First report and molecular analysis of Apple scar skin viroid in sweet cherry

Apple scar skin viroid (ASSVd) is a serious pathogen of pome fruits. Recently, it has been reported in Chinese apricot and Chinese peach. In the context of our research on fruit tree viroids in Greece, ASSVd was initially detected in a sweet cherry tree cv ‘Tragana Edessis’ from Florina (Macedonia) by RT-PCR and this finding was confirmed by direct sequencing. This tree is located at the edge of a newly established apple orchard, along with other sweet cherry and wild cherry (Prunus avium) trees. In order to verify this interesting finding, we examined for ASSVd four sweet cherry trees, two wild cherry trees and their neighboring apple trees in the same orchard.

The examination was done by imprint hybridization using an ASSVd-specific DIG-labelled probe at stringent hybridization conditions and by RT-PCR using two different ASSVd specific primer pairs. We obtained ASSVdpositive results for all 6 cherry trees. No ASSVd was detected in the apple trees of the orchard. Purified ASSVdpositive RT-PCR products from the cherries were directly sequenced or cloned into the pGEM-T vector and then sequenced. ASSVd sequences were obtained from 5 trees. These sequences are 327-340 nucleotides long and share 96- 99% identity with ASSVd isolates from Asian (Indian) apples. These results are similar to our data for other ASSVd variants from cultivated and wild pome fruit trees in Greece.

The cherry ASSVd variants differ from the ASSCS prototype isolate of ASSVd at 18-29 sites. There are 15 nucleotide changes (differences from ASSCS) common to all Hellenic ASSVd variants, including cherry and pome fruit tree variants. There are no cherry-specific nucleotide changes in the ASSVd sequences obtained. To our knowledge, this is the first published report of natural infection of cherry by ASSVd.