Theodor Otto Diener (born February 28, 1921) is the Swiss-American plant pathologist who, in 1971, discovered that the causative agent of the potato spindle tuber disease is not a virus, but a novel agent, which consists solely of a short strand of single-stranded RNA without a protein capsid, eighty times smaller than the smallest viruses.
He proposed to name it and similar agents to be discovered viroids.
Viroids displace viruses as the smallest infectious agents known.