Biological properties of apple scar skin viroid: Isolates, host range, different sensitivity of apple cultivars, elimination, and natural transmission

Studies conducted over the last 10 years have revealed that the disease cau sed by the apple scar skin viroid (ASSVd) is extremely rare in Europe. ASSV d was detected by molecular hybridization and indexing in field plots on th e apple indicators Starkrimson and Indo, which showed symptoms of dapple ap ple disease within 2 years, and rough scarred skin within 3 years, respecti vely, Results from both approaches were in agreement. In an attempt to impr ove the biological detection of ASSVd the Japanese PK13 isolate was inocula ted to 4 Prunus, 13 Malus, 17 Pyrus, and 17 other pomaceous species. All th e species tested of the Malus, Pyrus, Sorbus, Chaenomeles, Cydonia, and Pyr onia genera were susceptible to ASSVd based upon back indexing and hybridiz ation, but none developed leaf or bark symptoms during a 2-year period. The viroid was not detected in the tested members of genera Amelanchier; Aroni a, Cotoneaster, Crataegus, Prunus, and Pyracantha. Symptoms on fruit of 42 commercial apple cultivars experimentally inoculated with ASSVd fell into f ive groups ranging from inconspicuous spots to severely scarred skin and cr acking. ASSVd was eliminated from most of the infected apple plants when th ey were subjected to a dormant stage followed by thermotherapy and shoot ti p grafting. Analysis of more than 400 apple seedlings, originated from Star krimson and Indo fruits with typical ASSVd symptoms, showed that there is l ittle or no seed transmission of this viroid. However, ASSVd was transmitte d at a low rate under field conditions to adjacent trees.