Apple scar skin, one of the most destructive diseases affecting apple, is caused by Apple scar skin viroid (ASSVd). Fruit dappling appeared on several cultivars in Korea and has been distributed to major cultivated areas since 2001. ASSVd was identified from infected fruits by using nucleic acid sequence-based amplification with electrochemiluminescence (NASBA-ECL). NASBA-ECL method was faster and hundredfold more sensitive than reverse transcription-polymerase chain reaction (RT-PCR) for ASSVd detection in apple leaves/ stems. ASSVd was rapidly transmitted to the entire tree in the second year after artificial inoculation. The ASSVd could be transmitted efficiently by using contaminated pruning scissors to both lignified stems (60 to 70%) and green shoots (20 to 40%) of apple tree and young plants. Dipping of contaminated scissors in 2% sodium hypochlorite solution effectively prevented viroid transmission. In the ASSVd-infected fruits, the viroid was easily detected from fruit skin, seed coat, and embryo. Moreover, embryo and endosperm separately excised from the ASSVd-infected seeds were ASSVd positive in NASBA-ECL assay. Seedlings germinated from ASSVd-positive seeds showed 7.7% infection rate., which indicated that ASSVd is seed-borne.