Ascomycota, Ascomycetes & Sac Fungi

All members of the Ascomycota produce an ascus that contains ascospores. The class includes the largest group and most successful of all fungi, with over 44,000 known species. The group has existed for many millions of years and there is some evidence that they took their origin from Zygomycotous forms. The Ascomycota are highly important in the break down of organic matter in the soil, as plant pathogens and for the production of antibiotics and other industrial substances. Many species are purely saprophytic; some are obligate parasites and others facultatively saprophytic or parasitic. Most species have a well-developed septate mycelium, and septations contain a septal pore or perforate septum. Cells primarily have only a single nucleus per cell but there are some with more than one nucleus per cell.

Those species known as yeasts are distinguished by not forming mycelia and individual cells multiply by fission or budding processes. The yeasts are not primitive organisms but special forms that evolved from a retrogressive process in evolution.

A large proportion of the Ascomycetes have one or more means of vegetative reproduction. This imperfect stage produces conidia but never any zygospores. The perfect or sexual stage produces asci, which is the cardinal feature of the entire class. The production of asci terminates the relatively complicated sexual process. Asci occur in a spore sac, which may take various shapes.

The number of ascospores is variable, but 8 per ascus are typical in 98 percent of species. Typically, ascospores are forcibly discharged (sporangiospores are not). The ascus is the seat of meiosis, which is not true in sporangia. In 99 percent of cases the ascus is also the site of nuclear fusion.

In contrast to sporangiospore formation by progressive cleavage, ascospores are delimited in a process known as "Free-cell Formation". This results in a leaving of cytoplasm in the ascus, which is called epiplasm. Ascospores are almost always uninucleate (haploid with a single nucleus per spore), but the genus Neurospora is an exception with 2-4 nuclei per spore. The epiplasm largely disappears when the ascospores mature, which suggests that it serves to nurture the spores.

As the ascus develops there is a large 2N nucleus that divides to form 8 nuclei. These nuclei develop beaks. Cytoplasmic radiations form walls around each nucleus thereby delimiting ascospores.

Fruiting bodies (ascocarps) whose walls consist of closely interwoven hyphae may form various shapes. In contract there are no fruiting bodies in the Zygomycota.

But not all of the Ascomycota produce fruiting bodies. There are none in the small sub-class Hemiascomycetes while all members of the much larger subclass Euascomycetes produce fruiting bodies (= ascocarps).

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Please refer to the following plates for characteristic structures in the Ascomycota:

Ascomycota

Plate 102 = Conidiophore types: Phyllosticta, Dendrophoma, Monopodium, Verticillium, Aspergillus, Penicillium &

Isariopsis.

Plate 103 = Asexual fruiting bodies: Septoria, Marssonia, Epicoccum & Arthrobotryum.

Plate 104 = Sexual reproduction & ascus development in Ascomycota: Pyronema omphalodes.

Plate 105 = Types of asci: Globose, Ovate, Septate, Clavate, Cylindrical.

Plate 106 = Variety of ascospores (20 types).

Plate 107 = Four ways that Ascomycota bear asci.

Plate 108 = A section thru' the stroma revealing embedded ascocarps.

Plate 109 = Several types of openings (pores) in asci.

Plate 110 = Four stages in ascospore germination: Gelasinospora autosteira

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Two discussed orders in the Hemiascomycetes are: Endomycetales & Taphrinales Three families in the order Endomycetales are: Ascoidaceae, Endomycetaceae & Spermophtoraceae.

The Ascoidaceae is well represented by the Genus Dipodascus. Here the mycelium consists of multinucleate cells and thee is no imperfect stage. Asci are formed as follows:

Two gametangia arise on adjacent cells and nuclei migrate to gametangia where crosswalls are formed.

The female portion usually enlarges and gametangia fuse. The fusion cell enlarges into a sac, which is a transformed fused gametangium with many nuclei.

Fusion nuclei fuse but others disintegrate. Meiosis takes place to produce many nuclei. Ascospores are delimited from each of the many nuclei.

