Several terrestrial plant diseases and aquatic animal (fish, frog) diseases attributed to fungal attack are actually caused by Oomycetes. In plants they are often better known by their symptoms and referred to as root rotting fungi, seedling dampening mold, blister rusts, white rusts, downy mildew or late blights.

There are many identification or naming situations - where Oomycetes get lumped into "fungi" for gross morphological reasons, much the same way dolphins and whales were lumped together with fish. Yet the taxonomic difference, or divergence from the last common ancestor, is several magnitudes more extreme than that between mammals and fish. The divergence is the order of billions rather than millions of years.

Oomycetes are like fungi. They have the same filamentous, branching, indeterminate bodies and absorb food by excreting digestive enzymes and absorbing the resultant soup. This can be referred to as the fungal way of life or absorptive nutrition. But ecologically and by using ultra structural characters, the differences can be seen to be far more fundamental: