Viroid, << VY royd, >> is one of the smallest known agents of infectious disease. All known viroids are highly structured molecules of RNA (ribonucleic acid) that cause disease in certain plants. There are no known animal or human viroids.
Like viruses, viroids can reproduce only within living cells. But viruses typically consist of a core of a nucleic acid surrounded by a coat of protein (see Virus (Structure)). Viroids lack a protein coat. Also, viroids are at least 10 times as small as the smallest viruses.
Scientists have identified about 30 viroid diseases of plants. These diseases affect such diverse crops as potatoes, citrus fruits, tomatoes, palms, cucumbers, hops, chrysanthemums, and avocados. Many of these diseases cause significant economic damage. For example, a disease that later proved to be caused by viroids nearly destroyed the chrysanthemum industry in the United States in the early 1950’s.
The Swiss-born scientist Theodor O. Diener is credited with the discovery of viroids. In 1971, Diener published evidence that a particle that infected potatoes differed structurally from viruses. Diener proposed the term viroid to describe this submicroscopic particle.