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The family Endomycetaceae is characterized by a septate mycelium and the absence of a fruiting body, all the asci being scattered on the mycelium. All species form asci as a result of direct fusion of gametangia; but there is only a single nucleus per gametangium. Gametangia fuse and the nuclei fuse to produce one diploid nucleus. Meiosis occurs and the number of spores varies between 4 and 8. There is no degeneration of the nuclei and the duration of the 2N nucleus is short except in Saccharomyces cerevisiae.

Various species show distinct behaviors. In the Genus Eremascus there is only a perfect stage. But Endomyces has an imperfect stage also. In this species arthrospores are formed by disarticulation of hyphal cells. The process begins at the apex of hypha that have stopped growing and continues posteriorly. Arthrospores germinate directly or they may form crosswalls and multiply by fission.

In Endomycopsis blastospores are formed in a budding process. The blastospore gives rise to hypha that may produce secondary blastospores and are thus difficult to distinguish from yeasts.

Schizosaccharomyces and Saccharomyces are yeasts that grow primarily in single-celled form. They multiply by fission. In Schizosaccharomyces asci formation occurs when two cells come together (gametangia), fusion occurs and eight ascospores are delimited in one ascus.

Saccharomyces is a budding yeast, and S. cerevisiae is one of the most important species that is used for rising bread. There are individual cells and multiplication is accomplished by the formation of buds. A single nucleus divides with one nucleus going to the bud. In the sexual stage every vegetative cell under the right conditions can form ascospores within, and typically four are delimited. This process appears to be parthenogenetic but it is not.

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Details on Characteristics of Yeasts

Yeasts are not regarded as primitive and they multiply either by fission or budding. The many different types are distinguished as being sporogenous (produce asci), asporagenous (do not produce asci as in the Deuteromycota), haplobiontic, budding, apiculate, bipolar, film-forming, oxidative, diplobiontic, illegitimate diploid and multipolar.

A typical haplobiontic yeast is Schizosaccharomyces octosporus. A septum separates the cell, and a cell may function as a gametangium, which fuse to form an ascus. Fusion of nuclei occurs in the developing ascus, and meiosis follows to produce 8 haploid nuclei.

An example of a budding (multipolar) yeast is Saccharomyces cerevisiae. It is intermediate between haplobiontic and diplobiontic and budding is the process of multiplication. It is a multipolar budding type where a bud may appear on any side of the cell. Diploid cells result from the budding process after fusion of "A" and "B" asci. Four ascospores are delimited in a cell (ascus).

Ascospores multiply by a budding process, but are haploid and much smaller than the diploid cells. Under ordinary circumstances two of the haploid cells come together and fuse. Half of the ascospores in an ascus are of the "A" Type and half of the "B" Type. When two or one types fuse the result is an "illegitimate diploid." The budding of fused ascospores produces diploid cells:

S. cerevisiae is of great importance because under conditions of low aeration it can produce alcohol without the intervention of Oxygen, even though the yeast cells will not grow.

C₆H₁₂O₆ -----------► CO₂ + C₂H₅OH

Various strains are used in the fermentation of beer, wine, etc. The most tolerant strains will not tolerate more than 13-15 percent alcohol. Taxes from the sale of alcohol can surpass 6 billion dollars per year in North America, and the baking industry uses yeast for leavening and carbon dioxide. An estimated 300 tons of yeast cake is used daily in North America

If Oxygen is present, the reaction will be:

C₆H₁₂O₆ O₂ -----------► CO₂+ H₂O

This reaction is used in the Baking and Yeast Production Industries. The medium is acidified and aerated to stimulate rapid multiplication. In about 12 hours the yeast will multiply to five times its original volume.

Yeasts cannot use starch so they must have sugar. Sugar sources are usually molasses or grain mash (acid hydrolysis or enzyme or another microorganism breaks down the starch into sugar).

Yeasts are rich in protein and there is a possibility of using sawdust, etc. as a food source for yeasts, which are substituted for high protein diets. They are extensively used for livestock. Yeasts also are high in Vitamin B complexes and Vitamin D (Ergosterol).

