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Not to be confused with molds.
Water molds
Phytophtora reproduction.png
The reproductive structures of Phytophthora infestans
Scientific classification e
Kingdom: Chromalveolata
Phylum: Heterokontophyta
Class: Oomycota
Orders (& families)

Oomycota or oomycetes (/ˌəˈmst/[1]) 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. They 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. [2]


The oomycetes rarely have septa (see hypha), and if they do, they are scarce,[3] appearing at the bases of sporangia, and sometimes in older parts of the filaments.[4] Some are unicellular, but others are filamentous and branching.[4]

Phylogenetic relationships[edit]

This group was originally classified among the fungi (the name "oomycota" means "egg fungus") and later treated as protists, based on general morphology and lifestyle.[2] A cladistic analysis based on modern discoveries about the biology of these organisms supports a relatively close relationship with some photosynthetic organisms such as brown algae and diatoms. A common taxonomic classification based on these data, places the class Oomycota along with other classes such as Phaeophyceae (brown algae) within the phylum Heterokonta.

This relationship is supported by a number of observed differences in the characteristics of oomycetes and fungi. For instance, the cell walls of oomycetes are composed of cellulose rather than chitin[5] and generally do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Most oomycetes produce self-motile zoospores with two flagella. One flagellum has a "whiplash" morphology, and the other a branched "tinsel" morphology. The "tinsel" flagellum is unique to the Kingdom Heterokonta. Spores of the few fungal groups which retain flagella (such as the Chytridiomycetes) have only one whiplash flagellum.[5] Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes which differ.[5] The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.[5]

In spite of this, many species of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.


The group is arranged into six orders. Briefly:[4]


(The above after [4]).





A water mold from a stream, growing on substrate




Many oomycetes species are economically important, aggressive plant pathogens. Some species can cause disease in fish. The majority of the plant pathogenic species can be classified into four groups, although more exist.

  • The third group are the downy mildews, which are easily identifiable by the appearance of white, brownish or olive "mildew" on the leaf undersides (although this group can be confused with the unrelated fungal powdery mildews).


  3. ^ Kortekamp, A. (2005). "Growth, occurrence and development of septa in Plasmopara viticola and other members of the Peronosporaceae using light- and epifluorescence-microscopy". Mycological research 109 (Pt 5): 640–648. doi:10.1017/S0953756205002418. PMID 16018320.  edit
  4. ^ a b c d Sumbali, Geeta; Johri, B. M (January 2005). The fungi. ISBN 978-1-84265-153-7. 
  6. ^ Haas, BJ; Kamoun, S; Zody, MC; Jiang, RH; Handsaker, RE; Cano, LM; Grabherr, M; Kodira, CD et al. (2009). "Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans". Nature 461 (7262): 393–8. doi:10.1038/nature08358. PMID 19741609. 
  7. ^ A. M. Vettraino, O. Morel, C. Perlerou, C. Robin, S. Diamandis, A. Vannini (2005) Occurrence and distribution of Phytophthora species in European chestnut stands, and their association with Ink Disease and crown decline. European Journal of Plant Pathology. 111(2): 169-180

External links[edit]