Plant diseases change the function, structure or development of a plant. Plant diseases affect humans though the landscape, culture, food supply and mycotoxins that cause health problems, all of which cost money. The costs include healthcare costs, costs for replacing plants and costs for treating diseases. Three types of pathogens cause plant disease. Most microorganisms do not cause plant disease.
Fungi are multicellular heterotrophs that are toxic to plants. Fungi kill plants with toxins that breed spores in cool, wet conditions. Cultural management practices and fungicides control fungi, depending on the type. Some fungi are useful, in that they break down dead organic material and cycle the resulting nutrients through the ecosystem. Certain fungi are also used to create antibiotics, while others provide mushrooms, truffles and the bubbles in beer.
Yet other fungi cause disease in plants and animals. According to Berkeley, fungi are harder to treat because they are genetically and chemically similar to plants and animals. Fungi cause plant disease, such as smut, rust, stem rot, root rot and leaf rot. These diseases cause extreme damage to crops and landscape plants.
Bacteria are single-celled heterotrophs that are also toxic to plants. Bacteria grow in cool, wet conditions and reproduce by division. They can only enter the plant through openings and other wounds. Bacteria that cause plant disease are spread via rain, wind, insects and birds. Even people can unknowingly spread bacteria from one plant to another. If the plant is wounded, the bacteria enter the plant and cause disease, such as leaf spot, by killing the host cells as they grow. Bacteria, depending on the type, overwinter on plants and in the soil. Cultural management practices and bactericides can control bacteria.
A virus is a submicroscopic genetic material that hijacks the plant's healthy cells. A virus is one of the smallest pathogens that cause disease in plants and is made of nucleic acid and protein. Of the 2,000 known viruses, 500 cause plant disease, according to the University of Florida. Most viruses cause diseases that leave mosaic patterns, deformed growth, flowerbreak, stunting, yellowing, chlorosis and ring spot on the plants. Often, the homeowner does not know that a virus is causing a problem until the disease symptoms manifest. Cultural management practices can control viruses.