The most common causes of parasitic diseases are fungi, bacteria, viruses, and nematodes. A few seed-producing plants, such as the mistletoes, can also cause plant diseases. There are between 50,000 and 100,000 different species of fungi of many types and sizes, but not all are harmful. Most are microscopic in size, but some, such as the mushrooms, are quite large. Fungi reproduce by spores.
Bacteria are very small, one-celled plants that reproduce by simple fission.
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.