''Pythium ultimum'' is a plant pathogen. It causes the

damping off Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions. Symptoms There ar ...

and root rot diseases of hundreds of diverse plant hosts including corn,

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu a ...

potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Unit ...

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

, fir, and many ornamental species. ''P. ultimum'' belongs to the peronosporalean lineage of

oomycete Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...

s, along with other important plant pathogens such as ''

Phytophthora ''Phytophthora'' (from Greek (''phytón''), "plant" and (), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water molds), whose member species are capable of causing enormous economic losses on crops worldwide, a ...

'' spp. and many genera of downy mildews. ''P. ultimum'' is a frequent inhabitant of fields, freshwater ponds, and decomposing vegetation in most areas of the world. Contributing to the widespread distribution and persistence of ''P. ultimum'' is its ability to grow

saprotroph Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...

ically in soil and plant residue. This trait is also exhibited by most ''

Pythium ''Pythium'' is a genus of parasitic oomycetes. They were formerly classified as fungi. Most species are plant parasites, but ''Pythium insidiosum'' is an important pathogen of animals, causing pythiosis. The feet of the fungus gnat are frequen ...

'' spp. but not by the related ''Phytophthora'' spp., which can only colonize living plant hosts.

Pathology and disease management

Infections of seeds and roots are initiated by both the

mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...

and spores of ''P. ultimum''. Two spore types are made, depending on the strain. ''P. ultimum'' is a species complex that includes ''P. u.'' var. ''ultimum'' and ''P. u.'' var. ''sporangiiferum''. The major distinguishing feature is that

sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

and

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or ...

s (swimming spores) are produced only rarely by ''P. u.'' var. ''ultimum''. Both species make

oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia ...

s, which are thick-walled structures produced by sexual recombination. Both varieties are self-fertile (

homothallic Homothallic refers to the possession, within a single organism, of the resources to reproduce sexually; i.e., having male and female reproductive structures on the same thallus. The opposite sexual functions are performed by different cells of a si ...

), which means that a single strain can mate with itself. In addition to oospores, ''P. u.'' var. ''ultimum'' also makes hyphal swellings which germinate in a manner resembling

to form plant-infecting hyphae. One important ecological difference between the different types of spores is that sporangia and zoospores are short-lived, while the thick-walled oospores can persist for years within soil, surviving even winter freezes. Mycelia and oospores in soil can infect seeds or roots. This leads to wilting, reduced yield, and ultimately plant death. Common signs of a ''Pythium'' infection include stunting of the plants, brown coloration of root-tips, and wilting of the plant during the warm part of the day. Management of disease is challenging but focuses on

sanitation Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation syste ...

, fungicides, and

biological control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also i ...

. Fungicides include mefenoxam,

thiadiazole In chemistry thiadiazoles are a sub-family of azole compounds, with the name thiadiazole originating from the Hantzsch–Widman nomenclature. Structurally, they are five-membered heterocyclic compounds containing one sulfur and two nitrogen atoms. T ...

, etridiazole, propamocarb, dimethomorph, and

phosphonate In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing groups (where R = alkyl, aryl, or just hydrogen). Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorly ...

s. Biological control agents include the bacteria ''

Bacillus subtilis ''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacillus ...

'', '' Streptomyces griseoviridis'', and the fungi '' Candida oleophila'', '' Gliocladium catenulatum'', '' Trichoderma harzianum'', and ''

Trichoderma virens ''Trichoderma'' is a genus of fungi in the family Hypocreaceae that is present in all soils, where they are the most prevalent culturable fungi. Many species in this genus can be characterized as opportunistic avirulent plant symbionts. This re ...

''. Effective resistance in the plant host is generally not available. Sanitation is very important since the pathogen can be easily introduced into pasteurized soil or even soil-free potting mixes on dirty tools or pots. Especially in greenhouses, fungus gnats may also help move the pathogen from place to place. A recent study of greenhouses in Michigan revealed that the same pathogen populations were responsible for the root rot of all greenhouse ornamental plants over a two-year period. These results stress the importance of sanitation and encourage greenhouse growers to improve their scouting of all incoming plant material to prevent additional root rot.

Genetics

The genomes of both ''P. u.'' var. ''ultimum'' and ''P. u.'' var. ''sporangiiferum'' have been sequenced.Levesque, C. A., Brouwer, H., Cano, L., Hamilton, J. P., Holt, C., Huitema, E., et al. (2010) Genome sequence of the necrotrophic plant pathogen ''Pythium ultimum'' reveals original pathogenicity mechanisms and effector repertoire. Genome Biol. 11, R73. Analysis of the genomes suggest that the two species encode 15,290 and 14,086 proteins, respectively.

References

External links

''Pythium'' Genome Database

Index Fungorum

USDA ARS Fungal Database
{{Taxonbar, from=Q7263907 Water mould plant pathogens and diseases ultimum Soybean diseases