Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 18

Among the animals exploiting plants are many pests which cause damage to roots, leaves, shoots, flowers, and fruits. Usually these pests, which include insects such as aphids and leafhoppers, and some nematodes, spend relatively brief periods on individual plants before moving on to explore new food supplies. Other pests, such as leaf miners, gall‐forming sawflies, and endoparasitic nematodes, spend their entire life cycle, or a major part of it, on one plant. Pest attack may result simply in a drain on host nutrients or, alternatively, in extensive destruction of tissues. Aphids and whiteflies on leaves and stems extract sap from the phloem with almost clinical efficiency. Many caterpillars are simply small herbivores; nevertheless, they can consume large areas of the leaf lamina.

Figure 1.5 Agents responsible for plant disease, disorders, and damage. Highlighted factors are damaging when extremes occur.

Other pests cause more complex host responses or symptoms. Developing gall wasp larvae induce the formation of morphologically characteristic and often pigmented galls on leaves, while nematodes such as Meloidogyne spp. cause swellings, termed “knots,” on the roots of tomatoes and potatoes. When these root‐knot nematodes and the related endoparasitic cyst nematodes penetrate root tissues, host cells adjacent to the vascular system become enlarged and provide a specialized feeding site where nutrients are transferred to the sedentary worm. Such pests often show highly specialized adaptions to their respective hosts and, conversely, the plants mount defense reactions in response to attack which are similar to those induced by pathogenic microorganisms. Nematodes are of particular importance in the tropics where they damage numerous crop species, but some are also serious pests on temperate crops; for instance, cyst nematodes are the number 1 pest problem on potatoes in the UK and have infested around two‐thirds of the land on which the crop is grown.

Larger animals such as birds or mammals can also be destructive pests. Winter grazing by rabbits can seriously reduce the final yield of autumn‐sown crops such as wheat and oilseed rape. In Europe, pigeons also cause damage to oilseed rape, while in parts of Africa flocks of seed‐eating finches, such as Quelea, are a major threat to crops of sorghum and millet.