diseases of plants. Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots of growing plants; both are caused by bacteria and fungi. Any environmental factor that favors the growth of parasites or disease transmitters or that is unfavorable to the growth of the plants will lead to increases in the likelihood of infection and the amount of destruction caused by parasitic disease. Parasitic diseases are spread by dissemination of the agent itself (bacteria and viruses) or of the reproductive structures (the spores of fungi). Wind, rain, insects, humans, and other animals may provide the means for dissemination.

Most names for plant diseases are descriptive of the physical appearance of the affected plant, e.g., blight (a rapid death of foliage, blossom, or the whole plant); leaf spot, fruit spot and scab, and stem canker (localized death of an organ); wilt (loss of turgor); gall (overgrowth of cells); witches'-broom (growth of abnormal shoots); stunting (underdevelopment); and leaf curl, mosaic, and yellows (resulting from chlorosis, or lack of chlorophyll). Many of these abnormalities are caused by different agents on different plants; when parasites are involved, each individual parasite usually invades only certain plant species and specific organs. Dutch elm disease, chestnut blight, rust, smut, certain mildews, and ergot are caused by various fungi (see fungal infection). Clubroot diseases are caused by slime molds, and water molds cause downy mildew (a disease of grapes), blue mold of tobacco, and sudden oak death (also known as ramorum leaf blight or ramorum dieback). Sudden oak death, caused by Phytophthora ramorum, was identified in 1995 in California, where it caused the deaths of many oaks. The disease, which affects many plant species besides oaks, has since been found in Oregon, and is also found in Europe; there, it was identified in 1993 in Germany, where it affected rhododendrons and viburnums. The water mold P. infestans was the cause of the late blight of potatoes that resulted in the Great Potato Famine in Ireland (1845-49). Both slime molds and water molds are now usually considered protists, rather than fungi. Most mosaic diseases and many other types of chlorosis are caused by viruses (see virus).

Plant diseases are more often classified by their symptoms than by the agent of disease, because the discovery of microscopic agents such as bacteria dates only from the 19th cent. (see Louis Pasteur). The Irish potato blight stimulated the development of plant pathology. The identification of tobacco mosaic virus in 1892 was the starting point of all modern knowledge about viruses.

Control

Plant diseases are controlled by methods of cultivation (e.g., crop rotation and the plowing under or burning of crop residue); by application of chemicals, e.g., fertilizers (to correct mineral deficiencies in the soil), spray or dust fungicides, bactericides, and insecticides; by development of disease-resistant strains by genetic methods; by use of alternative species that are not susceptible to the disease; by eradication of diseased plants or of their alternate hosts (e.g., barberries, which harbor wheat-stem rust); and by quarantine measures by state and federal governments to prevent the introduction of foreign plant diseases. Field and orchard crops are more susceptible to destruction than are wild plants, because the close proximity of large numbers of a single species (monoculture) makes possible the rapid spread of disease to epidemic proportions.