Pythium It is a genus of oomycetes that contains many phytopathogenic species of economic importance in agriculture, since they cause the disease known as root rot in various plants. Some species are saprophytic and one of them affects different species of animals, including man, causing the disease known as pythiosis.
The species of this genus are characterized, among other things, by presenting a coenocytic mycelium, being capable of reproducing vegetatively (asexually) as well as sexually, producing a single oosphere per oogonium, presenting an elongated antheridium and producing flagellated zoospores..
Currently the genus is represented by more than 120 species around the world. Parasitic plant species do not show specificity for their host and are difficult to eradicate, since in addition to attacking various plant species, they are capable of surviving as saprophytes on decomposing plant matter..
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The species of the genus Pythium They all have coenocytic hyphae, hyaline in appearance, without transverse septa. The sporangium can be globose or filamentous. A characteristic of these oomycetes is that the zoospores do not develop directly in the sporangium, but rather in a vesicle that derives from the discharge tube of the sporangium..
The oogonium can be smooth or ornate, and the antheridium can be paragynous or hypogynous. Inside the oogonium a single oospore is formed, which can occupy its entire interior (plethoric) or a space may remain between the walls of the oogonium and the oospore (apletoric).
The taxonomy of oomycetes is complex both at the level of the higher and lower taxonomic categories. According to traditional taxonomy, these organisms were considered fungi, however, they have recently undergone various rearrangements due to molecular and biochemical studies..
According to these results, some authors place them in the Protista Kingdom, while others place them, together with the Chrysophyta and Phaeophyta algae, within the Chromista Kingdom, Pseudofungi Subdivision. Lastly, they have also been located in the Straminipila Kingdom.
The gender Pythium it is located within the Order Pythiales, Class Pythiaceae. This genus was originally described by Pringsheim in 1858, who selected P. monospermum as type species.
The internal ordering of the genus has also undergone numerous changes, with the description of various subgenera, or the inclusion or exclusion of species. Currently, taxonomists consider that there are two morphologically distinguishable groups based on the shape of the sporangium..
On the one hand, there are the species that present a filamentous sporangium, while in the other group the sporangium is globose, which suggests that new intrageneric studies should be carried out to elucidate the phylogenetic relationship between both groups..
Nonspecific phytopathogenic species of frequent appearance in greenhouse crops, it is responsible for the disease known as damping-off, root and stem rot, or fruit blight that affects numerous families and genera of plants..
The disease mainly affects plants in their early stages of development. In tobacco seedlings, for example, it causes growth retardation, chlorotic or partially necrotic leaves, as well as a diminished root system, with darker, necrotic roots..
It is also responsible for the disease known as lawn burn or cotton burn, which affects the lawns of golf courses, sports stadiums, parks and gardens. This disease is more active in the hottest seasons, with night temperatures above 20 ° C.
It is the causal agent of the disease known as soft seed rot, which can attack the plant at any stage of its cycle, but is more frequent in the sowing stage, before and after germination. Causes seedling drop as well as seed rot.
In the newly germinated seeds and in the smallest plants, it causes the total destruction of the host, but the more developed plants can survive showing as symptoms of the disease a change in the color of some leaves, which turn yellow..
It is a pathogen that causes pythiosis, a pyogranulomatous disease of the skin that is characterized by being diffuse, with multiple foci, exudative and that progresses very quickly. As this species lacks chitin and ergosterol in the cell wall and membrane, it cannot be controlled by antifungal agents that inhibit their production..
It affects various species of vertebrates, including dogs, cats, sheep, horses, cattle, migratory birds, and even man. It is considered an emerging disease.
Pythium spiculum It causes the radical rot of the holm oak and the cork oak. It is the cause of the decline of the holm oak populations in Spain and Portugal since the 90s of the last century.
Pythium ultimum It is a species that affects the cucumber, producing the wet rot of the fruit. The disease begins in a floral extreme and quickly invades the fruit. Pythium guiyangense, meanwhile, it is a parasite of mosquito larvae.
The sporangia in Pythium they can be filamentous or globose. When mature, these sporangia can germinate directly and give a hypha or can develop a vesicle in which motile zoospores are to form. These zoospores use their flagella to move in the water in search of a host..
Once the appropriate place is located, the zoospore encysts and germinates, forming a germination tube that will penetrate the host's tissues to start a new cycle..
Sexual reproduction can be carried out both by homothallic and heterothallic crosses, that is, the oogonium and antheridium may or may not develop in the same hypha. During reproduction, the antheridium emits a fertilization tube that penetrates the oogonium, transferring its gametic nucleus to it..
After karyogamy, the wall of the egg thickens to become an oospore. This oospore can germinate immediately or, if conditions are adverse, survive in dormancy for several years..
Pythium it can have a saprotrophic diet, feeding on the remains of dead plants. It can also have a parasitic diet, with plants being its hosts in most cases, however, at least two species can parasitize animals.
Additionally, some species have been identified as mycoparasites of fungi, as well as of other parasitic oomycetes, so they could be used as biological controllers. These species include, for example Pithium periplocum Y P. acanthicum.
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