This large and diverse assemblage of fungi can be grouped by only one character: The sexual spores of these fungi are borne inside a sac-like structure called an ascus.

 These spores are the product of sexual fusion of gametes and result from meiotic and mitotic divisions of a diploid zygote.

 According to the textbook the Phylum Ascomycota contains only one Class, the Ascomycetes, which is subdivided into three broad groups:

Each of these groups is distinguished by the type of sexual conjugation, the type of ascus (Prototunicate, Unitunicate or Bitunicate), the presence of an ascocarp, the type of ascocarp, the presence of an hymenium and the habitat.

Contains the genera Taphrina, Protomyces, Schizosaccharomyces, Saitoella and Pneumocystis. The most important features of this group are the lack of an ascocarp, the prototunicate asci and sexual conjugation by gametangial copulation, without ascogenous hyphae.

The genus Taphrina contains some very serious pathogens of fruit trees and other plants. These fungi cause leaf curl, witches broom diseases and other hyperplasia diseases of higher plants. The asci are borne naked on short ascophores, and are the result of gametic fusion between non-specialized gametes. Gametic copulation may occur between blastoporic conidia which bud from the ascospores or from the conidia themselves. Somatogamy may also occur, when mycelium is present in the host leaf, although these fungi grow in a yeast habit when grown in artificial culture. There is a rudimentary dikaryotic phase after mitotic divisions of the ascospore or blastospore nuclei, prior to plasmogamy and karyogamy occurring. The asci are formed on the surface of the leaf and generally contain 8 ascospores.

The Saccharomycetales contains 8 families and are commonly unicellular yeasts, usually without a mycelial stage. The most important family, the Saccharomycetaceae, contains the yeast Saccharomyces cerevisae used in the brewing and baking industries.

Plectomycetes:

The Plectomycetes are characterized as Ascomycetes which have an ascocarp without a hymenium. The ascocarp is typically a cleistothecium; the asci are usually globose and break down with maturation of the ascocarp. There are three orders in this group; the Ascosphaerales, Onygenales and the Eurotiales. Each of these groups are delimited on the basis of the structure of the cleistothecium, the habitat, the type of ascospore and the anamorph. Some orders are simply interesting curiosities, while other orders such as the Onygenales and the Eurotiales are very important in human affairs.

The Ascosphaerales produce their ascospores in the usual way with crozier formation, but the asci breakdown early and are borne in "spore-balls" (actually ascus-balls) within a sporocyst, which can be considered as a cleistothecium. These fungi are found in association with beehives. The Elaphomycetales form asci from ascogenous hyphae in the usual way but do not form croziers, no sex organs have been found in these fungi. The ascocarp of these fungi is a large cleistothecial complex, in which the individual cleistothecia lose their identity and become part of the gleba within a hardened peridium. The ascocarp is hypogeous and often eaten by animals.

The Onygenales contains two families, the Gymnoascaceae and the Onygenaceae which include some of the most potent pathogens of humans. These fungi are usually epigean and the anamorph stages are arthrosporic or aleuriosporic. The ascospores are non-dextrinoid and single celled. These fungi are often keratinophilic. These fungi often produce their cleistothecia on stalks and the ascospores usually disperse after drying. The Tinea pathogens of humans are classified here as are the cause of Histoplasmosis and Blastomycosis.

The Eurotiales consist of Plectomycetidae with anamorphs bearing conidia other than arthrospores or aleuriospores. The most common type of conidium in these fungi is the phialospore. This order includes the teleomorphs of some of the commonest fungi of all, the anamorph genera Penicillium and Aspergillus. The Eurotiales are characterized by having a definite cleistothecial peridium containing globose asci with single celled often ornate ascospores. Sexual organs are differentiated in these fungi with an antheridium and ascogonium, although not all species form croziers. While most teleomorph genera (those classified in the Eurotiales) have been associated with an anamorph species in the genera Aspergillus or Penicillium, there are many anamorph species in these genera which have never been associated with a teleomorph in the Eurotiales. The Anamorph - Teleomorph connection in the fungi, however is quite well defined. The teleomorph genera Talaromyces and Eupenicillium are always associated with Penicillium anamorphs. The teleomorph genera Eurotium and Emericella and Sartorya always have Aspergillus anamorphs. The taxonomy of the Eurotiales has to a large extent been developed by mycologists working on the anamorph genera and it is monographs on the genera Aspergillus and Penicillium that much work on the Eurotiales can be found.

