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Viruses - Submicroscopic bodies or entities that are infectious, potentially pathogenic, devoid of enzymes, and made up of nucleic acid and protein. Viruses can be crystallized. To be active, the virus must enter a cell through an injury that does not kill the cell. The genetic material or the nucleic acid portion enters the plant cell and in a sense, the virus takes over the cell and more virus entities are produced. Viruses not only can cause plant diseases but they also can infect bacteria and cause bacterial diseases. Viruses are spread by nematodes, fungi, birds, animals, and insects. The insects that have sucking mouth parts are the major vectors- leafhoppers, thrips, aphids. Viruses are common in trees. They are known to cause problems in orchard trees, especially species of Prunus. In English walnut, they are associated with a grafting problem called black line. In forest trees and amenity trees, viruses cause some leaf problems. In pecan, a disease called pecan bunch is associated with viruses. Other tree species have a similar type of problem and they are called witch's brooms. Some of the yellows type diseases that were first thought to be caused by viruses were later shown to be caused by other pathogens called mycoplasmas. The point here is to be aware of the many microscopic and submicroscopic bodies that can cause tree problems.