Plasticity in plant-microbe interactions: a perspective based on the pitch canker pathosystem

What we know about the life history of fungi that cause disease in plants is commonly based on studies of the pathogen's interaction with a susceptible host: how and when infection occurs, growth and reproduction within the host, and survival during the interval when a growing host is not available. This focus is appropriate, given the need for information that will facilitate management of disease affecting an economically important crop, but it can limit recognition of the full range of resources that may be utilized by fungi that we classify as plant pathogens. This was certainly the case for Fusarium circinatum, which causes a destructive disease of pines known as pitch canker. Although F. circinatum was initially known only as a necrotrophic, wound-infecting pathogen of coniferous trees, recent research has revealed that an isolate of this fungus that will kill shoot tissue when inoculated into a wound can also have a biotrophic relationship with roots of pine seedlings, infect and grow within grasses without causing symptoms, and cause ear rot of corn. Thus, although F. circinatum became known to science because it induced visible symptoms on pines, it has the capacity for a much broader range of ecological activities than is captured by its designation as a necrotrophic pathogen. The physiological plasticity manifested by F. circinatum illustrates the challenge of obtaining a comprehensive understanding of the life history of a plant pathogenic fungus.