Question: Which are the smallest known subviral pathogens of plants?

Context:
Since the discovery of non-coding, small, highly structured, satellite RNAs (satRNAs) and viroids as subviral pathogens of plants , have been of great interest to molecular biologists as possible living fossils of pre-cellular evolution in an RNA world.
PFOR2 analysis of the small RNA libraries from grapevine and apple plants led to the discovery of Grapevine latent viroid (GLVd) and Apple hammerhead viroid-like RNA (AHVd-like RNA), respectively.
We show that viroids from the two known families are readily identified and their full-length sequences assembled by PFOR from small RNAs sequenced from infected plants. 
Viroids are plant subviral pathogens whose genomes are constituted by a single-stranded and covalently closed small RNA molecule that does not encode for any protein. 
Viroids are small, circular, single-stranded RNA molecules that cause several infectious plant diseases. 
Viroids: petite RNA pathogens with distinguished talents.
Subviral pathogens of plants: viroids and viroidlike satellite RNAs.
Contrary to earlier beliefs, viruses are not the smallest causative agents of infectious diseases. Single-stranded RNAs as small as 246 nucleotides exist in certain higher plants and cause more than a dozen crop diseases. These RNAs have been termed viroids.
Subviral pathogens of plants: the viroids
Viroids, satellite RNAs, satellites viruses and the human hepatitis delta virus form the 'brotherhood' of the smallest known infectious RNA agents, known as the subviral RNAs.
Subviral pathogens of plants: the viroids.
Viroids, subviral pathogens of plants, are composed of a single-stranded circular RNA of 246-399 nucleotides.
During 1970 and 1971, I discovered that a devastating disease of potato plants is not caused by a virus, as had been assumed, but by a new type of subviral pathogen, the viroid. Viroids are so small--one fiftieth of the size of the smallest viruses--that many scientists initially doubted their existence. 
The database of the smallest known auto-replicable RNA species: viroids and viroid-like RNAs.
Viroids, satellite RNAs, satellites viruses and the human hepatitis delta virus form the &apos;brotherhood&apos; of the smallest known infectious RNA agents, known as the subviral RNAs.
 Replicating circular RNAs are independent plant pathogens known as viroids, or act to modulate the pathogenesis of plant and animal viruses as their satellite RNAs. The rate of discovery of these subviral pathogens was low over the past 40 years because the classical approaches are technical demanding and time-consuming.

Answer: Contrary to earlier beliefs, viruses are not the smallest causative agents of infectious diseases. Single-stranded RNAs as small as 246 nucleotides exist in certain higher plants and cause more than a dozen crop diseases. These RNAs have been termed viroids. Viroids are plant subviral pathogens whose genomes are constituted by a single-stranded and covalently closed small RNA molecule that does not encode for any protein.
Since the discovery of non-coding, small, highly structured, satellite RNAs (satRNAs) and viroids as subviral pathogens of plants , have been of great interest to molecular biologists as possible living fossils of pre-cellular evolution in an RNA world.
Exact answer: Viroids