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A class of fungi in the subdivision Mastigomycotina. They comprise a group of heterotropic, funguslike organisms that are classified with the zoosporic fungi (Mastigomycotina) but in reality are related to the heterokont algae. They are distinguished from other zoosporic fungi by the presence of biflagellate zoospores. Some taxa are nonzoosporic. Asexual reproduction involves the release of zoospores from sporangia; in some taxa the sporangium germinates with outgrowth of a germ tube. Sexual reproduction occurs when an oogonial cell is fertilized by contact with an antheridium, resulting in one or more oospores.
Oomycetes are cosmopolitan, occurring in fresh and salt water, in soil, and as terrestrial parasites of plants. Many species can be grown in pure culture on defined media. There are four orders: The Saprolegniales and Leptomitales are popularly known as water molds. Some species are destructive fish parasites. Many Lagenidiales are parasites of invertebrates and algae. The Peronosporales are primarily plant parasites attacking the root, stem, or leaf, and include some of the more destructive plant pathogens. See Eumycota, Fungi