In nature, resistance rather than susceptibility is the rule. The vast majority of fungi are strict saprophytes. The evolution of fungal phytopathogens has apparently proceeded from saprophytes to facultative parasites to obligate parasites. Physiological races of obligate parasites show the highest level of complexity. The apical growth of the hypha, its ability to perceive topographical signals and orient hyphal growth, its generation of mechanical force for entry through narrow openings, overcome host defense mechanisms by secreting enzymes capable of detoxifying plant secondary metabolites, stimulate nutrient leakage from infected cells and killing of host cells, secretion of toxic molecules that affect a fundamental biochemical process or neutralizes plant resistance or the production of enzymes capable of breaching structural barriers are among the attributes that makes a fungus a potential pathogen. Obviously one of these is not sufficient to make a virulent pathogen without the other capabilities. Likewise, a fungus may possess one or more of these properties but not be pathogenic.
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