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water mold

The Columbia Encyclopedia, 6th ed. | 2008 | Copyright

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.

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Chytridiomycota

A Dictionary of Biology | 2004 | Copyright

Chytridiomycota A phylum of microscopic organisms, the chytrids, that live in soil or fresh water and have affinities with true fungi. The body (thallus) is unicellular or a coenocyte and gives rise to threadlike hyphae or rhizoids; some species form a branching network (mycelium). The cell walls contain chitin, and some also have cellulose. Chytrids feed by secreting enzymes to digest material extracellularly in order to absorb the nutrients. They produce motile stages (zoospores) equipped with a single undulipodium. Sexual reproduction is the norm; fusion of the gametes results in a zygote, which either produces motile zoospores or germinates directly into a new thallus. Chytrids are sometimes classified as protoctists, sometimes as fungi.

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Oomycota

A Dictionary of Biology | 2004 | Copyright

Oomycota A phylum of the Protoctista that includes the water moulds, downy mildews, and potato blight (Phytophthora), formerly classified as a class of fungi (Oomycetes). They are coenocytic and the cell wall is made of cellulose. Oomycotes are either saprotrophic or parasitic; they feed by extending hypha-like threads into the food source or host's body. Asexual reproduction is by means of flagellated zoospores, which are released from a sporangium. Sexual reproduction involves the fusion of an antheridium and an oogonium and results in the production of a zygote, which can develop a wall of chitin and become a resistant oospore.

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Oomycota

A Dictionary of Plant Sciences | 1998 | | Copyright

Oomycota (water mould, downy mildews) Fungus-like organisms in which the zoospores have two flagella, one of the whiplash type and one of the tinsel type. The cell walls of oomycetes are atypical of fungi in that in most species they contain cellulose but not chitin. The majority of species are aquatic, but some are important pathogens of terrestrial plants. The class contains 4 orders.

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MICHAEL ALLABY. "Oomycota." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 12 Feb. 2012 <http://www.encyclopedia.com>.

MICHAEL ALLABY. "Oomycota." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (February 12, 2012). http://www.encyclopedia.com/doc/1O7-Oomycota.html

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water mould

A Dictionary of Plant Sciences | 1998 | | Copyright

water mould An aquatic fungus; sometimes specifically a fungus of the order Saprolegniales.

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Chytridiomycota

The Columbia Encyclopedia, 6th ed. | 2008 | Copyright

Chytridiomycota see water mold .

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Oomycota

Oomycota see water mold .

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