Many of the pathogenic organisms that cause disease in plants are transferred from plant to plant, from field to field and from region to region by another living organism. In fact, some diseases require for their propagation some such intermediary, for their causal pathogens are not adapted for dissemination by wind, water or other inanimate means. Many vectors of plant disease not only transport pathogens, but also introduce these organisms directly into the plant by biting or sucking or in other ways puncturing or rupturing the surface of the plant.
Insects are by far the most important vectors of plant disease. Other anthropods are also important. Among higher animals, birds and man are important in disseminating plant pathogens. A distinction can be made between man as a vector of plant disease and man as a disseminator of other vectors and diseased plants. Several plant viruses, notably that of American peach mosaic, are transmitted by mites (eriophyidae), and nematodes spread a number of plant viruses of economic importance. The spores of the fungus olpidium brassicae transmit two viruses that affect tobacco and lettuce plants.
The Encyclopedia of World Problems and Human Potential is a collaboration between UIA and Mankind 2000, started in 1972. It is the result of an ambitious effort to collect and present information on the problems with which humanity is confronted, as well as the challenges such problems pose to concept formation, values and development strategies. Problems included are those identified in international periodicals but especially in the documents of some 60,000 international non-profit organizations, profiled in the Yearbook of International Organizations.
The Encyclopedia includes problems which such groups choose to perceive and act upon, whether or not their existence is denied by others claiming greater expertise. Indeed such claims and counter-claims figure in many of the problem descriptions in order to reflect the often paralyzing dynamics of international debate. In the light of the interdependence demonstrated among world problems in every sector, emphasis is placed on the need for approaches which are sufficiently complex to encompass the factions, conflicts and rival worldviews that undermine collective initiative towards a promising future.
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