Oomycetes

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The oomycetes (Oomycetes) are a group of filamentous protists belonging to the pseudofungi group. The name means "egg fungi" and refers to the oogonium, a large, spherical structure that contains the female gametes. The group includes both saprophytic and parasitic species, closely linked to the aqueous environment. As parasites they act against aquatic animals and plants. They are of great economic importance, since they include parasites of vascular plants, many of them of agricultural interest. The species Phytophthora infestans ravaged the potato fields of Ireland in the XIX century, the base of the diet of the time. The ensuing famine killed a million Irish people and caused mass emigration to the US. About 700 species of oomycetes are known.

Notable species

The following oomycetes are of special importance in agriculture:

Gender Phytophthora includes numerous species that are relatively specific to the plants they attack: potatoes, tomatoes, oaks, alders, rhododendrons, coconut trees, pines, lice, etc. They cause diseases such as late tidings (P. infestans), sudden death of oak (P. ramorumetc.

Gender Pythium is even more prevalent than Phytophythorabecause their species individually cover a greater number of guests, usually causing less damage. It is a very common problem in greenhouses, killing the seedlings. Some species are micoparasite, that is, parasitizing other oomicetes and fungi (e.g., P. oligandrum), so they have been used as biological control agents. In addition, a species that also affects mammals is known (P. insidiosum).

The midiu group is easily identified by the appearance of a lanosous white growth on the surface of the plant organs (although this sign can be confused with the Erysiphales, ascomitants causing the dusty mildiu and which are true fungi). Very well known species are the mildiu of the vine (Plasmopara viticola) and the mildiu of the spinach (But we're Farinosa).

Gallery of infected species

Potatoes affected by the late tizone, Phytophthora infestans
Sudden death of oak, Phytophthora ramorum
Vid blade affected by the mildiu of the vine, Plasmopara viticola
Leaves affected by the mildiu of the spinach, But we're Farinosa
Plant affected by white roya, Albugo candida

Features

Saprolegnia: A=cigoto, B=zoosporangio, C= vegitativa, D=septo del zoosporango, E=septo del oogonio, F=oogonio (barra a escala = 0.2mm)

The common characteristic of the group is mycelial growth without septation and their life cycle is diplonte, that is, they present a cycle that alternates diploid mycelial phases with haploid sexual reproduction phases. The asexual reproduction phase is characterized by the presence of biflagellate zoospores with a mastigonemate flagellum directed forwards and another bare flagellum that is generally directed backwards. They are dispersed in water or by taking advantage of surface moisture (including precipitation on the surface of plants). Some species produce aerial asexual spores that are dispersed by the wind. The phase of sexual reproduction is by oogamy, by gametangiogamy (copulation-gametangial contact). The sexual spores, called oospores, spherical, translucent and double-walled, are used to survive in adverse environmental conditions.

The taxonomic classification of oomycetes has been much disputed. At first they were classified as a division belonging to the Fungi kingdom, but this has fallen out of use. For example, their cell walls are composed of cellulose, not chitin as in true fungi, and they generally do not have septation. Furthermore, in the vegetative state they have diploid nuclei, while fungi have haploid ones. Today Oomycetes is included as a class in the phylum Pseudofungi of the clade Heterokonta, along with brown algae, diatoms, and other saprophytic protists.