With virus diseases, the exact identification of the parasite is difficult. Viruses are so small that they cannot be seen through the ordinary microscopes used to detect and study fungous and bacterial parasites. They can be photographed by means of modern electronic microscopes, but even so they am something of a mystery. Undoubtedly, plants suffering from virus disease have some form et infectious agent in their sap, but in many cases im exact nature has not been identified. It is, however, known to be very small and to multiply within the plants' cells, so that it is usually distributed throughout the tissues.
Results of Virus Infection
Plants, unlike animals, do not seem to produce antibodies to fight viruses, although, in some cases, they are able to resist to a certain extent. Thus, more than one virus can exist in a plant at the same time. For a few plants virus attack means sudden death, but usually infected plants become more crippled and degenerate with the passing of each season.
Seeds of most virus-infected plants are usually free from virus, so, by saving seed, clean stock can be obtained again. This method is, however, suitable only in the case of fairly short-lived plants.Special care has to be taken, to exclude virus infection from clean fruit trees and bush fruits.
Symptoms of Virus Infection
Common signs of virus infection are stunted growth and mottled patterns on the leaves (often referred to under the general term "mosaic"). Other signs are ring-like markings on the leaves (ring spots); ozrrling or distortion of leaves and shoots; "breaking" of flowers (white streaking in the colour of the petals); abnormal production of shoots (proliferation); and many other abnormalities. Infection usually results in all the cells of the plants being waded by the virus, although shoots already developed are not usually much altered. New shoots arid leaves, however, begin to show abnormal symptoms as they grow.
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