ABSTRACT
The symptoms observed in a diseased plant depend on the effect of the pathogen on the physiology of the plant. Pathogen attack first initiates a series of rapid changes resulting in a decline in photosynthesis and an increase in respiration, photorespiration, and invertase enzyme activity. Pathogens infecting source tissue will encounter different conditions related to primary metabolism as well as to defence responses compared with those pathogens infecting sink or assimilate-producing tissue such as roots, flowers, and sink leaves. All living plant cells require an abundance of water and an adequate amount of organic and inorganic nutrients in order to live and to carry out their respective physiological functions. Changes in plasma membrane and organelle membrane permeability are often the first detectable responses of plant cells to infection by pathogens, and this often leads to loss of electrolytes (calcium and potassium ions in particular). The permeability change is a response to toxins produced by the invading fungus.
