Viroids are small, unencapsidated ssRNA molecules- the smallest known pathogen of plants.
The term viroid has recently introduced by T.O. Diener (1971).The first to be examined in detail was potato spindle tuber viroid (PSTVd) responsible for significant loss to the potato industry.There are some 25 viroids which vary in nucleotide sequence.
Viroids are subviral and they consists of only of naked strand of nucleic acid without a protein coat.
The RNA is covalently closed circle and ranges in size from 246 to 357 nucleotides in length. The RNA has a complex secondary and tertiary structure which gives it a rod like shape and resistance to nucleases. The RNA does not have a characteristic open reading frame and so does not act as mRNA. How this piece of RNA cause is largely unknown. It replicates in plant cells with the aid of host cell enzymes (e.g. RNA polymerase II).
Viroids cause several important disease include citrus exocortis, Chrysanthemum stunt and cucumber pale fruit. Viroids are spread by plant propagation (e.g. cuttings and tubers) through seeds and by manual mishandling with contaminated implements.
The only human disease known to be caused by a viroids is Hepatitis D
Learn more: Difference Between Virus and Viroids