Who discovered viroids?

Hint: In the five-kingdom classification of Whittaker there is no mention of lichens and some other acellular organisms like viruses, viroids, and prions. They did not belong to the kingdoms classified by Whittaker which included Monera, Protista, Fungi, Animals, and Plants Kingdom.

Step by step answer:In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and causes potato spindle Tuber disease. It was found to be free RNA. It lacked the protein coat that is found in viruses hence the name viroid. The RNA of the viroid is of low molecular weight.
Viroids are the smallest infectious pathogens known. Viroid is composed solely of a short strand of circular, single-stranded RNA that has no protein coating. All the known viroid are inhabitants of higher plants and most cause diseases, whose respective economic importance on humans varies widely.
All the viroid are composed of nucleic acid but did not code any protein. The viroid replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with the synthesis of messenger RNA from DNA which instead catalyzes rolling circle synthesis of new RNA using the viroid RNA as a template.
Some additional information :
Some differences between viroids and viruses. In viruses, a protein coat is formed but in viroids it is absent. In viruses DNA And RNA both are present. But in viroids only free RNA is present. Viruses cause disease in both plants and animals where viroids cause disease in plants.

Note: The size of viroids is smaller than that of viruses. The first disease that was reported of viroids was potato spindle tuber disease. The RNA is the genetic material of the viroid of low molecular weight.