Viruses are obligate parasites; that is, they require a living host in order to grow and multiply. Once in a wounded cell, the virus particle sheds its protein coat and the nucleic acid then directs the production of multiple copies of itself and related proteins leading to the development of new virus particles. Cell-to-cell movement of plant viruses occurs through the cytoplasmic “bridges” between cells called plasmodesmata and move systemically throughout infected plants via the phloem. Interestingly, many kinds of plants are infected with viruses and show no symptoms. Such infections are referred to as being latent. Some viruses, such as cucumber mosaic virus (CMV) and cowpea mosaic virus (CPMV), occur as a complex of multiple component particles, each containing different nucleic acid cores. In multi-component viruses, all components have to be present in a plant for infection and replication to take place.

Viruses are difficult to classify and, for want of anything better, they are given descriptive (and sometimes colorful) names based on the disease they cause, for example, tobacco ring spot, watermelon mosaic, barley yellow dwarf, potato mop top, citrus tristeza, sugar beet curly top, lettuce mosaic, maize dwarf mosaic, potato leaf roll, peach yellow bud mosaic, African cassava mosaic, carnation streak, and tomato spotted wilt. Many of these viruses also infect plants of other species. For example, tobacco ring spot virus causes a bud blight in soybeans; maize dwarf mosaic infects sorghum.