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Class Leotiomycetes

The Leotiomycetes consist of 10 orders, 60 families, 925 genera, and more than 10,000 species. They are mostly small cup fungi, producing apothecia (open, disc or cup-like fruiting bodies) with a hymenium (spore-producing layer) consisting of single-walled asci that do not have an apical lid (operculum) but instead release their hyaline, single-celled ascospores through an apical pore.

Four orders of Leotiomycetes have species within this database that are relevant to forest pathology: Chaetomellales, Helotiales, Phacidiales, and Rhytismatales.

Order Chaetomellales

The Chaetomellales consist of 2 families, 20 genera, and more than 170 species. They produce small, often brightly coloured apothecia in decaying or occasionally living plant tissues.

Family Marthamycetaceae

This family consists of species characterized by small, often brightly coloured apothecia that are partially immersed in their plant substrate. Asci in the hymenium are cylindrical to club-shaped, with a rounded apex, and usually contain eight ascospores. The hyaline ascospores vary from ellipsoid to thin, slender, and septate, and sometimes have a gelatinous sheath. Species are saprophytic on dead plant material, and rarely parasitic on conifer foliage.

Order Helotiales

The Helotiales are a very large order of fungi consisting of 42 families, 708 genera, and more than 8,400 species. They are characterized by cup-shaped ascocarps (apothecia) that are usually covered with long cylindrical hairs.

Family Cenangiaceae

This family of species consists of apothecial fungi whose hymenium is made up of cylindrical to club-shaped asci that usually have tips that stain blue when exposed to Melzer’s reagent. The asci have eight spores and are interspersed with septate, slightly clubbed paraphyses. Ascospores are colourless, ellipsoid to globose, and single-celled or up to 2-septate. When present, the conidia (asexual spores) are produced in pycnidia. Some species produce apothecia on conifer foliage, causing needle cast disease.

Family Drepanopezizaceae

This family of species consists of plant pathogens associated with foliar blight and twig blights of broadleaf trees and plants, and producing apothecia that are mostly embedded in host tissue. Ascospores are mostly ellipsoid, colourless, and single-celled or 1–2-septate. Conidia are produced in acervuli, which are saucer-shaped masses of conidiogenous cells where conidia break through the host epidermis. Two types of conidia are often formed in the acervuli: macroconidia that are ellipsoid to teardrop-shaped and often slightly curved; and tiny single-celled microconidia.

Family Erysiphaceae

This family of species consists of obligate plant parasites known collectively as powdery mildews because their conidial state, with superficial mycelia and chains of cylindrical colourless conidia, produces a white powdery coating on foliage. They obtain nutrients from plant cells by penetrating them with specialized hyphae called haustoria. The asci are produced in near-microscopic closed ascocarps called cleistothecia, which are often dark and ornamented with colourless appendages. The asci are broadly club-shaped, containing colourless to yellowish ellipsoid single-celled ascospores.

Family Gelatinodiscaceae

This family of species consists mostly of saprophytic, gelatinous, apothecial fungi, some of which are associated with wood decay. Asci stain blue when exposed to Melzer’s reagent, producing yellowish to brownish smooth-walled ascospores that usually have a gelatinous sheath. Ascospores are single-celled or 1-5-septate.

Family Godroniaceae

This family of species consists of plant pathogens causing shoot blights and stem cankers, and producing dark disc- or cup-shaped apothecia in clusters from a common base. The hymenial layer produces cylindrical asci bearing eight, colourless, septate ascospores that are ellipsoid with pointed ends (fusoid). Dark pycnidia also develop in the vicinity of the apothecia, producing colourless septate conidia that vary in shape from rounded to cylindrical and curved.

Family Lachnaceae

This family consists of saprophytic and parasitic species, some of the latter causing cankers on conifers. Most species form apothecia that are often stalked and externally hairy, with a yellow-to-orange hymenium. The cylindrical asci produce ascospores in a variety of shapes ranging from globose to filiform. Conidia are produced in pycnidia.

Family Ploettnerulaceae

This family consists of saprophytic and parasitic species, including some weak pathogens of conifer needles. Most species produce apothecial fruiting bodies, often externally covered with dark hairs. The cylindrical asci have conical apices that stain blue when exposed to Melzer’s reagent. Ascospores vary in morphology from ellipsoid to filiform, and aseptate to 3-septate. Conidia are produced either on conidiogenous cells on bare hyphae or in pycnidia.

Family Sclerotiniaceae

This family consists of saprophytic and parasitic species that produce apothecial fruiting bodies, usually forming on sclerotia (rounded masses of hyphae, often black, forming as resting structures in plant tissues). Some species cause “ink spot” disease of broadleaf foliage, named for the appearance of the sclerotia in the leaves. Asci usually stain blue when exposed to Melzer’s reagent, and produce ascospores that are usually ellipsoid, colourless, smooth-walled, and 1–3-septate, often budding within the ascus.

Order Phacidiales

The Phacidiales consist of 2 families, 36 genera, and 210 species. Members are either saprophytic or parasitic, and some are associated with needle diseases in conifers. They produce apothecia that are usually dark and covered with an upper layer of fungal tissue (excipulum) that ruptures to expose the hymenium. Conidia (asexual spores) are produced in pycnidia.

Family Hamatocanthoscyphaceae

This family consists of saprophytic and parasitic species associated with foliar and twig blights, forming stromata in plant tissue that develop into apothecia covered by a layer of dark fungal tissue (excipulum). Asci are cylindrical to club-shaped, and ascospores are hyaline, ellipsoid to fusoid in shape, and 0–1-septate. Conidia are produced in pycnidia and are single-celled, varying from nearly cylindrical to elliptical, and some have a mucilaginous appendage.

Order Rhytismatales

The Rhytismatales consist of 3 families, 79 genera, and more than 710 species of saprophytic or pathogenic fungi, many fruiting on the upper surfaces of the host foliage where they cause “tar spots” on broadleaf hosts, and needle cast diseases on conifers. Species in this order typically produce dark stromata or ascomata that are drought resistant, opening by splits in the upper surface of the leaf or needle during moist weather to expose the hymenium.

Family Rhytismataceae

Originally, the species in this family were collectively referred to as “tar spot” fungi because of the appearance of the apothecia of some of the genera on broadleaf foliage. Genetic studies have broadened the concept of this family to include many of the genera that cause needle cast diseases in conifers, as well as species with other types of fruiting bodies, some even long and stalked. The apothecia, regardless of form, open by splits or fissures to expose the hymenium composed of cylindrical asci and paraphyses (sterile hyphal filaments). Ascospores are highly variable within this family, but many are covered by a gelatinous sheath. Conidia (asexual spores), when present, form in pycnidia. Needle cast genera often form ascocarps that are elongated parallel to the long axis of the needle, known as hysterothecia.