Transmission of Plant Viruses and Viroids

Abstract

Plant viruses are horizontally or vertically or both ways are transmitted. The horizontal transmission takes place through a group of organisms as diverse as parasitic fungi, nematodes and most importantly arthropod insects. In vertical transmission, seed transmission plays a pivotal role in the spread and the survival of a number of important plant virus and viroid diseases. Some of these diseases are effectively transmitted vertically through pollen and ovule to the seed and seedlings. Nearly 231 virus and viroid diseases are found to be seed transmitted. Similarly viruses like Cucumber green mottle mosaic virus, Carnation mottle virus, Tobacco necrosis virus, Tomato bushy stunt virus and TMV are isolated from rivers, lakes, ditches and seawage water and found to be infectious. The insect vectors are aphids, leafhoppers, planthoppers, treehoppers, thrips, mealybugs, beetles, and whiteflies. Majority of the viruses are transmitted by aphids, thrips and whiteflies belongs to the families Potyviridae, Bunyaviridae and Geminiviridae respectively. These vectors may have non-persistent or semi-persistent or persistent virus - vector relationships. The spread of virus from the infected sources to a crop and also to remote areas takes place through aerial insect vectors and favarouble wind conditions cause long distance spread. Even some viruses have certain fungi, nematodes, and mites as their vectors. In Tropics, Tomato spotted wilt virus, Tomato yellow leaf curl virus, Potyviruses, and Cucumber mosaic viruses are wide spread on number of crop plants and are responsible for heavy yield losses. Among the thrips transmitted viruses Tospo and Ilarviruses are widely spread and have very wide host range. Among the thrips transmitted viruses the major viruses are Tomato spotted wilt virus, Impatiens necrotic spot virus, Iris yellow spot virus, Peanut bud necrosis virus, Watermelon silver mottle virus etc. In sunflower Tobacco streak virus is the major virus disease under Ilarvirus group. Whitefly transmitted viruses of genus Begomovirus (family: Geminiviridae) transmits about 200 accepted virus diseases and the most economically important whitefly transmitted virus diseases are African cassava mosaic virus, Tomato yellow leafcurl virus, Tobacco leafcurl virus, Squash leafcurl virus, Cotton leafcurl virus, Soybean crinkle leaf virus, Bean golden yellow mosaic virus, and Okra yellow vein mosaic virus. Throughout the world, stranded molecular tests are developed against almost all economically important virus and viroid diseases and suitable management measures are worked out. Viroid diseases of plants have no insect vectors. Their primary spread takes place mechanically through contaminated implements; through vegetative propagative material and true seeds. In tropical countries Potato spindle tuber viroid, Citrus exocortis, Avocado sunblotch, and Chrysanthemum stunt viroids are economically important and molecular characterization and diagnostic techniques are developed.