Diseases of Plants

Diseases of Plants
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.

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. See also Bacteria.

DESTRUCTIVE FUNGI -> The majority of plant diseases are incited by fungi. Fungus diseases have been observed and commented on since ancient times. 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.

VIRAL INFECTIONS -> 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. See also Virus.

NEMATODES -> Nematodes, or roundworms, 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.

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