Viroids are the smallest infectious pathogens known. They consist solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens (plant diseases), some of which are can cause crop loss. Viroid genomes are extremely small in size. They are about 80 times smaller than the smallest virus. The human pathogen (causes diseases in humans) hepatitis D virus is a defective RNA virus similar to viroids.
Viroids were the first "sub-viral pathogens" discovered and named by Theodor Otto Diener. He was a plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971. The first viroid to be identified was the Potato spindle tuber viroid (PSTVd). About 33 species have been identified.
- Hammond, Rosemarie W. ; Owens, Robert A. "Viroids: New and Continuing Risks for Horticultural and Agricultural Crops". The American Phytopathological Society. Retrieved 7 August 2015.CS1 maint: multiple names: authors list (link)
- Dasgupta, M K, Principles of Plant Pathology (New Delhi: Allied Publishers, 1988), p. 132
- A defective virus cannot make copies of itself outside of a living host cell
- Alves C, Branco C, Cunha C (2013). "Hepatitis delta virus: a peculiar virus". Adv Virol. 2013: 560105. doi:10.1155/2013/560105. PMC 3807834. PMID 24198831.
- Diener TO (August 1971). "Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA". Virology. 45 (2): 411–28. doi:10.1016/0042-6822(71)90342-4. PMID 5095900.
- "ARS Research Timeline – Tracking the Elusive Viroid". 2006-03-02. Retrieved 2007-07-18.
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