Viroids are a recently recognised unique class of pathogen which differ from viruses in the absence of dormant phase (virions) and in having much smaller genomes. They lack the protein component, covering the nucleic acid of a virus and consists of a RNA molecule only. The first viroid discovered was the Potato Spindle Tuber Viroid (PSTV) which causes a disease in potatoes (Diner and Raymer, 1967).