virulence

(pathogenicity) The capacity of a pathogen to cause disease. A pathogen often exists in a number of *physiological races , which, with respect to one particular crop cultivar, may be either virulent or avirulent. The release of resistant crop cultivars imposes a selection pressure on the pathogen to develop new genes for virulence. It has been shown there is a gene-for-gene relationship between crop resistance and virulence of the pathogen.