Since viruses are obligate intracellular parasites they must develop direct methods of transmission, between hosts, in order to survive.
However, the virus is dependent upon physical damage, generated naturally by the wind and feeding of vectors or by human intervention.
There are a variety of methods the virus can use to spread throughout the organism but the most common route use the vascular system, otherwise known as the xylem and phloem, and the plasmodesmata, which interconnect adjacent cells.
Consequently, the virus must modify the plasmodesmata as they, themselves, are too large to fit through the small and tightly regulated plant structure.
The main mechanisms involve expressing proteins which coat the virus and interact with the structure of the plasmodesmata.