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Class Sordariomycetes

The Sordariomycetes are a large class, with 48 orders, 180 families, 2,176 genera, and more than 25,100 species. Most species in this class produce asci in perithecia. They mostly grow in soil, dung, and forest litter, but several orders also contain forest pathogens causing vascular wilts, cankers and blights, and wood decay fungi. Examples of forest pathogenic Sordariomycetes are those causing blue stain, chestnut blight, Verticillium wilt, and hypoxylon canker.

Six orders of Sordariomycetes have species within this database that are relevant to forest pathology: Diaporthales, Glomerellales, Hypocreales, Microascales, Ophiostomatales, and Xylariales.

Order Diaporthales

The Diaporthales consist of 34 families, 275 genera, and more than 3,780 species. Ascocarps are perithecia that form either singly or in clusters, either immersed or erupting from plant host tissue or formed within fungal stromata. Perithecia range in colour from orange to brown to black, with short or long necks ending in a pore where spores are released. Ascus walls have a single layer, and their apices have a conspicuous refractive ring. Ascospore morphology is highly variable. Conidia (asexual spores), when produced, are single-celled or single-septate, and form in acervuli or pycnidial conidiomata.

Family Cryphonectriaceae

This family consists of species that are saprophytic or parasitic on forest trees as well as economic crop plants. Several perithecia are immersed in a single stroma, which itself is fully or partially immersed in plant host tissue. From above, the stromata appear circular, dotted with perithecial openings (ostioles). The ostioles form at the end of long perithecial necks. Cells in the upper surface of the stroma stain purplish when exposed to a potassium hydroxide solution. Asci are cylindrical with a refractive apical ring. Ascospores vary from single-celled to multiseptate. Conidia are usually formed within separate locules in each stroma. Conidia are tiny and colourless with varying morphology.

Family Diaporthaceae

This family consists of species that are saprophytic and parasitic on plants, producing perithecia in a “pseudostroma,” which is a brownish structure made up of a mixture of fungal mycelium and plant tissue. Perithecia often form in a ring around the conidial stage, which is pycnidial or acervular. Asci are cylindrical with a distinct refractive apical ring, producing ascospores that vary in morphology from ellipsoid, oblong to pointed at each end (fusoid), single-celled or 1-septate, constricted at the septum, with or without appendages at both ends, and hyaline to dark brown. A single species will often produce two types of conidia, one larger than the other.

Family Gnomoniaceae

This family consists of species that are pathogenic or saprophytic on leaves and bark, are characterized by dark perithecia embedded in or erupting from the host plant tissue, and produce a thick disc where the perithecial neck emerges. Asci vary in shape from oval to narrowly cylindrical with short stalks and distinct refractive apical rings, producing 8–32 ascospores. Ascospores are mostly oval to pointed at either end (fusiform), and single-celled or 1-septate. Conidia are produced in pycnidia or acervuli, and are mostly ellipsoid to oval, hyaline and single-celled.

Family Sydowiellaceae

This family consists of species that are saprophytic or parasitic on plants, and are characterized by stromata (sometimes rudimentary) producing immersed to erumpent dark perithecia, which open via individual ostioles that dot the surface of foliage. Asci vary in shape from cylindrical to almost globose, with short stalks and a prominent apical ring. Ascospores are filamentous to ellipsoid, single- to multiseptate, sometimes appendaged, and pale to dark brown. Conidia, which are produced in brownish pycnidia, are ovoid to ellipsoid, hyaline, smooth, and single-celled. The genus Dothidella is also described by some authors as belonging to Polystomellacae, a family that is currently not assigned to any order in the Dothideomycetes.

Family Tubakiaceae

This family consists of endophytic and pathogenic (leaf spot) species, characterized by the production of pycnothyria, which are tiny shield-like pale brownish structures formed from radiating fungal cells. Single-celled, colourless to pale brown, globose conidia are extruded from under pycnothyria and dispersed by rain.

Family Valsaceae

This family consists of saprophytic and pathogenic (canker, twig dieback) species, characterized by stromata that are formed from both fungal and host plant tissues. Convoluted pycnidia first form in cankered bark, extruding tendrils of small, sausage-shaped (allantoid) colourless single-celled conidia. Later, black perithecia form, their elongated necks converging to open via ostioles immersed in a common stromatic disc. Asci are club-shaped, with a single wall layer and a conspicuous refractive apical disc, detaching from the base to float free in the perithecial cavity. Ascospores are single-celled, allantoid, and colourless.

Order Glomerellales

The Glomerellales consist of 3 families, 38 genera, and more than 680 species, characterized by dark perithecia with walls that are several layers thick, and with periphyses (hair-like appendages) lining the neck and ostiole. Their hymenia consist of single-walled asci and thin tapered paraphyses (sterile thread-like hyphae). Tips of asci are either thickened or thin with a distinct apical ring.

