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. They not only influence the activities of the disease organism, but also affect how easily a plant becomes diseased and the way the disease develops. The fact that diseased plants are in some way different from normal plants indicates that there are ways of recognizing plant diseases. However, there are certain facts that help in the identification of plant diseases.