Apiculate yeasts are exemplified by the Genus Hanseniaspora, which is found on ripening fruits (apple, grapes). They may also occur on dust, leaves in the soil, etc. These yeasts multiply in the juice and carry on a kind of fermentation that is known as "Spontaneous Generation Fermentation." However, the alcohol content produced is low, in the range of 5-6 percent. In the commercial production of wine, the juice is first sterilized and then inoculated with a desired strain of yeast. The insect Genus Drosophila feeds mainly on the yeasts although the initial attraction may involve the alcohol. The apiculate yeasts bud only at the poles and are thus "bipolar."

The Genus Pichia typifies film-forming yeasts. They have elongated cells in a chain known as a pseudomycelium. They occur on the surface of liquids such as pickle brine and are poor in the ability to carry on alcoholic fermentation. In some cases they may actually be exclusively oxidative.

Characteristics of the family Spermophthoraceae are shown in the genera Spermophthora, Eremothecium and Ashbya. There is a mycelium and some species are the cause of "Cotton Stigmatomycosis," which damages the cotton boll. The Genus Nematospora is single-celled and causes "Yeast Spot of Pea." All these genera produce Rhiboflavin in large amounts. Eremothecium exceeds all others in production, but following the placing of a patent, several strains of Ashbya have been found that almost equal it in production.

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Please refer to the following plates for characteristic structures and Life Cycles in the Endomycetales:

Ascomycota: Hemiascomycetes: Endomycetales

Plate 50 = Ascomycota: Hemiascomycetes, Ascoidaceae: Dipodascus sp.

Plate 111 = Life Cycle -- Dipodascus uninucleatus.

Plate 112 = Life Cycle -- Eremascus fertilis.

Plate 113 = A yeast cell showing various structures.

Plate 114 = Chain of yeast cells (pseudomycelium) produced by budding.

Plate 115 = Five types of yeast ascospores.

Plate 116 = Life Cycles -- Schizosaccharomyces octosporus, Saccharomycodes ludwigii, Saccharomyces cerevisiae

Plate 185 = Life Cycle -- Endomycetaceae: Endomyces sp. & Endomycopsis sp.

Plate 186 = Life Cycle -- Endomycetaceae: Schizosaccharomyces octosporus

Plate 187 = Life Cycle -- Endomycetaceae: Saccharomyces cerevisiae; & Structures of Hanseniaspora sp. & Pichia sp.

Plate 189 = Example Structures: Endomycetales: Endomyces sp., Schizosaccharomyces octosporus & Endomycopsis sp.

Plate 190 = Example Structures: Endomycetales: Saccharomyces sp. & Ashbya gossypii.

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The order Taphrinales is represented here by two families: Taphrinaceae and Protomycetaceae. Asci arise from a proascus, not from two gametangia as in the Endomycetales. No fruiting bodies are present.

The family Taphrinaceae is represented by a single genus, Taphrina, which causes various plant diseases. Taphrina deformans causes "Peach Leaf Curl", T. communis and T. pruni cause "Plum Pockets." Also various species of Taphrina occur on many wild hosts of which about one-third are pathogens on ferns.

A naked layer, hymenium, bears asci on the surface. They may be subcuticular, breaking the host cuticle on emergence or they may reside in the epidermal cell interspaces. Each arises from a proascus. The mycelium is intercellular (= "High Type" parasite) and there are no haustoria. All cells have two nuclei (= dikaryotic mycelium), and this is the only place in all the Ascomycota where this occurs. In the Basidiomycota a dikaryotic mycelium is common; thus, the similarity. Ascospore budding may begin inside the ascus, which resembles a yeast on agar (Saccharomyces).

Taphrina deformans exemplifies the family. Hyphae accumulate underneath the host cuticle and are called proasci.

Nuclei fuse in the proascus to give a diploid condition.

The diploid cell pushes up through the host cuticle and forms a crosswall. The bottom cell is now the "foot cell" and the top one the ascus.

Meiosis occurs in the ascus and subsequent mitotic divisions produce an ascus with 8 haploid nuclei.

Each nucleus forms an ascospore and these may be forcibly discharged from the ascus.

