Natural infection of sweet cherry trees with Apple scar skin viroid.
Apple scar skin viroid (ASSVd) is a fruit-damaging pathogen that causes significant economic losses to pome fruit trees. In the context of a survey on fruit tree viroids in Greece, ASSVd was initially detected by RTPCR in two sweet cherry trees of cv. Tragana Edessis in an apple orchard in Florina (Macedonia, Greece). This finding was confirmed by direct viroid sequencing of the amplified RT-PCR products. In order to verify this finding, we further examined four sweet cherry trees cvs Tragana Edessis and Biggareau Burlat, two sweet cherry trees of undetermined cultivar, and fifteen neighboring apple trees in the same orchard for possible infection with ASSVd. The viroid assay was done by tissue print hybridization using an ASSVd-specific DIG-labeled probe at stringent hybridization conditions and by RTPCR using two different ASSVd-specific primer pairs. ASSVd was detected in the six sweet cherry trees, including symptomatic samples, but not in any of the 15 apple trees. Purified ASSVd-positive RT-PCR products from sweet cherries were sequenced either directly or after cloning into pGEM-T or pCR II plasmid vectors. Sixteen ASSVd sequences obtained from five trees were 327-340 nucleotide long and shared 96-99% identity with ASSVd isolates from Indian apples. There was no cherry-specific nucleotide changes in the ASSVd sequences obtained. The viroid was graft transmitted successfully from cherry trees to cherry rootstocks and the newly developed rootstock leaves were ASSVd-positive by RT-PCR. To our knowledge, this is the first molecular and biological analyses of ASSVd infecting sweet cherry trees.