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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 caused by the apple scar skin viroid (ASSVd) is extremely rare in Europe. ASSVd was detected by molecular hybridization and indexing in field plots on the apple indicators Starkrimson and Indo, which showed symptoms of dapple apple disease within 2 years, and rough scarred skin within 3 years, respectively. Results from both approaches were in agreement. In an attempt to improve the biological detection of ASSVd, the Japanese PK13 isolate was inoculated to 4 Prunus, 13 Malus, 17 Pyrus, and 17 other pomaceous species. All the species tested of the Malus, Pyrus, Sorbus, Chaenomeles, Cydonia, and Pyronia genera were susceptible to ASSVd based upon back indexing and hybridization, but none developed leaf or bark symptoms during a 2-year period. The viroid was not detected in the tested members of genera Amelanchier, Aronia, Cotoneaster, Crataegus, Prunus, and Pyracantha. Symptoms on fruit of 42 commercial apple cultivars experimentally inoculated with ASSVd fell into five groups ranging from inconspicuous spots to severely scarred skin and cracking. ASSVd was eliminated from most of the infected apple plants when they were subjected to a dormant stage followed by thermotherapy and shoot tip grafting. Analysis of more than 400 apple seedlings, originated from Starkrimson and Indo fruits with typical ASSVd symptoms, showed that there is little or no seed transmission of this viroid. However, ASSVd was transmitted at a low rate under field conditions to adjacent trees.