Water mould
Lagenidiales
Leptomitales
Peronosporales
Pythiales
Rhipidiales
Saprolegniales
Sclerosporales
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Water moulds are a group of filamentous
protists, physically resembling
fungi. They are microscopic, absorptive organisms that reproduce both sexually and asexually and are composed of mycelia, or a tube-like vegetative body (all of an organisms mycelia are called its
thallus). The name "water mold" refers to the fact that they thrive under conditions of high humidity and running surface water.
Water moulds were originally classified as fungi, but are now known to have developed separately and show a number of differences. Their cell walls are composed of
cellulose rather than
chitin and do not have
septations. Also, in the vegetative state they have
diploid nuclei, whereas fungi have
haploid nuclei.
Instead, water moulds are related to organisms such as
brown algae and
diatoms, making up a group called the
heterokonts. The name comes from the common arrangement and structure of motile cells, which typically have two unequal
flagella. Among the water moulds, these are produced as asexual
spores called zoospores, which capitalize on surface water (including precipitation on plant surfaces) for movement. They also produce sexual spores, called oospores, that are translucent double-walled spherical structures used to survive adverse environmental conditions. A few oomycetes produce aerial asexual spores that are distributed by wind.
The water moulds are economically and scientifically important because they are aggressive plant pathogens (see
plant pathology). They can be roughly broken down into three groups, although more exist. The
Phytophthora group is a genus that causes diseases such as
potato blight,
sudden oak death and rhododendron root rot. The
Pythium group is a genus that is more ubiquitous than
Phytophythora and individual species have larger host ranges, usually causing less damage.
Pythium damping off is a very common problem in greenhouses where the organism kills newly emerged seedlings. The third group is the downy mildews, which are easily identifable by the appearance of white "mildew" on leaf surfaces (although this group can be confused with the unrelated
powdery mildews).