The ascospores bud until reaching an optimum substrate, where they stop budding and send out a germ tube between the epidermal cells. Hyphae may occur throughout the leaf tissue.

Taphrina deformans is a homothallic organism and there is no imperfect stage. There are at least two heterothallic species of Taphrina in Europe. In this case "A" and "B" ascospores are produced. As "A" and "B" ascospores fuse and "A & B" nucleus is formed.

Other differences among the species of Taphrina may include the presence of foot cells, and the size of the ascus varies (e.g., T. coerulescens are very large). Budding may also occur in the ascus.

Taphrina species may affect the host by causing hypertrophy with or without hyperplasia (e.g., ferns show little hypertrophy). There may be a failure of tissue differentiation as in the case of the disease known as "Plum Pockets."

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The family Protomycetaceae is typified by the Genus Protomyces. These are plant invaders that incite gall production on ragweed. Thick-walled cells arise on the mycelium (= proasci).

Intervening hyphae break down leaving only the proasci. These are located down in the plant tissue. Ascospores bud on being released from the ascus. However, there is some doubt as to whether or not these are proasci. Cytological behavior in the "ascus" is quite unlike that of other members of the Ascomycota.

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Please refer to the following plates for characteristic structures and Life Cycles in the Taphrinales:

Ascomycota: Hemiascomycetes: Taphrinales

Plate 117 = Life Cycle-1 -- Taphrina deformans.

Plate 188 = Life Cycle-2 -- Taphrinaceae: Taphrina deformans "Peach Leaf Curl."

Plate 191 = Plant Host Symptoms -- Taphrinales: Taphrina spp.

Plate 192 = Example Structures: Ascomycota: Taphrinales

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In the Sub-Class Euascomycetes, Series: Plectomycetes three orders discussed are: Plectascales, Myriangiales and Erysiphales. In the Series: Pyrenomycetes seven orders discussed are Hypocreales, Sphaeriales, Pseudosphaeriales, Dothideales, Hemisphaeriales, Laboulbeniales and Hysteriales. In the Series: Discomycetes four orders are Helotiales, Lecanorales, Pezizales and Tuberales.

The Euascomycetes show different developmental patterns, but are generally similar throughout. Asci are produced in aggregates and formed in connection with a fruiting body, the ascocarp. A sexual process involving a female ascogonium and a male antheridium or spermatium initiates asci formation. Gametangia fuse followed by a transfer of nuclear material to the ascogonium. The ascogonium gives rise to ascogenous hyphae in which nuclei are paired (= dikaryon). Conjugate nuclear division is carried out in the ascogenous hyphae and these hyphae re dikaryotic. A ascogenous hypha branch bends over in a crozier. Three kinds of cells are formed: (1) ultimate, (2) penultimate and (3) antepenultimate. The penultimate cell becomes the ascus.

The ultimate cell may fuse with the antepenultimate cell to form a dikaryon and another crozier. This may then form another ascus.

Sterile hyphae are induced to grow and branch underneath the sexual structure. These sterile structures either form the main part of the ascocarp or the system of ascogenous hyphae may simply give rise to a structure of sterile hyphae below the sexual structures, which is still an ascocarp, however.

Three types of ascocarps are found in the three respective groups of the Euascomycetes, or series (Fig 277).

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The order Plectascales is characterized by the families Gymnoascaceae, Aspergillaceae and Elaphomycetaceae

In the Family Gymnoascaceae, the Genus Byssochlamys includes what are known as "Barnyard fungi." These are karotinophilic fungi that grow on feathers, hair, hoofs, nails, maize, etc. They are similar to Dermophytes in the Deuteromycota, and they can produce a mild skin disease in humans that is not as serious as in the Deuteromyccota.

They have a Racquet mycelium where the hyphae are racquet-shaped.

Two kinds of vegetative reproduction are by conidia or chlamydospores. The conidia occur in long chains at the ends of branched or unbranched conidiophores. The Deuteromycota have a genus Paescilomyces that produces conidia in the same manner.

