Thursday, 14 February 2019

The Nature of Plant Disease

The Nature of Plant Disease


Plants make up the majority of the earth’s living environment as trees, grass, flowers, and so on. Directly or indirectly, plants also make up all the food on which humans and all animals depend. Even the meat, milk, and eggs that we and other carnivores eat come from animals that themselves depend on plants for their food. Plants are the only higher organisms that can convert the energy of sunlight into stored, usable chemical energy in carbohydrates, proteins, and fats. All animals, including humans, depend on these plant substances for survival.

Plants, whether cultivated or wild, grow and produce well as long as the soil provides them with sufficient nutrients and moisture, sufficient light reaches their leaves, and the temperature remains within a certain “normal” range. Plants, however, also get sick. Sick plants grow and produce poorly, they exhibit various types of symptoms, and, often, parts of plants or whole plants die. It is not known whether diseased plants feel pain or discomfort.

The agents that cause disease in plants are the same or very similar to those causing disease in humans and animals. They include pathogenic microorganisms, such as viruses, bacteria, fungi, protozoa, and nematodes, and unfavorable environmental conditions, such as lack or excess of nutrients, moisture, and light, and the presence of toxic chemicals in air or soil. Plants also suffer from competition with other, unwanted plants (weeds), and, of course, they are often damaged by attacks of insects. Plant damage caused by insects, humans, or other animals is not usually included in the study of plant pathology.

Plant pathology is the study of the organisms and of the environmental factors that cause disease in plants; of the mechanisms by which these factors induce disease in plants; and of the methods of preventing or controlling disease and reducing the damage it causes. Plant pathology is for plants largely what medicine is for humans and veterinary medicine is for animals. Each discipline studies the causes, mechanisms, and control of diseases affecting the organisms with which it deals, i.e., plants, humans, and animals, respectively.

Plant pathology is an integrative science and profession that uses and combines the basic knowledge of botany, mycology, bacteriology, virology, nematology, plant anatomy, plant physiology, genetics, molecular biology and genetic engineering, biochemistry, horticulture, agronomy, tissue culture, soil science, forestry, chemistry, physics, meteorology, and many other branches of science. Plant pathology profits from advances in any one of these sciences, and many advances in other sciences have been made in attempts to solve plant pathological problems.

As a science, plant pathology tries to increase our knowledge about plant diseases. At the same time, plant pathology tries to develop methods, equipment, and materials through which plant diseases can be avoided or controlled. Uncontrolled plant diseases may result in less food and higher food prices or in food of poor quality. Diseased plant produce may sometimes be poisonous and unfit for consumption. Some plant diseases may wipe out entire plant species and many affect the beauty and landscape of our environment. Controlling plant disease results in more food of better quality and a more aesthetically pleasing environment, but consumers must pay for costs of materials, equipment, and labor used to control plant diseases and, sometimes, for other less evident costs such as contamination of the environment.

In the last 100 years, the control of plant diseases and other plant pests has depended increasingly on the extensive use of toxic chemicals (pesticides). Controlling plant diseases often necessitates the application of such toxic chemicals not only on plants and plant products that we consume, but also into the soil, where many pathogenic microorganisms live and attack the plant roots. Many of these chemicals have been shown to be toxic to nontarget microorganisms and animals and may be toxic to humans. The short- and long-term costs of environmental contamination on human health and welfare caused by our efforts to control plant diseases (and other pests) are difficult to estimate. Much of modern research in plant pathology aims at finding other environmentally friendly means of controlling plant diseases. The most promising approaches include conventional breeding and genetic engineering of disease-resistant plants, application of disease-suppressing cultural practices, RNA and gene-silencing techniques, of plant defense promoting, nontoxic substances, and, to some extent, use of biological agents antagonistic to the microorganisms that cause plant disease.

The challenges for plant pathology are to reduce food losses while improving food quality and, at the same time, safeguarding our environment. As the world population continues to increase while arable land and most other natural resources continue to decrease, and as our environment becomes further congested and stressed, the need for controlling plant diseases effectively and safely will become one of the most basic necessities for feeding the hungry billions of our increasingly overpopulated world. 

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