Risk potential of international fruit trade for viroid spreading - case study on hop viroids in Europe

Most hops are produced in Europe; therefore, it is alarming that the citrus bark cracking viroid (CBCVd), the causal agent of the severe hop stunt disease, was detected in different nonadjacent hop growing countries. It is still unclear how the initial infection occurred since CBCVd is typically found in citrus and not in hops. To extent data for a viroid risk assessment, potential hosts were tested for the presence of viroids in grocery stores in the hop producing areas of Slovenia and Germany. Samples positive for hop-pathogenic viroids were further used for infection studies. The surveys covered CBCVd, hop stunt viroid (HSVd), citrus exocortis viroid (CEVd), citrus dwarfing viroid (CDVd), citrus viroid V (CVdV), and citrus bent leaf viroid (CBLVd). The results show that all tested viroids can be found in fruits sold in grocery stores, thus there is a risk of introducing CBCVd, HSVd, and other viroids into the hop growing regions via imported fruits and their remains. Furthermore, the transmission study reveals that CBCVd and HSVd infected citrus fruits can lead to infected plants, irrespective of the type of inoculum whether in the form of RNA extract, injected sap, or fruit peel in the soil. Finally, the phylogenetic analysis showed that the sequence diversity within viroid samples is high and that CBCVd and HSVd sequence variants can be found, which are almost identical to variants confirmed in hop. We assumed that fruit imports contribute to international viroid spreading and inappropriate handling like fruit waste deposition to agricultural lands is a serious risk factor.