Plant viruses spread by insects like aphids, or by infected seeds, asexual propagation. Plasmodesmata are holes in plant cell walls for cytoplasmic connections. Virus moves thru these. Reduced size, productivity, changes in leaves, flowers. Agriculture looks for viral resistant varieties.
Viroids have no associated proteins.
Genome does not code for proteins Very short strand of RNA, copied by host polymerase with viroid template Plant and animal diseases, may interfere with gene regulation in host cell nucleus. Transmissible Spongiform Encephalopathies. Brain gets holes. Scrapie in sheep and goats, lose coordinaiton, become irritable, itch. Bovine spongiform encephalopathy - ate feed that had been mixed with sheep brains and other organs.