Plant Pathology and Plant Pathogens. John A. Lucas

Plant Pathology and Plant Pathogens - John A. Lucas


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target="_blank" rel="nofollow" href="#ulink_0caa2034-8494-5ac4-9a4c-8e90c1f9baa4">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.

      Parasitic Plants and Weeds

Family, common name, genus Geographic area Crops attacked
Convolvulaceae Dodder (Cuscuta) Europe, North America Alfalfa, clover, potatoes, sugar beet
Lauraceae Dodder (Cassytha) Tropics and subtropics Citrus trees
Loranthaceae Dwarf mistletoe (Arceuthobium) Worldwide Gymnosperms
American true mistletoe (Phoradendron) North America Angiosperm trees
European true mistletoe (Viscum) Europe Angiosperm trees, especially apple
Orobanchaceae Broom rape (Orobanche) Europe Tobacco, sunflower, beans
Scrophulariaceae Witchweed (Striga) Africa, Asia, Australia, North America Maize, sorghum, rice, cowpea

      The deleterious effects of other higher plants are due to competition for space, light, water, and nutrients. Species which are vigorous competitors with crop plants are usually described as weeds. As well as affecting crop development, weeds may interfere with harvesting and their seeds can contaminate grain samples. They may also be important as alternative hosts for pests or pathogens which can subsequently spread to crops. In addition to direct competitive effects, some plants produce chemicals which inhibit the growth of neighboring plants. This phenomenon, analogous to microbial antibiosis, is known as allelopathy. Plant roots release a diverse range of chemicals which can act as potential inhibitors or defense compounds, but it is difficult to determine the extent to which these interactions operate in nature. Allelopathy is believed to influence plant succession and distribution in natural communities, and may also have significant effects in agricultural systems. The chemicals involved are of interest both as potential herbicides and as signal molecules affecting the growth and behavior of other organisms. The suppression of Striga by some legumes, described earlier, has been shown to be due to a combination of compounds that stimulate “suicidal germination” in the absence of the host and inhibitors that interfere with infection of roots.

Image described by caption.

      Source: Photo provided by John Lucas.

      Abiotic Agents

      Green plants, in common with all other living organisms, only flourish within a relatively narrow range of environmental conditions. Inside the plant, individual cells are able to exert control over their internal environment and thereby maintain conditions suitable for normal metabolism. However, the extent to which living cells can withstand alterations in the external environment is limited. Fluctuations in environmental conditions outside an acceptable range are therefore harmful and may result in irreversible damage. Green plants, unlike animals, are particularly susceptible to the effects of inanimate agents because they are sedentary and so are unable to escape from local changes in the environment. Plants also lack the sophisticated homeostatic mechanisms possessed by higher animals.


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