Mycoherbicides

The potential of fungal pathogens to control unwanted plant species has often been underestimated, primarily because the impact of a pathogen on its host plant within a given region is usually subtle, and only the final population equilibrium is observed (1 ). Moderate levels of pathogenicity, in combination with host tolerance or resistance, facilitate survival of both the pathogen and the host. As a rule, few pathogens risk completely decimating their host population, and therefore eliminating their ecological niche. However, potato late blight, Dutch elm disease, chestnut blight, and coffee rust are just a few examples of devastating plant disease epidemics that have resulted when this fragile equilibrium has been disrupted. These epidemics all resulted from the accidental introduction of an exotic pathogen into areas where the host plant had been cultivated in the absence of the pathogen. In the absence of this selective pressure, little resistance or tolerance was present in the host population, favoring development of severe disease in the plant population. Thus, the challenge of biological control is to artificially shift the ecological balance in favor of the pathogen through the introduction of new strains, or by artificially increasing the concentration or virulence of an indigenous pathogen within a given area.