| I | INTRODUCTION |
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 |
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 |
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 |
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.
| V | NEMATODES |
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.
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