple scar skin viroid (ASSV)-specific cRNA probes demonstrated that the pathogen associated with dapple apple disease is a viroid that is closely homologous to ASSV. Dapple apple viroid (DAV) consists of fewer than 359 nucleotides and is systemically distributed in apple seed, fruit, bark, leaf, and root tissues of infected apple trees. Molecular hybridization assays using 32P-labeled ASSV cRNA probes have been developed and applied for the detection of DAV or ASSV in small amounts of infected apple tissue (0.2-2.0 g). These assays are accurate, easy to perform, and applicable for screening DAV or ASSV in improved apple cultivars. These viroids now can be positively identified from infected apple tissue in a few days instead of a few years by fruit symptoms on grafted woody indicators.