ABSTRACT
Since the beginning of time, humans have observed the effects of plant pathogens on plants, although they did not know about plant pathogens or how they caused the observed effects. In many cases, the processes that result in symptom development are not completely understood. Plant disease has been defined as alteration in the normal physiology of a plant, but this definition obviously requires extensive knowledge of normal plant physiology. Normal plant physiology includes many processes such as photosynthesis, respiration, absorption, and translocation of water, transport of photosynthetic products, production of compounds, such as plant growth hormones, enzymes, proteins, carbohydrates, lipids, and nucleic acids by the plant; and movement of materials between individual cells. Changes in any of these processes may have an effect on the overall appearance of the plant, which is observed as symptoms of disease. How do plant pathogens interrupt these processes? Responses may be something as small as the alteration of the metabolism within a cell or the movement of materials across a cell membrane to an overall plant response such as reduction in crop yield. We have learned much about the effects of plant pathogens on the normal physiology of
plants, but still many questions remain, especially related to the exact sequence of events that result in the expression of symptoms.
