Plant Virus Diseases Viral Diseases of Plants Viruses are sub-microscopic, infectious particles that multiply only inside living host cells.
Viruses are for the most part beyond the resolution capabilities of a light microscope.
Consequently, virus structures have been determined primarily by electron microscopy and X-ray defraction. They vary in size from about 20 millimicrons to 1200 millimicrons. Plant viruses are made up of two components - a protein coat and the nucleic acid center. The nucleic acid is the infectious component of a virus. Viruses are obligate parasites, meaning that they must be within living tissue before they can reproduce themselves.