Viruses
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Viruses
 Among plant pathogens causing infectious diseases the smallest are viruses and virus like organisms, ranging in size from 0.1 to 0.01 microns (1 micron = 0.001 mm). Viruses attack every form of life on earth. Viruses are inactive outside of living cells. Once inside a live cell, though, these pathogens use the cell’s ‘machinery’ to multiply themselves, upsetting the cell’ metabolism and causing disease. A single infected plant cell may become home to over a million virus particles. Causes- Many different groups of living organisms can act as vectors and spread viruses from one plant to another. Seeds may transmit virus infection either due to external contamination of the seed with virus particles, or due to infection of the living tissues of the embryo. Viruses are invisible to the naked eye, exist in living plant material where they may not necessarily cause symptoms. They are spread by sap-sucking insects such as aphids, by contact (on hands, on tools, especially cutting tools such as pruners), by birds , and by propagation from infected plants Vegetative propagation/grafting are cheap and easy methods of plant propagation, but provide the ideal opportunity for viruses to spread to new plants. Virus particles may contaminate soil for long periods and may be transmitted to the leaves of new host plants as wind-blown dust or as rain-splashed mud. Susceptible Plants- Almost all plants are vulnerable to to some type of virus. Symptoms- are many and varied, including stunting, mottled, and mosaic- patterned leaves, distorted fruits, and even death of the host plant. Yield of perennial crops will fail. Other symptoms of virus infections include:: Rosetting is a term used to describe bushy growth caused by some viruses. Peach rosette is an example of a virus that causes stunting by telescoping down the distance from one leaf to the next along the stem. Leaves or side branches are so close together they grow in a rosette. Ring spot viruses, including those that cause peony or dahlia ring spot disease, show up as pale, yellow spots on leaves. Leaf Curling- or deformed leaves is another common virus-produced symptoms. · Prevention and Control- Control the agent that transmits the virus. Grow resistant cultivars. Plant certified virus-free planting material. Use a crop rotation for viruses transmitted by soil-living organisms. There is no cure for infected plants. Dig up and dispose of infected plants. Burn woody plants. Many viruses attack weeds. Good weed control can also help control the spread of viruses. Also see Cucumber mosaic virus and Zucchini mosaic virus.
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