Phytophthora is a genus of phytopathogenic oomycetes. Its name comes from the Greek words phyton (plant) and phthora (destroyer), that is, destroyer of plants. This genus includes both necrotrophic species (they cause the death of the hosts) and hemibiotrophs (they need the host alive for a time).
They mainly attack dicotyledons, being in many cases a specific species within the genus. That is, a kind of Phytophthora attacks only one species of plant. Some attacks on these crop plants have been the cause of considerable economic losses and even human deaths, as has come to cause the P. infestans.
Their reproduction occurs both sexually and asexually. In the first case, it is carried out through oospores, while in the second, chlamydospores and zoospores intervene.
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Representatives of the genus have a coenocytic mycelium and hyaline in appearance. Eventually they can present partitions, separating the old parts that do not have protoplasm from the rest of the mycelium. Inside the mycelium there are drops of oil.
They are facultative phytopathogens that can live by feeding on decomposing organic matter (saprophytes), or by infecting plants, especially dicots..
They reproduce asexually or sexually, although in some species their sexual reproduction is not known and in others it occurs very rarely. During sexual reproduction, round oospores are produced, with a smooth or moderately warty surface and a yellow or transparent color..
In asexual reproduction, biflagellate zoospores or chlamydospores can be produced that are generally rounded and with a wall of more than 2 microns thick..
Phytophthora belongs to the class of the Oomycetes, a group of organisms that had traditionally been included within the fungi kingdom, but which have currently been relocated within the Chromista by some authors, while others place them in the Protista kingdom.
The classification at the family level has also undergone changes. Phytophthora was originally located among the Pythiales, next to Pythium and other related genera, however, molecular biology analysis has shown that it has a greater affinity with the Perennosporales, where they were relocated.
On the other hand, this relocation also requires careful reviews, according to some authors..
The gender Phytophthora was described by de Bary in 1876 based on P. infestans as a type species. It comprises about a hundred species divided into 10 clades, however, there are still many species without describing, some located within complexes of species such as the complex Phytophthora megasperma.
This is one of the species responsible for chestnut ink, a disease that affects the tree of the same name. In addition to parasitizing the chestnut tree, it can also be found in the soil, where it feeds on decomposing organic matter.
As a parasite it affects the roots of the plant and, depending on the level of the root where the infection begins, it can cause the rapid death of the host (near the trunk) or it can survive for several years (secondary roots).
Phytophthora cinnamoni it is another species that causes chestnut ink. Like P. cinnamomi has saprophytic and parasitic habits, however, its effects on the host are much more devastating, to the point of being included among the 100 most harmful invasive species.
This fungus begins by invading the host through the secondary roots and from there they advance, via primary roots, to the trunk through the mycelium. The infection has no cure and causes the death of the plant.
Oomycete responsible for the disease known as potato late blight or potato mildew, even though it can attack different species of plants, including tomatoes and soybeans. He was responsible for the potato epidemic that caused the Great Famine in Ireland and Scotland (1845-1849).
The species of the genus Phythophthora can reproduce sexually through oospores (oogamy) or asexually, with the formation in this case of two types of spores, chlamydospores and zoospores, both produced by the sporangium.
Asexual reproduction is carried out by means of spores that are produced in sporangia. The sporangiophores, in most cases are indistinguishable from the hyphae, although in some cases they may be of a different diameter. The sporangia are colorless or may be a very pale yellow hue.
These structures are generally inserted terminally in the sporangiophore, although they may also occasionally be intercalated. They can remain in the sporangiophore (not deciduous), or detach easily (deciduous), in which case they act as scattering structures.
The sporangiophores will produce two different types of spores called chlamydospores and zoospores. Free water is necessary for zoospores to form. These zoospores can be dispersed by rain, air, irrigation waters, by humans and by some invertebrates.
Chlamydospores, on the other hand, are resistant spores characterized by having thick walls (> 2 microns thick). These chlamydospores can remain in the soil for a long time as infective units until the conditions are suitable for their germination..
The Phythophthora They present, as already said, both asexual and sexual reproduction, however, some species are very inconsistent with this last type of reproduction and they have only been identified in the laboratory and others are even completely unknown sexual reproduction.
In those in which sexual reproduction is known, it is carried out by means of oospores. Gametes are produced in the oogonium (female) and the antheridium (male). Some species can be homothallic, that is, they produce oogonia and antheridia in the same mycelium (they are self-fertile).
On the contrary, other species are heterothallic, that is, each mycelium produces only one type of gametangium, either male or female. In both cases, the fusion of the gametes will result in the formation of oospores, which in this genus are produced individually, that is, a single oospore per oogonium..
Oomycetes of the genus Phytophthora they are facultative phytoparasites. In free life, they are saprophytes that secrete enzymes to digest decomposing organic matter extracellularly, and then absorb the molecules of already digested materials..
Parasites, for their part, can be necrotrophs or hemibiotrophs. The necrotrophs will secrete toxins to kill the cells and tissues of the hosts and then obtain the nutrients from them..
Hemibiotrophs, in the early stages of their parasitic life, absorb nutrients from the plant without killing it, but then act and feed as necrotrophs.
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