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.