Final answer:
The statement that viroids cause diseases in plants is true. Viroids are composed of RNA, lack a protein coat, and must replicate within a host cell.
Step-by-step explanation:
Viroids are known to be plant pathogens that can cause diseases in plants. They are very small particles, composed of a single-stranded, circular RNA molecule without a protein coat, making them different from viruses in that they lack a capsid or any outer envelope. Unlike viruses, which can contain either DNA or RNA as their genetic material, viroids are solely composed of RNA. Therefore, they do not produce any proteins and they replicate only within the host cell. No human diseases caused by viroids have been identified so far. Regarding the structure and characteristics of viruses, it is important to note that a virus is composed of DNA or RNA surrounded by a protein coat, and glycoproteins on its surface can help the virus attach to the host cell.