The biflagellate water molds, downy mildews, and other microorganisms classified as members of the Oomycota (also termed Oomycetes) have long puzzled taxonomists. Previously, investigators thought them to constitute a group within the true fungi (of the division or phylum Eumycota), primarily because of their filamentous hyphal growth and the fact that they feed on decaying matter. However, they are now considered to comprise a distinct group of microscopic fungus-like eukaryotes. Some taxonomists assign a class status to the Oomycota within the phylum Heterokontophyta, which otherwise contains mostly algal forms; in contrast, others designate Oomycota as a distinct phylum. See also: Algae; Eukaryotae; Fungal genomics; Fungi; Mycology

Regardless of the exact taxonomic scheme, conclusive phylogenetic and morphological analyses have indicated a number of features that distinguish these microorganisms from fungi. Two characteristics are particularly important. First, individual members of the Oomycota produce motile zoospores (asexual spores) that possess two flagella: a shorter anterior one designated tinsel and a longer posterior one designated whiplash. Fungi, in contrast, typically have no flagella or only one whiplash posterior flagellum. Second, the main components of oomycetous cell walls are cellulose and beta-glucans (the cell walls of fungi are made predominantly of chitin). Based on past tradition, however, the study of oomycete microorganisms is still included in fungal investigations. See also: Cellulose; Cilia and flagella; Fungal β-glucans; Fungal phylogenetic classification

Oomycete microorganisms are typically divided into five groups: Lagenidiales, Leptomitales, Peronosporales, Rhipidiales, and Saprolegniales. About 800 species occur worldwide in fresh and salt waters and in soil, and many are capable of being grown in pure culture on defined media. Some water mold species also grow in stagnant or polluted waters. Numerous species are harmful parasites of plants, invertebrates, or algae, among them Phytophthora infestans, one of the most aggressive plant pathogens in the world. This species infects and destroys the roots, stems, leaves, fruits, and tubers of potatoes and tomatoes, and it was the causative agent of the blight of potatoes that triggered the great Irish famine in the mid to late 1840s. The oomycete microorganisms called downy mildews are especially detrimental to a variety of agricultural crops, including cruciferous vegetables, grapes, tobacco, melons, squashes, and cucumbers. A number of water molds belonging to the Saprolegniales and Leptomitales groupings are destructive fish parasites. These pathogenic microorganisms can infect salmon hatcheries, often reaching epidemic levels and leading to major economic losses. See also: Cause of great Irish potato blight identified; Fisheries ecology; Fungal ecology; Marine fisheries; Parasitology; Pathogen; Plant pathology; Potato, Irish; Salmon; Salmon farming