Water molds
The reproductive structures of Phytophthora infestans
Scientific classification
Domain: Eukarya
(unranked): SAR
Superphylum: Heterokonta
Class: Oomycota
Arx, 1967[1]
Orders and families

Oomycota or oomycetes (/ˌəʊəˈmst/[4]) 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 mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi.[5] 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.[6]


The oomycetes rarely have septa (see hypha), and if they do, they are scarce,[7] appearing at the bases of sporangia, and sometimes in older parts of the filaments.[8] Some are unicellular, but others are filamentous and branching.[8]

Phylogenetic relationships

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.[6] 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[9] 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.[9] Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes that differ.[9] The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.[9]

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


Previously the group was arranged into six orders.[8]

However more recently this has been expanded considerably.[10][11]










Peronosporomycetes s.s.





"Oomycota" means "egg fungi", referring to the large round oogonia, structures containing the female gametes, that are characteristic of the oomycetes.

The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water, which is characteristic for the basal taxa of the oomycetes.


A culture of an oomycete from a stream


Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile spores called zoospores, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind. They also produce sexual spores, called oospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.


Many oomycetes species are economically important, aggressive plant pathogens. Some species can cause disease in fish, and at least one is a pathogen of mammals. The majority of the plant pathogenic species can be classified into four groups, although more exist.


  1. Arx, J.A. von. 1967. Pilzkunde. :1-356
  2. Winter, G. Rabenhorst’s Kryptogamen-Flora, 2nd ed., vol. 1, part 1, p. 32, 1880 [1879].
  3. Dick, M. W. (2001). Straminipilous fungus. Dordrecht, the Netherlands: Kluwer Academic Publishers, p. 289.
  4. "oomycete". Collins English Dictionary. HarperCollins. Retrieved 4 June 2014.
  5. Vallance, J.; Le Floch, G.; Deniel, F.; Barbier, G.; Levesque, C. A.; Rey, P. (2009). "Influence of Pythium oligandrum Biocontrol on Fungal and Oomycete Population Dynamics in the Rhizosphere". Applied and Environmental Microbiology. 75 (14): 4790–800. doi:10.1128/AEM.02643-08. PMC 2708430Freely accessible. PMID 19447961.
  6. 1 2 "Introduction to the Oomycota". Retrieved 2014-07-07.
  7. 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.
  8. 1 2 3 Sumbali, Geeta; Johri, B. M (January 2005). The fungi. ISBN 978-1-84265-153-7.
  9. 1 2 3 4 Van der Auwera G, De Baere R, Van de Peer Y, De Rijk P, Van den Broeck I, De Wachter R (July 1995). "The phylogeny of the Hyphochytriomycota as deduced from ribosomal RNA sequences of Hyphochytrium catenoides". Mol. Biol. Evol. 12 (4): 671–8. PMID 7659021.
  10. Ruggiero; et al. (2015), "Higher Level Classification of All Living Organisms", PLoS ONE, 10 (4): e0119248, doi:10.1371/journal.pone.0119248, PMC 4418965Freely accessible, PMID 25923521
  11. Silar, Philippe (2016), "Protistes Eucaryotes: Origine, Evolution et Biologie des Microbes Eucaryotes", HAL archives-ouvertes: 1–462
  12. 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.
  13. Vettraino, A. M.; Morel, O.; Perlerou, C.; Robin, C.; Diamandis, S.; Vannini, A. (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. doi:10.1007/s10658-004-1882-0.
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