During sexual reproduction antheridia and ascogonia fuse. Ascogenous hyphae branch out from the ascogonium and the ascus forms as a result of crozier formation from the ascogenous hyphae. Many asci are produced, which are typically naked, each with 8 ascospores. There is no real ascocarp although there may be a weak suggestion of one.

In the Genus Gymnoascus a sexual process initiates the building the building up of a weak wall, and spines occur on the tips of swelled hyphae. These are considered to be a "weft of specialized hyphae."

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Principal Characteristics of Euascomycetes

Asci are clustered and borne in a fructification (ascocarp). The asci are formed in connection with an ascogenous hyphal system. Ascogenous hyphae arise as outgrowths of an ascogonium. Nuclei pass from an antheridium to an ascogonium but do not fuse. Instead, conjugate nuclear division increases each nucleus respective to set up a dikaryotic phase in the ascogenous hyphal system. The dikaryophase is ended when the asci form, which involves a crozier.

Classification of the groups (series) is based on the form of the ascocarp, the form of the asci and the distribution of asci in the ascocarp. The Plectomycetes have a closed fructification (cleistothecium), scattered asci and globular asci. The Pyrenomycetes have a flask-shaped fructification (perithecium), asci occur in a definite layer (hymenium) and they are elongated. The Discomycetes have an open fructification (apothecium), asci occur in a definite layer and they are also elongated.

The order Plectascales is the most characteristic order in the group. Most ascocarps are very small although the family Elaphomycetaceae is an exception (some doubt about its classification).

The family Gymnoascaceae has poorly developed ascocarps (thin wefts of hyphae), but all other typical features of Euascomycetes are present. A scattered group of more or less globular asci each containing from 6-8 spores is formed. The genus Byssochlamys has relatively undeveloped ascocarps. The family also has karytinophilis forms. It is related to a group of Fungi Imperfecti (Dermatophytes) ecause of the peculiarity of the racquet mycelium.

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The family Aspergillaceae has very small ascocarps with well-defined peridial walls and a well-developed cleistothecium. The asci are globular, scattered in the ascocarp with 8 ascospores per ascus, and thus are similar to the Gymnoascaceae.

The Genus Aspergillus has a characteristic pattern in the conidial stage. A conidiophore arises anywhere on the mycelium and forms a bulbous structure at its apex (= vesicle). Sterigmata are formed, and conidia are produced in chains at the ends of the sterigmata. Sometimes there are secondary sterigmata also. Similar forms in the Fungi Imperfecti bear the name Aspergillus, but they never possess the added names of Eurotium, Emericella, and Sartorya.

Members of the genus possess the ability to grow where there is a high osmotic concentration (high sugar, salt, etc.). Species that best represent the genus are Aspergillus ametelodami, with both sexual and asexual stages; and Aspergillus niger, A. fumigatus and A. flavus that do not have a perfect stage.

Examples of species with economic importance are Aspergillus alliaceus (Bulb Rot of Onion), A. flavus (starch-digesting enzyme), A. fumigatus (causes Aspergillosis), A. niger (produces citric acid.)

The Genus Penicillium is a companion genus of Aspergillus, both occurring in the soil. However, Penicillium exceeds Aspergillus in abundance. The ascocarpic stage is similar to Aspergillus, but the conidial stage differs. Here conidiophores do not form a vesicle, instead a penicillus of where there are three types (Fig 284). The structure bearing the chains of conidia are called sterigmata.

The abundance of Penicillium spp. In nature is extensive. They are primarily saprophytic forms and plan an important role in the breakdown of organic matter. They also may spoil many foodstuffs. There is no perfect stage. Penicillium chrysogenum secretes penicillin. It acts on gram-positive organisms, which are very common in nature. Although other species also produce antibiotics, P. chrysogenum is widely used for antibiotics production due to its secretions being low mammalian toxicity. Other species are P. expansum (Apple Rot), P. digitatum and P. italicum (Citrus Fruit Rot), P. roqueforti (Blue Cheeses).

