Withered areas are called necroses . Chloroses are caused by a breakdown of the chlorophyll resulting in a decreased rate of photosynthesis. Heavy infections are characterized by a complete local loss of chlorophyll. Affected areas have a yellowish look as only the carotenoids remain. In nature, such strains are pretty rare since such a strong damaging of the host does also impair the virus chances of replication and spreading.
Some viruses multiply within the plant without causing symptoms.
This phenomenon is called latent infection . In contrast, wound tumor virus cause the development of tumors. The symptoms of most viruses are dependent on both virus and host, and do thus present an important diagnostic feature. As a result, fully differentiated, old leaves and roots, and young leaves are equally infected. Isolated TMV-RNA itself is infectious.