ABSTRACT
Plant pathogens are organisms that have evolved to occupy specialized niches. The complexity of extracting nutrients from living hosts is such that we can assume that a significant portion of the pathogen's genotype is concerned, at least peripherally, with pathogenicity. The expression of these pathogenicity genes can be quantitated, with the relative amount of damage caused to the host being a reflection of virulence. Low virulence, or hypovirulence, may result from mutations of one or more pathogenicity genes. The discovery of morphological markers of hypovirulence has proven to be a great boon to the study of this phenomenon. The persistence of hypovirulence in natural populations of E. parasitica is certainly the result of its transmissible nature. The term hypovirulence has been applied to a cultural decline observed in various isolates of the plant pathogen R. solani. Some strains of this fungus were found to degenerate with time when placed into culture.