Family Plectosphaerellaceae

This family consists of soil-inhabiting, plant pathogenic species that produce ascospores from perithecia or cleistothecia when the sexual reproductive states are known. Asci are unitunicate (have a single wall layer), and are cylindrical, clavate or saccate, lacking any distinctive apical structure. Ascospores are ellipsoid to egg-shaped, single-celled or single-septate, colourless to pale brown, and occasionally ornamented. The sexual reproductive states of some genera have not been observed. Conidia (asexual spores) are produced in cushions (sporodochia), in acervuli, or from conidiogenous cells on hyphae. Conidia are single-celled or single-septate, forming slimy clusters or chains. Some species form thick-walled dark resting hyphae (chlamydospores), or larger structures called microsclerotia.

Order Hypocreales

The Hypocreales consist of 13 families, 470 genera, and more than 5,840 species. Species are often brightly coloured (red, yellow, or orange), and range in size from single perithecia less than 1 millimetre in diameter to large stromata several centimetres tall, bearing many perithecia. Some families are plant pathogenic, causing cankers and blights, while others are parasitic on other fungi or insects.

Family Nectriaceae

This family consists of species that are plant pathogenic fungi producing single or clustered, usually reddish or purplish, waxy perithecia that change colour when exposed to a potassium hydroxide solution. Asci are cylindrical and thin-walled, usually with a small colourless apical ring. Ascospores are variable in shape but usually have a single transverse cross wall. Conidial states are mould-like, often producing copious clumps of conidia (asexual spores) that aggregate in brightly coloured masses. Conidia are variable, but often slender, multiseptate, and curved (e.g., Fusarium species). Species in this family cause cankers on forest trees, while others are damaging agricultural crop pathogens.

Order Microsacales

The Microascales consist of 5 families, 134 genera, and more than 980 species. Species in this order produce single, usually dark perithecia of varying shapes from globose to pear-shaped or with long slender necks. Asci are thin-walled, usually dissolving to release the ascospores while they are still in the perithecium. Ascospores are single-celled, often with wall thickenings. Conidial stages are mould-like.

Family Ceratocystidaceae

This family consists of plant pathogenic (cankers, root disease, and vascular wilts) or saprophytic species characterized by single, usually dark perithecia with globose bases and long slender necks that are apically fringed or setose. Asci are formed in chains, and are globose and thin-walled, soon dissolving to release the ascospores while still in the perithecium. Ascospores are small, single-celled, and colourless, with varying morphologies, but often eccentrically shaped with wall thickenings. Conidia (asexual spores) are mould-like, with conidiogenous flask-shaped cells, extruding conidia from within (phialides). Conidia are also small, colourless, and usually rod-shaped. Various shapes of thick-walled resting cells (chlamydospores) are also formed on the hyphae. Members of this family, especially Ceratocystis species, are similar to members of the Ophiostomatales but are genetically and physiologically distinct, with morphologically differing conidial states.

Order Ophiostomatales

The Ophiostomatales consist of 2 families, 27 genera, and more than 300 species. Species in this order include plant pathogens (e.g., wood stains and vascular wilt fungi), as well as saprophytes. Many produce spores that are attractive to insects such as beetles, which act as vectors transporting these fungi to susceptible hosts.

Family Ophiostomaceae

This family of species consists of plant and animal pathogens and saprophytes, producing individual perithecia not associated with stromata. Perithecia are usually dark and range from globose to flask-shaped, some with long slender necks. Asci are usually globose, thin-walled and soon deliquesce (becoming fluid). Ascospores are single-celled and variously shaped. Conidia (asexual spores) may be formed simply, from conidiogenous cells on hyphae, or they may be formed on complex stalked structures, accumulating in a slimy glob that is attractive to insects. Many species are associated with insects such as bark beetles, which act as vectors helping to transport the fungi to susceptible plant hosts. Although they are often morphologically similar to members of the Ceratocystidaceae (Microascales), they are genetically and physiologically distinct. Unlike Ceratocystidaceae, they are resistant to the antibiotic cycloheximide, which can be added to agar to produce a selective medium to differentiate between the two families.

Order Xylariales

The Xylariales consist of 28 families, 261 genera, and more than 3,200 species. Species in this order have widely variable types of ascostromata, ranging from small and globose with a single perithecium, to large club-shaped stromata several centimetres tall. Asci are cylindrical and eight-spored, often with a distinct stalk, and an apical ring or plug, which stains blue when exposed to Melzer’s reagent. Ascospores are usually one-celled and dark, and have a germination slit or pore. When present, the conidia are produced either as a palisade of conidiogenous cells on immature stromata, or on mould-like mycelium.

Family Diatrypaceae

Species of this family are characterized by stromata that are usually dark and embedded in their host tissues, causing spots or patches on woody bark. Embedded within their stromata are perithecia that produce asci with long slender stalks and allantoid (sausage-shaped) ascospores. Some species are associated with canker diseases of trees and shrubs.

Family Xylariaceae

This family consists of species whose stromata usually have a carbonaceous dark outer layer and a paler interior. Dark perithecia are embedded in the outer stromatal layer and are usually flask-shaped with a central opening. Asci are usually cylindrical and stalked, with an apical plug, which usually stains blue when exposed to Melzer’s reagent. Ascospores are dark and usually single-celled, with a longitudinal germ slit or pore. When present, the conidia (asexual spores) are produced either as a palisade of conidiogenous cells on immature ascostromata, or on mould-like mycelia. They are associated with stem cankers, root disease, and wood decay (white rot).