Oomycota or oomycetes () form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous, microscopic, absorptive organisms that reproduce both sexually and asexually. Oomycetes occupy both saprophytic and pathogenic lifestyles, and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens. The Oomycota have a very sparse fossil record. A possible oomycete has been described from Cretaceous amber.

Oomycota or oomycetes () form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous, microscopic, absorptive organisms that reproduce both sexually and asexually. Oomycetes occupy both saprophytic and pathogenic lifestyles, and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens. The Oomycota have a very sparse fossil record. A possible oomycete has been described from Cretaceous amber.