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Septobasidium is a fungal genus within the family Septobasidiaceae. Approximately 175 described species are associated with this genus. 227 records are listed by Species Fungorum.
Septobasidium species are known to be entomopathogens.
Fruiting bodies form a crust (resupinate) and range in color and size, from small patches (1 mm in diameter) to 2 meters wide. Species of this genus are often distinguished based on the thickness of the fruiting body. Some species form elaborate chambers and tunnels that house with top and bottom layers while others form a very thin network. Microscopic characteristics, such as the number of produced on a Basidium, presence of pillars supporting the top layer (if applicable), number of cells in a basidia, and shape of Haustorium (infectious cells) that form within the scale insects are used to distinguish species. Septobasidium is unique in that it is one of a few genera within the family Septobasidaceae that exists in symbiotic relationships with scale insects ranging from obligately Parasitism to mutualistic.
This type of fungus is fairly unique for having a mutualistic relationship with scale insect hosts, rather than killing them. Although it weakens the insects it parasitizes, it does not kill them and it benefits the population as a whole, helping provide protection from parasitoid wasps by forming a mycelial mat that helps conceal the insects. The fungus benefits from the relationship, as it is nourished by the waste products the insects produce.
During the spring months, Basidium gives rise to sexual spores known as that are capable of infecting a first instar scale insect walking across the surface of the fruiting body. The infected insect will either 1) settle with other scale insects within the same fruiting body it was infected by, 2) travel to another fruiting body and settle, or 3) move to an un-infected plant tissue and settle, forming a new colony of the fruiting body. Scale insects are mobile during the first instar after hatching, which co-occurs in the spring when basidiospores are released. Scale insects begin to feed on plant sap and settle into one location, eventually molting and shedding legs. Hyphae emerge from natural openings of infected insects and form a Mycelium mat above the infected and non-infected insects. The life cycle completes itself when hyphae gives rise to new basidiospores on the surface of the mycelial mat during spring rain events, and infects the next generation of scale insects.
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