question: Which are the smallest known subviral pathogens of plants?
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.