There are more than 500 species in the Oomycota -- these include the so-called water molds and downy mildews. They are filamentous protists which must absorb their food from the surrounding water or soil, or may invade the body of another organism to feed. As saprophytes, oomycetes play an important role in the decomposition and recycling of decaying matter. Other parasitic species have caused much human suffering through destruction of crops, fish and aquatic invertebrates. They are also human pathogens.
Their greatest impact, however, comes from the many species of water mould which are parasites on flowering plants. These include root rotting fungi, seedling dampening mold, blister rusts, white rusts (Albugo) and the downy mildews that affect grapes, lettuce, corn, cabbage, and many other crop plants. They are major economic pests and Phytophthora infestans that causes late blight of potato (and tomato) had a major impact on world history.
This group is recognised in both the fungal and algal kingdoms. The Oomycota were originally classified as fungi, because of their filamentous growth, and because they feed on decaying matter like fungi. Most recent description of the ultrastructure, biochemistry and molecular sequences of these organisms indicates that they belong rather with the Chromista -- a group of the algae.