Viruses comprise one of the groups of causative agents of plant infections, the others being fungi, bacteria, some parasitic higher plants, and nematodes. Plant viruses, other than those few that may cause disease in both the insect vectors and the plant, do not infect humans or other animals.
Like some plant pathogens, including rust fungi, plant viruses are obligate parasites - "parasitic organism(s) that cannot live independently of (their) host(s)" - ("Obligate parasite", the Wikipedia).
There are two types of virus disease of plants: the mosaic diseases and the leaf curl and yellows diseases. Each of these two types of virus disease of plants is distinguished by the types of harm or injury it causes on plants. Of the two, mosaic diseases make up the largest and most important group in microbiology.
In mosaic diseases, there occurs mottling of leaves; yellowish blotches and necrotic spots appear on the leaves or sometimes on the blossoms of the diseased plant. Tomato spotted wilt and tobacco and cucumber mosaic diseases are two economically important examples of such plant infections.
In ornamental horticulture, the effects of some such plant infections have been utilized to produce rare variegations on blossoms or foliage. An example of such variegation - called breaking - is seen in certain tulips.
In the leaf curl and yellows diseases, there occur curling and yellowing of leaves, dwarfing, and at times excessive branching, too. This type of virus disease of plants includes such diseases as peach yellows, strawberry stunt, and sugar beet curly top.
Tumors may also develop in stems and roots of plants infected by the leaf curl and yellows diseases. An example of this is the wound-tumor disease, "a disorder induced by a virus that affects many plant species..." - ("The Nature of Disease in Plants", p. 28, by Robert P. Scheffer, online version).
Pathogenic plant viruses are spread by various means, including the following: by insects, by infected seed, or by infected vegetative parts of plants used for propagation. Insects are the sole vectors of many plant viruses. They include aphids, leafhoppers, mealybugs, and whiteflies. These insects transmit infected plant juices from plant to plant. Likewise, when plants are propagated by budding, grafting, roots, shoots, tubers, or other vegetative means, any virus existing in the original plant is transmitted to the new plant.
Many plant infections or diseases caused by viruses cause considerable losses to the agricultural economy. Foremost in the list of economic importance are virus diseases that affect potatoes, stone fruits, sugar beets, sugar cane, and tomatoes.
A great deal of time and research has been spent and made in the study of virus disease of plants, notably those classified as mosaic diseases and those under the leaf curl and yellows diseases group; this provided vital information about the nature of plant viruses in general and how exactly they affect the different plant species.
1. Agri Overview, "Viral diseases of economic crops", by M. Mithal Jiskani, Assistant Professor (Plant Pathology), Sindh Agriculture University, Tandojan - http://www.pakissan.com/english/agri.overview/viral.diseases.of.economic.crops.shtml
2. "Diseases of Plants", from The Columbia Encyclopedia, Sixth Edition, 2008 (online) - http://www.encyclopedia.com/doc/1E1-diseases.html
3. "Mosaic Virus Diseases of Vine Crops, HYG-3109-95", by Stephen T. Nameth, The Ohio State University Extension Fact Sheet, Plant Pathology - http://ohioline.osu.edu/hyg-fact/3000/3109.html