Pyrenomycetes

The pyrenomycetes contains the majority of ascocarpic Ascomycetes. These fungi produce unitunicate asci in perithecial ascocarps with a hymenium and usually a well defined peridium. They are divided on the basis of the type of ascocarp, and the structure and chemistry of the ascus apex. The centrum, or the whole interior of the ascocarp, is an important character used to classify the group. The ascocarp is a perithecium in most species. There are 9 orders of Pyrenomycetes including some very important plant pathogens.

The Order Microascales contains the devastating plant pathogens in the genus Ceratocystis. This order is divided into two families the Microascaceae and the Ceratocystidaceae. They have dextrinoid ascospores with germ pores and annelosporic or aleuriosporic anamorphs. Croziers and ascogenous hyphae are usually not formed and the ascocarps are often covered in loose mycelia and have a long neck in the genus Ceratocystis. The Ceratocystidaceae have been well studied since it contains the fungus Ceratocystis fagacearum, the Oak wilt pathogen. The anamorphs of these fungi usually produce enteroblastic annelospores which may be produced singly, or in globose heads held together by mucus.

The order Ophiostomatales contains the infamous pathogen Ophiostoma novo-ulmi the cause of Dutch Elm Disease, an absolutely devastating disease. This order has been confused with the Microascales, but differs in that the Ophiostaomateles has rhamnose and cellulose in the cell walls, they are resistant to cyloheximide and they have non-phialidic , holoblastic anamorphs, such as Leptographium.

The Xylariales, Sordariales and Phyllachorales are often produce their ascocarps in a stroma, although many from their ascocarps free on somatic hyphae. They all have a Xylaria - type centrum characterized by the outgrowth of hyphae from ascogonial stalk cells or subtending hyphae to form the wall of the ascocarp, which is quite distinct from the cells of a stroma , if present. The Xylaria - type centrum then develops by the growth of paraphyses from the walls of the ascocarp into the cavity which will hold the ascospores. It is thought the paraphyses actually enlarge the cavity by their growth, and continued growth of the paraphyses causes the formation of an ostiole. Asci develop from a basal hymenium and grow upwards through the paraphyses which may persist or breakdown during the maturation of the ascospores. The Xylariales include all perithecial ascomycetes which have unitunicate asci borne in dark, leathery or carbonaceous ascocarps, usually with abundant paraphyses,a nd may be borne in stroma of fungal or host origin. These fungi are typically saprophytes and mild pathogens and most do not have an imperfect state (anamorph). Sexual reproduction in the Xylariales is quite varied although ascogonia and antheridia have been found in many species. The xylariales have clavate asci with a single apical pore and often some thickening around the apex. These fungi are common and important saprophytes and plant pathogens. The ascocarps can be quite large and macroscopic or small and inconspicuous. The Sordariales contains the important genetics experimental organisms Neurospora and Sordaria. These fungi are easy to grow and fruit in the laboratory and provided a valuable model systems for studying the development of ascospores and the phenomena of cross-overs and segregation during meisosis. (Remember: meiosis occurs in the ascus mother cell after the diploid nucleus is formed by karyogamy and results in the formation of 8 ascospores after a single mitotic division).

