Viruses are the smallest pathogens presented in these information sheets about pests and diseases. The infectious viral particle is called a virion, which is a stable, non-multiplying stage by which the virus is transferred from one plant to another. Viruses multiply in the host plant, and transmission may occur via several means: by insects and mites (especially sucking insects, such as aphids), by nematodes, by seed, by pollen, by fungi, by soil and mechanically.

Viral diseases are often difficult to detect because infected hosts may not display visible symptoms, or symptoms may closely resemble those of various physiological disorders or genetic abnormalities. Identification can be facilitated by determining which vectors are present and the host range; in many cases, positive identification requires the use of an electron microscope and serological techniques.