Oomycetes are a diverse group of fungus-like eukaryotic microorganisms that form the phylum Oomycota. First classified within the kingdom Fungi, oomycetes are now unambiguously recognized as distinct from fungi and more closely related to heterokonts, such as brown algae, phylum Phaeophyta.

The more than five hundred species of oomycetes, commonly known as water molds, white rusts, or downy mildews, are essentially saprophytic but include pathogens of plants, insects, crustaceans, fish, vertebrate animals, and various microrganisms.

Plant pathogenic oomycetes cause devastating diseases on several crop, ornamental, and native plants. Animal pathogenic oomycetes can cause severe losses in aquaculture and fisheries. Both have had a significant impact on human history.

Evolutionary History

Traditionally, and due essentially to their filamentous growth habit, oomycetes have been classified in the kingdom Fungi. However, modern molecular and biochemical analyses as well asmorphological features suggest that oomycetes share little taxonomic affinity with filamentous fungi but are more closely related to brown algae (phylum Phaeophyta) in the kingdom Protista.

This position is supported by molecular phylogenies based on ribosomal RNA(ribonucleic acid) sequences, compiled amino acid data for mitochondrial proteins, and protein encoding chromosomal genes.

The oomycetes also display a number of biochemical and morphological characteristics that distinguish them from the fungi and confirm their affinity to brown algae and other heterokonts.

The cell walls of oomycetes are composed mainly of glucans and cellulose and, unlike fungal cell walls, contain little or no chitin. The zoospores display two flagella, with an ultra structure similar to that of the flagella of the motile spores of heterokont algae. The oomycetes also contain the energy storage chemical mycolaminarin, a molecule that is also found in kelps and diatoms.

Taxonomic Classes

Oomycetes on a leaf
Oomycetes on a leaf

The subdivison of oomycetes into taxonomic classes remains under debate. Typically four classes of oomycetes are identified. These include Saprolegniales, Leptomitales, Lagenidales, and Peronosporales.

Some authors elevated the plant pathogenic genera Phytophthora and Pythium to a separate class, named Pythiales. Recent molecular phylogenetic studies using ribosomal and mitochondrial sequences have started to unravel the evolutionary relationships between the different classes of oomycetes.

The Peronosporales and Pythiales, which together account for the majority of plant pathogenic genera, form an ancient monophyletic group, suggesting that acquisition of plant pathogenicity probably occurred early in the evolution of this lineage.

Most of the saprophytic and animal pathogenic species are restricted to the other classes. Aphanomyces, a genus with strong affinity to the Saprolegniales, includes both animal and plant pathogenic species.

General Features

The oomycetes inhabit primarily aquatic and moist soil habitats. They are often very abundant and can be easily cultured from both freshwater and saltwater ecosystems, as well as from a variety of agricultural or natural soils. However, several species are mainly terrestrial, including obligate biotrophic pathogens of plants that depend on air currents to disperse their spores.

The basic somatic structure of a majority of oomycete species is an extending fungus like thread, the hypha, that grows into a branched network of filaments, the mycelium. Oomycetes are known as coenocytic organisms; that is, their mycelium lacks septa or crosswalls that divide the hypha, except to separate it from the reproductive organs.

Both asexual and sexual reproductive structures occur. The primary asexual reproductive organ is the sporangium that differentiates at the tip of a vegetative hypha to produce and release motile zoosporeswith two flagella.

The zoospores can germinate directly or indirectly to produce a vegetative mycelium or can differentiate into secondary zoospores. Sexual reproduction involves the interaction of a male antheridia with a female oogonia through a fertilization tube that allows themale nuclei tomigrate into the oogonium.

Some oomycetes are self-fertile or homothallic, whereas others are self-sterile or heterothallic and require that strains with different mating types come into contact to achieve sexual reproduction. The sexual spores are the oospores, which can survive desiccation and starvation over long periods of time.

Under favorable environmental conditions, the oospores germinate to form vegetative mycelium or to release zoospores. Oospores are also the structures that gave the oomycetes their name of egg fungi. Oomycetes are diploid in the dominant vegetative phase with meiosis occurring only during gametogenesis.

Economic Importance

Saprophytic oomycetes play an important role in the decomposition and recycling of decaying matter in aquatic and soil environments. In addition, both plant and animal pathogenic oomycetes can cause serious economic impact by destroying crop, ornamental, and native plants as well as fish and other aquatic organisms.

Typically, oomycete diseases are difficult to manage and require the use of specific chemicals (fungicides or oomycides). Sources of sustainable genetic resistance in plants to oomycetes are limited. In addition, most oomycetes, such as Phytophthora, exhibit tremendous ability to adapt to chemical and genetic resistance through the development of new resistant strains.

For example, the appearance in the 1990’s of Phytophthora infestans strains resistant to the chemical metalaxyl resulted in potato late blight epidemics in the United States that were severe and destructive. Most modern research focuses on innovative approaches for the management of oomycete diseases, including the use of plant breeding, genetic engineering, and genomic technologies.

Plant-Pathogenic Oomycetes

Plant-associated oomycetes may be facultatively or obligately pathogenic. Pathogenic oomycetes form specialized infection structures, also found in fungi, such as appressoria (penetration structures) and haustoria (feeding structures).

Plant-pathogenic oomycetes include about sixty species of the genus Phytophthora, several genera of the biotrophic downy mildews, and more than one hundred species of the genus Pythium. Phytophthora species cause some of the most destructive plant diseases in the world and are arguably themost devastating pathogens of dicotyledonous plants.

The most notable plant-pathogenic oomycete is Phytophthora infestans, the Irish Potato Famine pathogen,which causes late blight, a disease of potato and tomato.

Introduction of this pathogen to Europe in the mid-nineteenth century resulted in the potato blight famine and the death and displacement of millions of people. Today, Phytophthora infestans remains a prevalent pathogen causing multi billion dollar losses in potato production worldwide.

Other economically important Phytophthora diseases include root and stem rot caused by Phytophthora sojae, which hampers soybean production in several continents, and black pod of cocoa caused by Phytophthora palmivora and Phytophthora megakarya, a recurring threat to worldwide chocolate production.

The introduction of exotic plant pathogenic oomycetes to natural ecosystems can also cause devastating effects. For example, Phytophthora cinnamomi has decimated native plants in Australia and South America.

More recently, sudden oak death, a disease caused by a new species, Phytophthora ramorum, has emerged as a severe disease of oak trees along the Pacific Coast of the United States.

Other notorious oomycete pathogens include the obligate biotrophs Plasmopara viticola, the agent of downy mildew of grapevine, as well as Albugo and Peronospora species, which cause white rust and downy mildews on several crops.

Animal-Pathogenic Oomycetes

Animal-pathogenic oomycetes are common. At least one oomycete species, Pythium insidiosum, is known to infect mammals, sometimes including humans, fatally.

However, most economic impact on animals is caused by oomycetes that infect fish, fish eggs, and crustaceans. Examples include Saprolegnia parasitica, a ubiquitous pathogen of fish that is common in aquaria and can cause severe losses in aquaculture, particularly when fish density is too high or fish diet is unbalanced.

Another important animal pathogenic oomycete is Aphanomyces astaci, the agent of crayfish plague that decimated European crayfish populations following its introduction from North America.

Animal-pathogenic oomycetes can also have beneficial effects. At least one species of oomycetes, the insect pathogen Lagenidium giganteum, has been commercialized by a California company as a biocontrol agent for mosquitoes.