The forms causing plant diseases are rod-like or cylindrical in shape and reproduce by fission. This means that one bacterium can divide to form two new cells, and these two can spit to form four cells. In this manner, millions of bacteria may be produced in a short period of time unless controlled. Bacteria are carried from place to place by insects, rain, and tools. Most orchardists are familiar with fireblight.
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.