Setosphaeria rostrata
| Setosphaeria rostrata | |
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| Scientific classification | |
| Kingdom: | Fungi |
| Phylum: | Ascomycota |
| Class: | Dothideomycetes |
| Subclass: | Pleosporomycetidae |
| Order: | Pleosporales |
| Family: | Pleosporaceae |
| Genus: | Setosphaeria |
| Species: | S. rostrata |
| Binomial name | |
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Setosphaeria rostrata K.J. Leonard (1976) |
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Setosphaeria rostrata is a thermophilic fungus with an asexual reproductive form treated in the genus Exserohilum (as Exserohilum rostratum). This fungus is a common plant pathogen, causing leaf spots as well as crown rot and root rot in grasses. It is also found in soils and on textiles in subtropical and tropical regions.Exserohilum rostratum is one of the 35 Exserohilum species implicated uncommonly as opportunistic pathogens of humans where it is an etiologic agent of sinusitis, keratitis and CNS vasculitis as well as cutaneous and subcutaneous mycoses. Infections caused by Exserohilum are most often seen in regions with hot climates particularly Israel, India and Southern USA.
Setosphaeria rostrata produce mature conidia with a distinct protruding hilum. The conidia are either straight, curved or bent and the septum above the hilum is thickened and dark. The walls are typically roughened and brown to olive in colour and there are typically 7–9 septa, however, some have 4–14.Exserohilum species typically exhibit rapid growth producing darkly pigmented colonies. Two other closely related species, E. longirostratum, and E. mcginnisii display a high homology with E. rostratum; however these can be differentiated by conidial morphology.Exserohilum ongirostratum is characterized by larger conidia (up to 228 x 12–19 μm), with 6–16 distosepta centrally curved. In contrast, E. mcginnisii has slightly clavate conidia, which are smooth-walled and brown and measure 44–76 x 11–18 μm with 4–8 distosepta lacking darkened bands.
In vitro studies of E. rostratum growth show sporulation to be completely inhibited by light at 34 °C (93 °F) but not at lower temperatures. Optimum sporulation in continuous light occurs 28–31 °C (82–88 °F) in vitro. At these lower temperatures, sporulation shifts from conidiophore induction to conidial formation. At 34 °C (93 °F) under continuous light, only conidiophores are formed. Sporulation of Exserohilum species can be enhanced by growth on cellulose medium for better identification in culture. In samples of naturally infested soil, overlaying plates with filter paper promotes sporulation and enables detection at low population levels.
In 1952, a study of the French Sahara and desert zone of Kizil Koum in Turkestan, identified a wide range of micromycetes in the uppermost layers of the soil. Many of the species present were saprophytic flora of dark pigmentation, including Helminthosporium, Curvularia, Alternaria and Stemphylium .Helminthosporium was the genus name of S. rostrata at the time. These fungi were mainly found in the mobile surface layer of the basin, becoming sparser with increasing depth. During dry periods, the Micromycetes in the mobile surface layers of the basin became mobilized by wind which accounted for the relatively uniform distributions of the surface microflora in the surrounding regions. It was discovered that the microfungal population of these superficial desert soils and sands had developed an adaptive morphology to protect them against unfavourable soil conditions. The dark pigmentation of these species is thought to protect them against light and desiccation, it can be seen in both the vegetative and reproductive states. As well, the spores of most of the Dematiaceae were reported as multicellular, septated transversely in the case of Helminthosporium which increases compartmentalization and protection. Another common characteristic of fungi isolated from desert sands is their rapid development and vast reproductive capacity.
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