Diseases of Plants, deviations from the normal growth and development of plants incited by microorganisms, parasitic flowering plants, nematodes, viruses, or adverse environmental conditions. In the United States alone, known plant diseases attributable to these causes are estimated to number more than 25,000; the estimated annual losses therefrom add up to several billion dollars. Injuries to plant life due primarily to insects, mites, or animals other than nematodes are not regarded as plant diseases.
II BACTERIA-INDUCED DISEASES
Oak Apple GallGalls are swellings formed on living plants in response to insect or bacterial parasites. The gall, or cecidium, is formed when chemicals secreted by the invading parasite stimulate swelling or rapid growth of plant tissues into an enlarged, thickened covering around the site of infection or parasitic invasion. Galls come in many shapes depending on the type of invading parasite, and although they may occur at any location on a plant, their formation is common in areas of active plant growth.Avril Ramage/Oxford Scientific Films
Bacterial diseases are marked by various symptoms, including soft rot, leaf spot, wilt of leaves and stems, canker, leaf and twig blight, and gall formation. Fire blight, a disease of apple and pear trees, is historically interesting because it was the first plant disease in which a bacterium was shown to be the inciting agent. Infected trees exhibit a blackening of the flowers, leaves, and twigs, and the disease finally may involve the entire tree, causing serious damage and even death. Citrus canker, an Asian disease of the orange tree and its relatives, is characterized by corky growths on the fruit, leaves, and twigs. Common scab of potato, bacterial canker of tomato, angular leaf spot of cotton, and black rot of crucifers are a few of the bacterial plant diseases prevalent in the U.S. Crown gall, or plant cancer, which occurs in a wide range of woody plants and some herbaceous groups, is a striking example of bacteria-induced disease. See also Bacteria.
III DESTRUCTIVE FUNGI
Fungal Diseases of PlantsMost types of plant-related diseases are caused by fungi. The leaves of this plant have been infected by tar-spot fungus. Fungi can infect all parts of the plant including leaves, stems, flowers, roots, and fruit. The physical manifestations of fungal diseases of plants include wilting, club root, root rot, wood rot, cankers, various types of mildews, blights, lesions, and leaf spots. The effects of fungal diseases can be devastating as evidenced by the potato blight that destroyed the Irish potato harvest of 1845 and caused a widespread famine in Ireland.Dorling Kindersley
The majority of plant diseases are incited by fungi. Fungus diseases have been observed and commented on since ancient times. Biblical records mention blights and mildews on the cereal and vine crops of the ancient Hebrews. Fungus diseases were responsible for several major catastrophes in various parts of the world. Prominent among these fungal diseases was the late blight, a disease of the potato, which invaded Europe after 1845 with particularly devastating results in Ireland. Powdery mildew of the grape, native to America, became established in France and nearly wrecked the French wine industry. A parasitic root fungus, Hemileia vastatrix, destroyed the coffee plantations of Sri Lanka and other Oriental countries. In the United States the American chestnut, an important timber, nut, and tannin-producing tree, was subsequently virtually eliminated by an introduced Asian fungus. More than 1400 species of rust fungi, all parasitic, and several hundred species of smut fungi occur in North America. Equally large numbers of fungi in other groups produce a large array of diseases characterized by leaf spots, ulcerous lesions, blights, powdery and downy mildews, cankers, wood rots and stains, root rots, wilts, club root, and various other symptoms.
IV VIRAL INFECTIONS
Calico Virus of the PeachThe calico virus of the peach is one of a number of infectious plant viruses transmitted by insects. Since the virus itself is very difficult to treat, the best method of treatment is to control the insects that carry the disease.Kathy Merrifield/Photo Researchers, Inc.
The viruses cause as wide a range of host reactions as do the bacteria and the fungi. The number of recognized virus diseases of plants has increased rapidly in recent years. Typical symptoms of viral infections include mosaic patterns, yellowing of foliage, veinclearing, ring spots, stunting and premature death, malformations, and overgrowth. Under some conditions the symptoms may be masked. Such virus diseases as peach yellows, tobacco mosaic, potato-leaf roll, and curly top of beets have been studied intensively because of heavy losses to U.S. crops afflicted with these diseases. All economic plants suffer from one or more of these obscure but potentially dangerous diseases. Virus diseases are infectious and are transmitted largely by insects. Control of these insects is the best means of reducing the disease incidence. Virus infections also may be transmitted in the process of budding or grafting, by contamination of the soil, and less commonly by means of seed or parasitic flowering plants. Among the flowering plants or so-called higher plants are a few true parasites that cause injury or death to their hosts. The mistletoes, dodders, and root parasites of the genera Striga and Orobanche (broomrape) are the more common of these parasitic plants. See Virus.
Nematodes, or roundworms (see Roundworm), are an important cause of disease in plants. For many years attention was focused on the root-knot nematodes, which cause fleshy root knots or galls on plants. More recent investigations were concerned with other species, including the stem or bulb nematodes, which live in the leaves, stems, bulbs, and roots of narcissus, phlox, and many other plants, and the leaf nematodes, growing in herbaceous plants including the begonia and chrysanthemum. The golden nematode of the potato plant and of related plants and the soybean cyst nematode are recent introductions into the U.S. that are causing increased concern.
VI ENVIRONMENTAL ENEMIES
Nonparasitic diseases attributable to adverse environmental conditions are numerous and include many of economic importance. Major causes of these diseases are excessively high or low temperatures, soil-moisture disturbances, atmospheric impurities, lightning, and nutritional disorders. Low temperatures, for example, are responsible for winter injury to fruit trees and potatoes, and high temperatures produce such disturbances as water core of apple and heat canker of flax. Excessive or irregular water supplies cause a variety of troubles, such as blossom-end rot of tomatoes. Disease-producing atmospheric impurities include escaping illuminating gas and smelter fumes; the latter in particular may be responsible for widespread killing of crops and forests. Lightning frequently causes injuries to plantings of cotton, bananas, sugarcane, potatoes, and many other field crops. Excessive soil acidity adversely affects many plants; on the other hand, high alkalinity may be deleterious. An excess of nitrogen or any other substance required for normal growth may cause abnormalities in plant development. Mineral deficiencies also cause diseases, and the characteristic symptoms produced by lack of each of many minerals are well established.
For diseases of specific plants, see articles on the individual plants.
John A. Stevenson