Scar skin and dapple apple viroids are seed-borne and persistent in infected apple trees

The closely related apple scar skin viroid (ASSV) and dapple apple viroid (DAV) were identified in whole seeds from infected pome fruits by hybridization of extracted nucleic acids with a 32P-labelled ASSV cRNA probe. Viroid amounts were greater in seed coats and subcoats than in seed cotyledons and embryos. ASSV or DAV was also detected in nucleic acid extracts from infected seeds, cotyledons and embryos by reverse transcription/polymerase chain reaction with viroid-cDNA-specific primers followed by Southern blot hybridization analysis of the amplified products with an ASSV cRNA probe. These results indicate that ASSV and DAV are seed-borne. ASSV and DAV were also found in the anthers, petals, receptacles, leaves, bark and roots of infected trees. The results suggest that viroid-infected trees constitute potential sources of the viroid in field spread. ASSV and DAV infections have been observed sporadically in commercial orchards in the United States and Canada and the infected trees have been eliminated. The use of viroid-free sources of seeds, seedlings, rootstocks and budwood should greatly reduce the risk of the future spread of the viroid.