Often referred to as the oomycetous water molds, the oomycetes (Peronosporomycetes) form the largest and most ubiquitous group of heterotrophic stramenopiles. Free-living forms occupy diverse ecological niches ranging from freshwater and marine to terrestrial environments. Both terrestrial and marine forms include saprophytic and obli-gately, or facultatively, parasitic genera.

Hosts include a range of nematodes, arthropods, molluscans, algae, and plants. Several of the plant pathogens have impacted the cultural and economic history of humans. These include the causative agents for a variety of root rots, downy mildews, white rusts, and late blights. Downy mildews of grapes (Plasmopara) and tobacco (Peronospora) were responsible for the near-decimation of the French wine industry and the Cuban tobacco industry in the late 1870's and the 1980's, respectively. Similarly, Phytophthora infestans, the causative agent of potato late blight, was responsible for the Irish Potato Famine of the mid-1840's and, during World War I, for the starvation of German civilians in 1915-1916.

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