water mold, common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista. There are two important phyla (divisions) of water molds, the chytrids (Chytridiomycota) and the oomycetes (Oomycota).
The chytrids live in salt- and freshwater and in moist soil. They live as saprobes, obtaining their metabolic energy from decaying plant and animal material, or as parasites, attacking plants, fungi, and algae. They typically take the form of small coenocytic (multinucleated) masses, called sporangia, from which many hairlike rhizoids protrude. Like roots, the rhizoids absorb nutrients. Reproduction can be by simple division of a sporangium into individual motile, flagellated spores or by more complicated sexual processes that yield flagellated gametes. There are approximately 900 species of chytrids.
The oomycetes resemble fungi, taking the form of coenocytic filaments (hyphae). They differ from fungi, however, in that cellulose is present in their cell walls. The hyphae of oomycetes have specialized regions that can produce distinct male and female gametes. Oomycetes can also reproduce asexually. Many oomycetes are aquatic. Many of the others live in water in certain stages of the life cycle. Most of the 800 species of oomycetes are saprobes, but those that are parasitic are of great significance: they cause downy mildew, a disease often affecting grapes; late blight of potatoes, an outbreak of which led to the Great Potato Famine in Ireland (1845–49); sudden oak death syndrome (also known as ramorum leaf blight or ramorum dieback; redwoods, Douglas firs, and other plants also are harmed by the same water mold); and blue mold of tobacco. Other parasitic water molds cause diseases of fish and fish eggs.