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The Leotiomycetes includes non-lichenized fungi producing a generally small apothecium with an exposed hymenium and an inoperculate, unitunicate ascus that has an apical perforation pore for releasing ascospores (Dennis 1968, Korf 1973, Nannfeldt 1932, Pfister and Kimbrough 2001). Recent molecular studies suggest that several groups of fungi with simple, cleistothecial ascomata belong to the Leotiomycetes, including the Erysiphales, Myxotrichaceae, and Thelebolales (Blackwell et al. 2006; de Hoog et al. 2005; Gernandt et al. 2001). Conversely, other groups traditionally included in the Leotiomycetes, such as the Geoglossaceae and Orbiliaceae, have been proved to be phylogenetically distinct (Spatafora et al. 2006; Wang et al. 2006). Currently, five orders, 21 families, and about 510 genera (115 with an uncertain position) are accepted in the Leotiomycetes on the basis of both traditional classification and molecular phylogenetic studies (Eriksson 2005, Kirk et al. 2001, Hibbett et al. 2007).
Morphologically and ecologically, the Leotiomycetes is a highly diverse group of the Pezizomycotina. For example, species of Cyttaria (Cyttariales) produce brightly colored globose fruiting bodies of "ping-pong ball" size on hard wood trees, while apothecia of Lophodermium (Rhytismatales) often mature as small, dark dots on conifer needles. The Erysiphales and Thelebolales are associated with distinct ecological characters and nutritional modes, in addition to morphology almost unique for each group. Members of the Helotiales thrive in various ecosystems and cover a broad range of niches, and helotialean fungi have been described as plant pathogens, endophytes, mycorrhizae, fungal parasites, terrestrial saprobes, aquatic saprobes, root symbionts, and wood rot fungi.
Top left and center: SEM (scanning electron micrographs) of Microsphaera palczewskii Jacz. (Erysiphales) showing appendages branching dichotomously multiple times from cleistothecia. Plant Disease 87: 451. © Claudia Nischwitz and George Newcombe. Top right: Aero-aquatic fungus Mitrula elegans (Heliothales) Berk. © Michael Wood. Bottom left: Chlorociboria aeruginascens (Nylander) Kanouse from Viry, Haute-Savoie, France. © Laurent Francini. Bottom right: Wood decay caused by Chlorociboria aeruginascens © Gary Emberger
Many species of the Leotiomycetes are described from the temperate Northern Hemisphere, but some members within the Leotiomycetes are patchy in their broad geographic distribution. For example many fungi of the Sclerotiniaceae (Helotiales) are plant pathogens, and these fungi might have evolved primarily in the Northern Hemisphere. However, other genera, such as Cyttaria spp. and Chlorovibrissea, are only found in the Southern Hemisphere.
Two fungi only known from the Southern Hemisphere (from left to right): Cyttaria gunnii; Chlorovibrissea phialophora.© Clive Shivley