Topic # 5
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.
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. 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.
blight is a 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., chestnut blight), and protists (e.g., potato blight). Other plant afflictions (caused by insects or unfavorable climatic conditions) that display similar symptoms are also called blights.
clubroot, disease of cabbages, turnips, radishes, and other plants belonging to the family Cruciferae (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. Clubroot can be partially or in some cases completely controlled by the application of lime (if the soil is very acid), by rotation of crops, and by soil sterilization. The disease is also called finger-and-toe from the swollen shape it gives to roots. Plasmodial slime molds (phylum, or division, Myxomycota) are classified in the kingdom Protista.
3. dry rot
A 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. Because the fungi require moisture for growth, dry rot occurs most often in places where the ventilation is poor or humidity is high or when the wood has been improperly seasoned. In the United States it is most frequently caused by a pore fungus (Poria incrassata) and by the dry-rot, or house, fungus (Merulis lacrymans). It may be prevented by application of creosote or other preservatives. Dry rot sometimes attacks standing conifers. The name is also used for other fungus diseases that attack the roots or stems of plants.
gall, 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. Crown gall, which attacks peach and other fruit trees, grapes, and roses, is caused by bacteria. Despite its name (the crown is the head of foliage), the tumorous growths usually occur on the stem below ground level. The gall insects (e.g., certain aphids, wasps, moths, beetles, and midges) deposit their eggs in the plant tissues, which begin to swell as the larvae hatch. Sometimes the larvae feed on the gall and pupate within it. The irritant is released by the female at the time of oviposition or by the developing larva itself. Each species of gall insect has its favorite host and forms galls of a characteristic shape; some are large and woody and others may be soft, knobby, or spiny. They may be formed on any part of a plant but generally occur in areas where cells are actively growing. In the United States, Galls are commonly seen on oak and willow trees and on rose bushes, goldenrod, and witch hazel. The Hessian fly, the wheat midge, and the mites and midges that attack fruit trees are the most damaging economically of the gall insects. Galls are rich in resins and tannic acid and have been used in the manufacture of permanent inks and astringent ointments, in dyeing, and in tanning. A high-quality ink has long been made from the Aleppo gall, found on oaks in the Middle East; it is one of a number of galls resembling nuts and called gallnuts or nutgalls.
smut, 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 covered by a smooth membrane (covered or kernel smuts). Certain smuts are edible and are considered a delicacy in some countries. As a disease, smuts lower the vitality of the host plant and often cause deformities. There is no alternation of hosts. Smuts are a most serious threat to cereal grain crops. Among those that cause severe annual losses to crops are corn smut, oat smut, bunt or stinking smut, and loose smut of wheat. Bunt is probably the most serious disease that attacks wheat at the young or seedling stage and spoils the grain. It has the odor of sour herring and is caused by either of two smut fungi. The fungus may be present on the wheat seed or in the soil in which the seed is sown, or it may be blown into a field by the wind. Smuts are classified in the kingdom Fungi, phylum (division) Basidiomycota, order Ustilaginales.
viroid, microscopic infectious agent, much smaller than a virus, that infects higher plants such as potatoes, tomatoes, chrysanthemums, and cucumbers, causing stunted or distorted growth and sometimes death. It can be transmitted by pollen, seed, or farm implements. Viroids are single strands of RNA and lack the protein coat of viruses. They do not code for any specific protein but are able to replicate themselves in the nuclei of infected cells. Some scientists believe viroids are parts of normal RNA that have gone awry. Potato spindle tuber viroid was the first to be identified.