Thielavia basicola occurs in the soil and was once erroneously thought to be pathogenic on tobacco. The disease "Thielaviopsis" was actually caused by one of the Fungi Imperfecti. Ascocarps are similar to Aspergillus and Penicillium, but they are dark and football shaped. The ascospores spill out into the cavity of the cleistothecium and the ascus is evanescent.

Monascus spp. Cause the disease "Purple silage mold." This name was derived from a misjudgment as it was once believed that the scattered ascospores occurred in one large ascus.

The family Elaphomycetaceae has ascocarps far different from any previous form. They are the size of a hickory nut and hypogenous. They are associated with tree roots and believed to be a mycorrhizal fungus by occurring on tree roots. The Genus Elaphomyces may actually be a highly modified Discomycete.

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Please refer to the following plates for characteristic structures and Life Cycles in the Plectascales:

Ascomycota: Euascomycetes: Plectascales

Plate 118 = Conidiophores & hypha of Aspergillus.

Plate 119 = Cleistothecia cross sections: Aspergillus sp.

Plate 120 = Conidiophores: Penicillium thomii, P. lanoso-coeruleum & P. wortmanni.

Plate 121 = Life Cycle -- Penicillium vermiculatum.

Plate 193 = Life Cycle -- Plectascales: Gymnoascaceae: Byssochlamys sp.

Plate 194 = Life Cycle -- Plectascales: Aspergillaceae: Penicillium vermiculatum

Plate 195 = Life Cycle -- Plectascales: Aspergillaceae: Aspergillus sp.

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The order Myriangiales has short and wide asci, which are scattered. There is no typical cleistothecium. Species in the genus Myriangium are parasitic on scale insects. They build a dense mass of compacted hyphae (stroma), which flatten out to form capsules. The whole structure is stromatic. The asci are borne scattered in the capsules. The ascospores are muriform where multicellular spores appear as bricks in a wall.

The Genus Elsinoe also has a stromatic form. Ascospores are scattered through an undifferentiated stromatic layer. The genus includes pathogens of cultivated plants and causes "Spot Anthracnose" especially in tropical regions. The imperfect stages is referred to as "Sphaceloma. A stroma appears as a layer on or under the host epidermis. Ascospores are not muriform but are simply divided by a few crosswalls.

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Please refer to the following plates for characteristic structures and Life Cycles in the Myriangiales:

Ascomycota: Euascomycetes: Myriangiales

Plate 122 = Life Cycle -- Elsinoe veneta.

Plate 123 = Structures of Myriangium bambusae.

Plate 196 = Life Cycle -- Myriangiales: Elsinoe sp.

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Please refer to the following plates for characteristic structures and Life Cycles in the Plectascales & Myriangiales:

Plate 197 = Plectascales & Myriangiales Example Structures: Aspergillus amstelodami, Penicillium Byssochlamys nivea, carpenteles, P. frequentans, Thielavia basicola.

Plate 198 = Plectascales & Myriangiales Example Structures: Aspergillus fumigatus, Byssochlamys nivea, Elaphomyces sp., Elsinoe wisconsinensis, Monascus sp., Myriangium sp.

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The order Erysiphales, which is almost equivalent to the group formerly known as "Perisporiales", contains several families, the members of which are usually found growing on leaves, stems and fruits of seed plants. The mycelium is largely, if not entirely, confined to the surface of the suscept organ involved and may be either white or dark colored. There are both obligate parasites, as in the Erysiphaceae, or forms merely growing as saprophytes on honeydew that is deposited by insects. The ascocarps are mostly cleistothecia.

The family Erysiphaceae includes the Powdery Mildews. All are common parasites. The mycelium is primarily superficial on leaves, stems and even fruit, but may be internal also. Haustoria anchor the mycelium to epithelial cells, which forms a whitish powdery mass with profusion of conidia. The mycelium is composed of short, uninucleate cells, and the haustoria are bulbous usually with a single nucleus. They may be elongated or they may even occur uncommonly in the subepidermal layer. The Meliolaceae and Capnodiaceae have a dark superficial mycelium, and these forms are common in the tropics.

