ABSTRACT
Plant viruses are transmitted mechanically and through a wide range of agents like vegetative propagative organs, grafting, seeds and pollen, fungi, and through insects and nematodes. Grafting plays a significant role in propagation of fruit and ornamental trees, as also the spread of their viruses. A notorious example is the tristeza disease of sweet orange plants. Many plant viruses are experimentally mechanically transmissible, but only a few of these exploit this mechanism for natural spread in the field. Viruses which occur in soil and are spread from one plant to another through soil are the soilborne viruses. They can be spread by mechanical means, by transmission through nematodes, and/or by transmission through fungi. Successful infection of healthy plants by a virus released earlier into soil from the roots of an infected plant presupposes that such a virus is stable and can persist in soil in the infectious state for short-to-long periods.
