ABSTRACT
Interestingly, it is not only the plant whose fitness may be affected by a pathogenic fungus. The interactions between pathogens on a plant may affect the fitness of the pathogenic fungi themselves. In a study of rust fungi on wheat leaves, Newton et al. (1997) showed that the relative fitness of a number of strains of the rust Puccinia graminis was controlled by density-dependent relationships. For example, relative fitness of the fungal strain SR22 was much greater at low spore densities on the leaf than at high density. At these low densities, which were well below the carrying capacity, the high infection efficiency of SR22 gave it a competitive edge. As spore density of a mixed spore inoculum on the leaf increased, however, the strong competitive abilities of strain SR41 allowed it to dominate in the community. In the natural ecosystem, the effect of
fungal pathogens on individual plants thus may depend upon the outcome of competition of the fungal pathogens within their own community as much as the competition between saprotrophic fungi and pathogens.
