Oomycetes are fungus-like Stramenopiles

The oomycetes form a diverse group of fungus-like eukaryotic microorganisms, also known as water molds, that include saprophytes as well as pathogens of plants, insects, crustaceans, fish, vertebrate animals, and various microorganisms. A multitude of saprophytic oomycetes primarily inhabit aquatic and moist soil habitats and play key roles in decomposition and recycling of organic matter. However, plant pathogenic species, notably those of the genus Phytophthora, are the best studied oomycetes. Species of the genus Phytophthora (the 'plant destroyer' in Greek) are arguably the most devastating pathogens of dicotyledonous plants. They cause enormous economic damage on important crop species such as potato, tomato, pepper, soybean, and alfalfa, as well as environmental damage in natural ecosystems. Virtually every dicot plant is affected by one or more species of Phytophthora, and several monocot species are infected as well.

Traditionally and due to their filamentous growth habit, oomycetes have been classified in the Kingdom Fungi. However, modern molecular and biochemical analyses indicate that oomycetes share little taxonomic affinity to fungi, but rather form a diverse and well defined group of eukaryotes with affinities to brown algae and diatoms. Subsequently, oomycetes are now classified as Stramenopiles, one of several major eukaryotic Kingdoms.

• A tree illustrating the five major eukaryotic kingdoms, NY Times, Aug. 31, 1999.


• Mitochondrial phylogenetic tree from B.F. Lang Lab (Univ. Montreal)


• For an up to date phylogeny of the eukaryotes, check Figure 6 of Haldar et al. (Nature Reviews Microbiol 2006).

Oomycetes cause devastating diseases

The oomycetes comprise numerous destructive plant pathogens, including over 60 species of the genus Phytophthora, several genera of the biotrophic downy mildews, and more than 100 species of the genus Pythium. Many of these pathogens cause devastating diseases on several crop and ornamental plants that are notoriously difficult to manage. Other oomycetes cause economically important diseases in animals.

Plant diseases caused by oomycetes

• Potato and tomato late blight caused by Phytophthora infestans.


• Phytophthora root rot of soybean caused by Phytophthora sojae.


• Black pod of cocoa caused by Phytophthora palmivora.


• Blue mold of tobacco caused by Peronospora tabaci.

Animal diseases caused by oomycetes

• Saprolegnia spp. are reemerging parasites that form lesions on fish, including salmon and trout. Read our cDNA sequencing study of Saprolegnia parasitica


• The soil inhabiting Pythium insidiosum can cause fatal lesions in farm animals, pets and humans (phytiosis).


• Lagenidium giganteum parasitizes the larval stage of mosquitoes..

Review articles on oomycetes