The Phytopathological Significance of Mycelial Individualism

Phytopathologists have long been aware of the considerable intraspecific variation that can exist in natural populations of phytopathogenic mycelial fungi. However, the significance of this variation has largely been perceived in terms of fungal-plant rather than interfungal relationships. Corresponding­ ly, questions about variation in the pathogenicity and virulence of different fungal lineages have received more attention than questions about the way in which these lineages have the potential to interact antagonistically or syn­ ergistically with one another (55). The purpose of this article is to explore the many ways in which mycelial interactions may, nonetheless, be relevant to our understanding of the etiology and control of plant disease. An important step in understanding mycelial interactions is to have a clear perspective of the units of selection in natural fungal populations. Historical­ ly, this issue has been clouded both by the indeterminate nature of fungal mycelia, which have the potential to continue extension indefinitely, and by the concept that hyphal anastomosis could allow extensive cooperation be­ tween colonies of diverse genetic origin (22, 57, 58). The resultant view that members of the same species could pool their genetic resources in a versatile