Plant and 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 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 type 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 human, 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 he methods of preventing or controlling diseased 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 disease can be avoided or controlled. Uncontrolled plant disease may result in less food and higher food prices or in food of poor quality. Diseased plant product 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 produce 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-suppression 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 disease effectively and safely will become one of the most basic necessities for feeding the hungry billions of our increasingly overpopulated world.
Because it is not known whether plants feel pain or discomfort and because，in any case，plants do not speak or otherwise communicate with us，it is difficult to pinpoint exactly when a plant is diseased. It is accepted that a plant is healthy，or normal，when it can carry out its physiological functions to the best of its genetic potential. The meristematic（cambium) cells of a healthy plant divide and differentiate as needed，and different types of specialized cells absorb water and nutrients from the soil；translocate these to all plant parts； carry on photosynthesis，translocate，metabolize，or store the photosynthetic products；and produce seed or other reproductive organs for survival and multiplication. When the ability of the cells of a plant or plant part to carry out one or more of these essential functions is interfered with by either a pathogenic organism or an adverse environmental factor，the activities of the cells are disrupted，altered，or inhibited，the cells malfunction or die，and the plant becomes diseased. At first，the affliction is localized to one or a few cells and is invisible. Soon，however，the reaction becomes more widespread and affected plant parts develop changes visible to the naked eye. These visible changes are the symptoms of the disease. The visible or otherwise measurable adverse changes in a plant，produced in reaction to infection by an organism or to an unfavorable environmental factor，are a measure of the amount of disease in the plant. Disease in plants，then，can be defined as the series of invisible and visible responses of plant cells and tissues to a pathogenic organism or environmental factors that result in adverse changes in the form，function，or integrity of the plant and may lead to partial impairment or death of plant parts or of the entire plant.
The kinds of cells and tissues that become affected determine the type of physiological function that will be disrupted first. For example，infection of roots may cause roots to rot and make them unable to absorb water and nutrients from the soil；infection of xylem vessels，as happens in vascular wilts and in some cankers，interferes with the translocation of water and minerals to the crown of the plant；infection of the foliage，as happens in leaf spots，blights，rusts，mildews，mosaics，and so on，interferes with photosynthesis；infection of phloem cells in the veins of leaves and in the bark of stems and shoots，as happens in cankers and in diseases caused by viruses，mollicutes，and protozoa，interferes with the downward translocation of photosynthetic products；and infection of flowers and fruits interferes with reproduction. Although infected cells in most diseases are weakened or die，in some diseases，e. g.，in crown gall，infected cells are induced to divide much faster （hyperplasia) or to enlarge a great deal more（hypertrophy) than normal cells and to produce abnormal amorphous overgrowths (tumors) or abnormal organs.
Pathogenic microorganisms，i. e.，the transmissible biotic agents that can cause disease and are generally referred to as pathogens，usually cause disease in plants by disturbing the metabolism of plant cells through enzymes，toxins，growth regulators，and other substances they secrete and by absorbing foodstuffs from the host cells for their own use. Some pathogens may also cause disease by growing and multiplying in the xylem or phloem vessels of plants，thereby blocking the upward transportation of water or the downward movement of sugars，respectively，through these tissues. Environmental factors cause disease in plants when abiotic factors，such as temperature，moisture，mineral nutrients，and pollutants，occur at levels above or below a certain range tolerated by the plants.
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