There are two common mechanisms that plant viruses use to spread:
The virus is received from an external source. In order to "invade" the plant, the virus must penetrate the plant's outer protective layer known as the epidermis (epi-, -dermis). Plants that have been damaged by the weather, insects, etc. are typically more susceptible to the virus.
In vertical transmission, the virus is inherited from a parent. This transmission can occur in both asexual and sexual reproduction.
In most cases, scientists have been unable to find cures for plant viruses, so scientists have been focusing on reducing the occurrence and transmission of the viruses.
Viroids are tiny strands of RNA, usually only a few hundred nucleotides long. Viroids can interfere with a plant's metabolism.
Generally speaking, where viroids come from and how they can disrupt the host cell are not known.