The Nectria - type centrum is found only in the Hypocreales and can be recognized by the downward growth of a palisade of paraphyses into the lumen of the perithecium, which disintegrates as the asci grow upward from the hymenium below. There are several important families in the Hypocreales, including the Nectriaceae and the Hypocreaceae. Perithecia may be superficial or immersed or stromatic. They are often brightly colored and obvious. The genus Nectria is the largest in the order and includes many serious pathogens of trees. The anamorphs of many of these fungi are in the form genus Fusarium and as such are responsible for many very serious plant diseases and animal mycotoxicoses. The majority of these fungi are saprophytes and are found commonly on many type of substrata. These fungi follow the usual ascomycete life cycle and each ascus contains 8 ascospores, which are usually one septate. The ascospores and ascocarps are usually light colored or hyaline. The family Clavicipitaceae contains some of the most serious of plant pathogens, in an historical sense. These fungi have perithecia which are borne in a stipate stroma. The order contains two main genera: Claviceps and Cordyceps. Both genera produce their perithecia in stalked ascostromata and have thread-like ascospores. The genus Claviceps is a specialized pathogen of small grains such as Rye and colonizes the host during the season finally colonizing the developing embryo. The embryo and endosperm are then replaced by a sclerotium or "ergot" which remains in the head on the grain plant. The sclerotium is slightly larger than the Rye grain and can be detected with careful observation. However during the seventeenth to the nineteenth century in Europe, many of these sclerotia were mixed with the rye grain and made into rye bread, with tragic results. The sclerotia of this fungus contain various alkaloids which are toxic to humans and animals, and estimates of over 2 million people dead from ergotism over the past 300 years have been made. The genus Cordyceps is a mild mannered cousin to Claviceps and is parasitic on the hypogean ascocarps of the Elaphomycetaceae. The ascocarps of Cordyceps are large and conspicuous and can be found fairly easily. The fungus Cordyceps militaris is very obvious having a bright red, erect stroma.

These fungi have a cleistothecium as the main type of ascocarp, and they are sometimes called "Plectomycetes" for this reason. However they are not in the Plectomycetidae because they have a well formed hymenium and peridium. there are two orders in this group: the Erysiphales and the Meliolales. They are distinguished from other Hymenoascomycetidae by having a Phyllactinia - type centrum, which is a centrum which begins as a pseudoparenchymatous mass filling the immature ascocarp. As the globose asci develop and fill the center of the ascocarp this pseudoparenchymatous mass disintegrates and is replaced by maturing asci, which break down by the time the cleistothecium is fully mature. The cleistothecia of these fungi are usually cream to white when young and brown to black when mature.

The fungi in this group are biotrophic plant pathogens and the members of the Erysiphales are known as the powdery mildews while the Meliolales are known as the black mildews. The two orders can be easily distinguished by the hyaline mycelium and lack of hyphopodia in the Erysiphales and the dark mycelium with hyphopodia present in the Meliolales. The Erysiphales also usually produces basocatenate hyaline conidia whereas asexual reproduction sis unknown in the Meliolales.

The Eysiphales can be identified to genus very easily if the Teleomorph (Cleistothecial stage) is present, which unfortunately is not always the case. The teleomorph genera are distinguished by the ornamentation on the cleistothecium. Teleomorph species are distinguished by the anamorph conidia and conidiophores, although it is possible to use only anamorph features to identify Teleomorph genera, it is always better to do this with the cleistothecia. One thing to note is that Erysiphe spp. are usually associated with herbaceous plants whereas Microsphaera spp. and Uncinula spp. and Phyllactinia spp. are mostly associated with woody plants.

The Erysiphales grow intercellularly in the host and penetrate the host cell wall but not plasmalemma with lobate feeding structures called haustoria.

The Erysiphales are both homothallic and heterothallic and sexual reproduction usually occurs at the end of a favorable period for growth. There are antheridia and ascogonia formed but the role of the antheridium, ascogonium and trichogyne is not yet clear, in the sexual reproduction of many of these fungi.

The Meliolales form a dark mycelial covering over the leaves of susceptible plants. There is a single family the Meliolaceae and the genus Meliola is the largest genus in the family. These biotrophic fungi produce hyphopodia which may act as attachment structures, and some but not all hyphopodia have been shown to produce infection pegs which develop into haustoria in host cells.

The cleistothecia of the Meliolales are similar to those produced by the Erysiphales in most respects.

