diseases of plants

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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 rotdry rot,
fungus disease that attacks both softwood and hardwood timber. Destruction of the cellulose causes discoloration and eventual crumbling of the wood. This frequently results in the collapse of wooden structures such as house flooring, mine shafts, and ship hulls.
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 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., blightblight,
general term for any sudden and severe plant disease or for the agent that causes it. The term is now applied chiefly to diseases caused by bacteria (e.g., bean blights and fire blight of fruit trees), viruses (e.g., soybean bud blight), fungi (e.g.
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 (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); gallgall,
abnormal growth, or hypertrophy, of plant tissue produced by chemical or mechanical (e.g., the rubbing together of two branches) irritants or hormones. Chemical irritants are released by parasitic fungi, bacteria, nematode worms, gall insects, and mites.
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 (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, rustrust,
in botany, name for various parasitic fungi of the order Uredinales and for the diseases of plants that they cause. Rusts form reddish patches of spores on the host plant. About 7,000 species are known.
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, smutsmut,
name for an order of parasitic fungi (Ustilaginales) and the various diseases of plants caused by them. Smuts produce sootlike masses of spores on the host. The spore masses may break up into a dustlike powder readily scattered by wind (loose smuts) or remain more or less
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, certain mildewsmildew,
name for certain fungi and protists, for the diseases they cause in various crops, and for the discoloration (and sometimes the weakening and disintegration) they cause in such materials as leather, fabrics, and paper.
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, and ergotergot
, disease of rye and other cereals caused by the fungus Claviceps purpurea. The cottony, matlike body, or mycelium, of the fungus develops in the ovaries of the host plant; it eventually turns into a hard pink or purple body, the sclerotium, or ergot, that resembles
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 are caused by various fungi (see fungal infectionfungal infection,
infection caused by a fungus (see Fungi), some affecting animals, others plants. Fungal Infections of Human and Animals

Many fungal infections, or mycoses, of humans and animals affect only the outer layers of skin, and although they are sometimes
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). Clubrootclubroot,
disease of cabbages, turnips, radishes, and other plants belonging to the family Cruciferae (or Brassicaceae; mustard family). It is induced by a plasmodial slime mold that attacks the roots, causing, in the cabbage, undeveloped heads or a failure to head at all.
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 diseases are caused by slime moldsslime mold
or slime fungus,
a heterotrophic organism once regarded as a fungus but later classified with the Protista. In a recent system of classification based on analysis of nucleic acid (genetic material) sequences, slime molds have been classified in a major group
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, and water moldswater mold,
common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista.
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 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 virusvirus,
parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.
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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 PasteurPasteur, Louis
, 1822–95, French chemist. He taught at Dijon, Strasbourg, and Lille, and in Paris at the École normale supérieure and the Sorbonne (1867–89).
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). 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.


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.


See books on plant pathology by G. C. Ainsworth (1981), J. G. Manners (1982), R. Wood and G. Jellis, ed. (1985), and G. N. Agrios (1988).