The powdery mildews: a review of the world's most familiar (yet poorly known) plant pathogens.

The past decade has seen fundamental changes in our understanding of powdery mildews (Erysiphales). Research on molecular phylogeny demonstrated that Erysiphales are Leotiomycetes (inoperculate discomycetes) rather than Pyrenomycetes or Plectomycetes. Life cycles are surprisingly variable, including both sexual and asexual states, or only sexual states, or only asexual states. At least one species produces dematiaceous conidia. Analyses of rDNA sequences indicate that major lineages are more closely correlated with anamorphic features such as conidial ontogeny and morphology than with teleomorph features. Development of molecular clock models is enabling researchers to reconstruct patterns of coevolution and host-jumping, as well as ancient migration patterns. Geographic distributions of some species appear to be increasing rapidly but little is known about species diversity in many large areas, including North America. Powdery mildews may already be responding to climate change, suggesting they may be useful models for studying effects of climate change on plant diseases.