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. They require a wound to gain entrance to a plant cell. In nature, they depend primarily on biological agents such as nematodes, insects and man for their dissemination. Once duplication starts, the virus is translocated from cell to cell through the plasmodesmata and to distant plant parts by the phloem.
In general, viruses are seldom lethal to plants, but do severely affect the host both in quantity, quality and longevity. Symptoms may often be very characteristic for a specific virus on a specific host. Symptoms along with other criteria are used to identify virus diseases. An advanced array of symptoms can be recognized today as expressions of viral diseases in plants.
Some of these would include abnormal leaf color, abnormal vein patterns of leaves, mottling in leaves [figure 1], [figure 2], spotting patterns in leaves, and abnormal leaf shape [figure 4], and [figure 7]. There are also abnormalities of flower color, fruit size, shape and color. [figure 3][figure 5][figure 6] With some virus diseases, the symptoms are masked.
Viruses can be spread from plant to plant by several means. Some of these would include transmission from the parent plant to an offspring through the genetic structure of the plants. Other ways in which viruses can be transmitted are through vegetative propagation, grafting and budding, seed transmission and mechanical spread by insects and man.
At the present time, no effective chemicals will control virus diseases. Therefore, sanitation and use of resistant varieties of plants has been the most effective means of controlling plant viruses.
In some instances, when the spread of the virus is slow, loss from disease can be reduced by removing diseased plants and replacing them with healthy replants. This method has been used to reduce losses from peach mosaic. Reducing the population of insect vectors by insecticides or by other means, such as elimination of host plants for the insects, has given at least partial control of some virus diseases.