The order consists mainly of organisms that occur in fresh water. There are, however, many representatives that occur in soil and a few in the marine environment. A number of the Chytridiales are parasitic on algae, other fungi and higher plants. Only a few are economically important plant pests such as Synchytrium endobioticum. Thallus types range from holocarpic to eucarpic, monocentric and polycentric (rhizomycelial) forms. When present, the tips of the rhizoids taper to diameters of less than 0.5 µm. Some commonly encountered genera are Chytridium, Chytriomyces, Rhizophydium, Allochytridium, Cladochytrium and Nowakowskiella. This is the largest and least understood order of the Chytridiomycetes, containing nearly 80 genera. Many species are poorly described and families and genera are not phylogenetically based.
Zoospore characteristics that help in defining the group are: ribosomes packed at the center of the cells and partially separated from the cytoplasm by a double membrane system; usually a single, laterally-placed lipid globule; the presence of a rumposome; microtubules originating from one side of the kinetosome and frequently running parallel to one another to the edge of the rumposome that is often associated with a microbody-lipid complex; a nucleus that is not intimately associated with the kinetosome and appears to occupy whatever space is left to it in the zoospore; and a non-flagellated centriole that is connected to and lies parallel to the kinetosome.
References and Keys:
Barr, D.J.S. 1980. An outline for the reclassification of the Chytridiales, and for a new order, The Spizellomycetales. Can. J. Bot. 58: 2380-2394.
Karling, J.S. 1977. Chytridiomycetarum Iconographia. Lubrecht and Cramer, Monticello, New York.
Longcore, 1996. Chytridiomycete taxonomy since 1960. Mycotaxon 60: 149-174.
Sparrow, F.K. 1960. The aquatic Phycomycetes, 2nd edn. Rev. Univ Michigan Press, Ann Arbor.