Water molds are always found in wet environments, especially in fresh water sources and near the upper layers of moist soil.
Officially named Oomycota, they are also known as downy mildews and white rusts.
Water molds were long considered fungi because they produce fungi-like filamentous hyphae and feed on decaying tissue like rotting logs and mulch.
The Oomycota species Phytophthora infestans caused the Great Potato Famine that killed nearly a million people in Ireland in 1846–1847. The water mold virtually wiped out the country’s potato crops, which were an essential staple in the Irish diet (sometimes the only food on the table.)
In addition to widespread starvation and malnutrition, the potato blight led more than 1.5 million Irish to flee the country. Because nearly all of the country’s potato crops were clones of a few original imports from South America, they had no natural ability to resist the pathogen.
Another water mold nearly wiped out the entire French wine industry. Plasmopara viticol (also known as downy mildew of grapes) was brought to Europe in the late 1870s on vines from America meant to be bred with French vines in hopes of yielding hybrids with a greater ability to ward off attacks by aphids.
The infestation led to the use of the first fungicide, a mixture of copper sulfate and lime, which became known as the Bordeaux mixture for its role in saving the French vines.
Other water mold species can cause disease in fish.