Viroids are the smallest known plant pathogens.They are single-stranded, covalent closed circular RNAs with the size range of 246-400 nucleotides.Viroids do not encode any pathogen-specific proteins and therefore viroids propagation is fully dependent on the host biochemical machinery.They exclusively infect plants and autonomously replicates via RNA/RNA pathway using host machinery.As might be expected from their highly base-paired structure and RNA-RNA mode of replication,viroids have been shown to induce RNA silencing.