Sources and patterns of diversity in plant-pathogenic fungi.

Plant-pathogenic fungi include a very large and heterogeneous group of organisms that occupy positions of great importance in both agriculture and natural plant communities. They show an enormous diversity in life-history strategies and the ways in which they interact with their hosts. These interactions range from species that establish perennial, systemic infections that castrate their hosts to pathogens that kill their hosts with considerable rapidity to pathogens that form discrete lesions whose individual effects are very limited (9). Many pathogenic fungi can survive for long periods of time on dead host tissue or saprophytically in soil; others rely entirely on living host cells for sustenance. Collectively, the pathogens can attack virtually any plant part, although individually they may be highly specialized. As a consequence, the range of pathogens found on different hosts also shows considerable diversity that may be associated with the evolutionary history of their hosts (16) or ecological criteria like the host’s architectural complexity or the extent of the pathogen’s natural range (35). Understanding the role pathogens play in shaping the genetic structure of plant populations and communities requires an understanding of the pathogens’ diversity, their origins, and the evolutionary interplay that occurs between pathogens and their hosts. Here we review sources of variation that contribute to the diversity of pathogen populations and some of the mechanisms whereby this diversity is maximized and maintained.