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 ...
