They divide into two equal halves, each of which becomes a fully developed bacterium. Plant diseases are frequently given their common names based on the plant part affected, by the way the plant is affected, or by the cause of the disease. Viruses are so small they cannot be seen with an ordinary microscope. Many of the viruses that cause plant diseases are transmitted from one plant to another by insects, usually aphids or leafhoppers. Viruses are also very serious problems in plants that are propagated by bulbs, roots, and cuttings because the virus is easily carried along in the propagating material.
Nematodes are small eel-shaped worms that reproduce by eggs.
The number of eggs produced by one female nematode and the number of generations in a season depends largely on soil temperature. Therefore, nematodes are usually more of a problem in warmer areas of the country. Most nematodes feed on the roots and lower stem of plants, but a few attack the leaves and flowers. The life cycles of all disease organisms are greatly influenced by environmental conditions. Temperature and moisture are probably the most important factors that affect the severity of plant diseases.