Viruses are submicroscopic, cylindrical or spherical bodies. They are not living organisms, at least not in the same sense as bacteria and fungi, but rather particles composed of nucleic acids and other compounds that are similar to the chemical make-up of chromosomes of the host cells. When introduced into plant cells, the virus particles are multiplied, often causing severe ill effects to the plant. An example of a common virus disease which strikes many home gardens each season is a strain of squash mosaic virus which causes yellow squash to form large green blotches. Virus particles may be transmitted by infected transplants, seeds, transmission of sap from one plant to another through pruning, insects, or even nematodes.
Nematodes are thread-like round worms.
Those attacking trees and garden plants are microscopic in size and live in the small roots of their host plants or the surrounding soil. With it, they deliver a digestive juice that predigests the root cell materials, which are then sucked into the nematode. When nematode populations are high, their feeding can result in stunted plant growth. Nematodes are transmitted in a number of different ways. One of the most common means is through infected nursery stock and vegetable transplants.