Oomycetes were originally grouped withfungi due to similarities in morphology and lifestyle. However, molecular and phylogenetic studies revealed significant differences between fungi and oomycetes which means the latter are now grouped with thestramenopiles (which include some types ofalgae). The Oomycota have a very sparse fossil record; a possible oomycete has been described fromCretaceousamber.[6]
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.
The oomycetes rarely have septa (seehypha), 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, while others are filamentous and branching.[8]
Previously the group was arranged into six orders.[8]
TheSaprolegniales are the most widespread. Many break down decaying matter; others are parasites.
TheLeptomitales have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and often reproduce asexually.
TheRhipidiales use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies.
TheAlbuginales are considered by some authors to be a family (Albuginaceae) within the Peronosporales, although it has been shown that they are phylogenetically distinct from this order.
ThePeronosporales too are mainly saprophytic or parasitic on plants, and have an aseptate, branching form. Many of the most damaging agricultural parasites belong to this order.
TheLagenidiales are the most primitive; some are filamentous, others unicellular; they are generally parasitic.
However more recently this has been expanded considerably.[9][10]
This group was originally classified among thefungi (the name "oomycota" means "egg fungus") and later treated asprotists, based on general morphology and lifestyle.[6] Acladistic analysis based on modern discoveries about the biology of these organisms supports a relatively close relationship with somephotosynthetic organisms, such asbrown algae anddiatoms.[11] A commontaxonomic classification based on these data, places theclass oomycetes class along with other classes such as the brown algae within thephylumStramenopile.
This relationship is supported by a number of observed differences between the characteristics of oomycetes and fungi. For instance, thecell walls of oomycetes are composed ofcellulose rather thanchitin[12] and generally do not haveseptations. Also, in the vegetative state they havediploid nuclei, whereas fungi havehaploid nuclei. Most oomycetes produce self-motile zoospores with twoflagella. 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 theChytridiomycetes) have only one whiplash flagellum.[12] Oomycota and fungi have different metabolic pathways for synthesizinglysine and have a number of enzymes that differ.[12] The ultrastructure is also different, with oomycota having tubular mitochondrialcristae and fungi having flattened cristae.[12]
In spite of this, manyspecies of oomycetes are still described or listed as types of fungi and may sometimes be referred to aspseudofungi, or lower fungi.
Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motilespores calledzoospores, 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, calledoospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.
Unidentified water mold on dead larval mayfly.Aculture ofAchlya sp. (Saprolegniales) isolated from a stream.Sea trout suffering fromUDN with secondarySaprolegnia infections.
Many oomycetes species are economically important, aggressive algae andplant pathogens.[13][14] Some species cancause 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.
TheparaphyleticPythium group is more prevalent thanPhytophthora and individual species have larger host ranges, although usually causing less damage.Pythiumdamping off is a very common problem in greenhouses, where the organism kills newly emerged seedlings. Mycoparasitic members of this group (e.g.P. oligandrum) parasitize other oomycetes and fungi, and have been employed as biocontrol agents. OnePythium species,Pythium insidiosum, also causesPythiosis in mammals.
The third group are thedowny 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 fungalpowdery mildews).
The fourth group are thewhite blister rusts,Albuginales, which cause white blister disease on a variety of flowering plants. White blister rusts sporulate beneath theepidermis of their hosts, causing spore-filled blisters on stems, leaves and theinflorescence. The Albuginales are currently divided into three genera,Albugo parasitic predominantly toBrassicales,Pustula, parasitic predominantly toAsterales, andWilsoniana, predominantly parasitic toCaryophyllales. Like thedowny mildews, thewhite blister rusts are obligatebiotrophs, which means that they are unable to survive without the presence of a living host.
^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.PMID16018320.
^abcdVan 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.doi:10.1093/oxfordjournals.molbev.a040245.PMID7659021.
^Agrios, George N. (2005).Plant Pathology. 5th ed. Academic Press.link.
^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.S2CID2041934.
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