Water molds
Phylum Oomycota
The oomycetes, or water molds, are a group of fungal-like protists, that are more closely related to diatoms and brown algae. This group plays an important role in the decomposition and recycling of decaying matter. They are also known for causing well-known plant diseases such as downy mildew and root rot. In addition, the genus Phytophthora causes Sudden Oak disease, Ink Disease in American Chestnut (Castanea dentata), and the infamous Potato blight that caused massive starvation (Irish Potato Famine) in Ireland in the 1800s.
Ecology & form
Terrestrial and aquatic (marine and freshwater)
Saprotrophic & parasitic
Non-photosynthetic; store glycogen
Filamentous with cellulosic cell walls (similar to plants)
Oomycetes are able to reproduce asexually
They produce flagellated asexual zoospores which can easily spread through wet and saturated soils
Asexual production of zoospores probably lead to the catastrophic and rapid blight of potatoes during the 1840s
Geologic Range
Neoproterozoic(?) or Cretaceous - present
Reproduction
Water molds have a diplontic life cycle
They are oogamous, in which they possess oogonia (with eggs) & antheridia (with sperm)
Diversity
~500 species
e.g. Saprolegnia, Phytophthora, Cryphonectria
Above: Downy mildew, caused by a water mold (oomycete); powdery mildew is caused by a true fungus (ascomycete)
Above: Potato blight, Phytothora infestans
Above: sudden oak disease, Phytophthora ramoru
Above: Ink Disease (Cryphonectria) in American chestnut
Above: an oogonium of a water mold
Below: crown infection of summer squash