The Diaporthe - type centrum is found in a single order the Diaporthales and can be distinguished by a pseudoparenchymatous centrum that is partly or totally deliquescent as the ascospores mature. Only one order has a Diaporthe - type centrum the Diaporthales. There are several families in the Diaporthales and they have a typical perithecium or cleistothecium as an ascocarp. The perithecia have a well developed neck, and both fruiting bodies may be either immersed, stromatic or superficial. The life cycle of these fungi follows the typical ascomycete pattern and there are 8 ascospores per ascus. Ascospores are 0-1 septate, filiform to fusiform and often dark. There are a number of important genera in this order, such as, Melanospora, Gaumannomyces, Diaporthe and Endothia parasitica. These are serious plant pathogens of various important crop and tree species.

The discomycetes produce asci in apothecium the disk-shaped open ascocarp which bears asci on the exposed upper hymenial layer. Some discomycetes, such as the Tuberales, have a secondarily closed ascocarp which is considered analogous to an apothecium. The discomycetes includes many of the macroscopic ascomycetes, such as the morels, the truffles, and the cup fungi. The asci may be unitunicate or bitunicate. Unitunicate in all free living discomycetes and bitunicate in the lichenized discomycetes. The asci may be operculate or inoperculate and inoperculate asci may have a variety of different apices. The discomycetes with unitunicate, inoperculate asci which form their ascocarps above ground are placed in three orders: The Phacidiales, The Ostropales and the Helotiales. These can be distinguished by the presence of stromata and sclerotia and the shape of the ascospores. The discomycetes with unitunicate, operculate asci which form their ascocarps above ground are placed in two orders: the Cyttariales and The Pezizales. The asci are operculate or have a slit at the apex and the Cyttariales are restricted to the southern hemisphere as parasites of the Antarctic Beech (Nothofagus cunninghami). The Pezizales are a very large cosmopolitan group of discomycetes, which are common throughout the world. This order includes some 90+ genera and is home for the morels, the false morels and the cup fungi, as well as the large number of saprophytic apothecial fungi found in forests and elsewhere.

The hypogean discomycetes with unitunicate asci includes the famous truffles (Order Tuberales). In this order the apothecium is turned inside out and the hymenium is enclosed in an enclosed peridium. these are mycorrhizal fungi and have a well deserved culinary reputation. The ascospores are spiny and are formed without crozier formation, although the life cycle is typical of the ascomycetes.

The order Coronophorales and Coryneliales do not fit neatly into the existing scheme of Ascomycete taxonomy. These fungi have unitunicate asci in an ascostroma. Hence they fall between the Hymenoascomycetidae and the Loculoascomycetidae. There are not common and are unlikely to cause much concern.

These fungi are specialized parasites of insects and red algae. They produce asci in perithecia and lack a true mycelium. They have unitunicate asci and would ordinarily fall into the Hymenomycetidae except for the fact they do not have a well defined mycelium. The thallus of these fungi is small and confined entirely to the body of their host throughout much of the life cycle.

These fungi are Ascomycetes which produce their asci in a ascostroma and have bitunicate asci. The latter point provides a ready means by which these fungi can be differentiated from perithecial ascomycetes. There are five orders in this subclass and they may be distinguished by the type of ascostromata. The Myriangiales have a single globose ascus in each locule; the Dothidiales have a polyascal locule without pseudoparaphyses; the Pleosporales have and ostiolate perithecioid ascostroma; and the Hysteriales and Hemisphaerales have saucer shaped ascostromata which may only be partially covered with a ascostromatic wall.

There are a large number of common and important fungi in this subclass, including many plant pathogens and cosmopolitan saprophytes. The life cycle of these fungi can be complex, since many of them produce both an anamorph and spermagonia (for sexual fertilization. The anamorph may bear conidia naked on hyphae or in a structure called a pycnidium, which looks similar to a perithecium, but contains only conidia not ascospores. Spermatia may or may not be produced but when present are unicells and quite small. Conidia are usually single cells but may be 1-septate to muriform. Ascospores can be 0-1 septate or muriform. These fungi are very common and relatively easy to identify.