The Erysiphaceae incite a great variety of diseases of cultivated and wild plants. The grape industry in France was threatened before with the surge of the Downy Mildews (Zygomycota). Dusting with sulfur onto the vines successfully controlled the infection that was the first successful use of a fungicide. But in humid areas the powdery mildews may be reduced by moisture on the leaf surfaces.

The Genus Phyllactinia is only one of the six common genera in North America. It has both a superficial and an internal mycelium. There is no form with only an internal mycelium. There is an Oidium Imperfect Stage, where the fungus bears unbranched, upright conidiophores with catenulate conidia.

The conidiophore typically has only one nucleus. The nucleus keeps dividing in the conidiophore and migrates to the terminus to form a new conidium with an indefinite number of conidia. In some species the conidia are seldom found in long chains (e.g., Clover Mildew). Spores are carried by wind to a new host where they may germinate without water being present. This explains why powdery mildews thrive under dry conditions. Spraying with water may even control these fungi.

The Ascocarpic Stage has a diagrammatic cleistothecia with a well-defined wall of interwoven hyphae. The color is usually dark. Appendages grow out from the outside layer.

A delicate inner layer is present and there is usually only one ascus inside the ascocarp with less than 8 ascospores. Although there were originally also 8 nuclei the others disintegrate. Alternatively there may be several asci in a hymenium.

Ascocarps are the overwintering structures and they are produced in abundance toward the end of the growing season of plants. The cleistohecium may explode in the spring followed by an explosion of the asci, which aides in scattering the spores.

Taxonomy is based on appendages and the number of asci per ascocarp as is shown in Fig 292:

The Genus Spaerotheca has a Perfect Stage with distinctive characteristics. Many species are heterothallic; and an ascogonium and antheridium are each produced on separate hyphae. Each possesses a single nucleus.

The male nucleus passes into the ascogonium through a pore to form the dikaryophase. The ascocarpic wall begins to form by sterile hyphae moving out from either adjacent cells, the ascogonium or even the antheridium. This process begins before fusion of nuclei (karyogamy).

As the cell grows it digests the hyphae that are in the interior of the fructification. These hyphae are the sertoli layer. There are eight nuclei in the ascospore, but not all may develop into ascospores.

When more than one ascus is delimited, the ascogonium has previously sent out a system of ascogenous hyphae.

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The families Meliolaceae and Capnodiaceae include the "Sooty Molds." They are not closely related to the Erysiphaceae and are uncommon in North America, being rather more tropical in distribution. Information about them is scanty. The mycelium is largely superficial but there may be a partial internal mycelium that produces haustoria. The mycelium is always dark in color. There are no obligate parasites and some species even grow as saprophytes on insect honeydew. When on fruit they can disfigure the surface. Meliolaceae is the most important family of sooty molds and is one of the most common maladies in tropical areas.

The term "Sooty Molds" may also be applied to other fungal groups as well. In North-central North America the most prevalent "sooty mold" is a Pyrenomycete Apiosporina.

Key To The Common Genera of Erysiphaceae

(Plate 200)

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Please refer to the following plates for characteristic structures and Life Cycles in the Erysiphales

Ascomycota: Euascomycetes: Erysiphales

Plate 124 = Erysiphaceae: Host cells & mycelia relationships.

Plate 125 = Life Cycle -- Sphaerotheca castagnei.

Plate 126 = Erysiphaceae: Taxonomic characteristics.

Plate 199 = Life Cycle -- Erysiphales: Erysiphaceae: Spaerotheca sp.

Plate 200 = Key to The Common Genera of Erysiphaceae

Plate 201 = Example Structures & Plant Host Symptoms: Plectomycetes: Erysiphales

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In the Sub-Class Euascomycetes, the Series Pyrenomycetes is a large group with few stable, ordinal characteristics. Their classification has been in a continuous state of flux with various new alignments being proposed. The changes which have been suggested from time to time originate from the continuing attempt to make classification of the fungi as natural as possible.They produce perithecia or perithecia-like hyphae. A perithecium possesses a peridium while the "perithecium-like" forms are without such a peridium. The ostiole is an opening in the perithecium.

Among the gametes the female is the ascogonium and the male the antheridium or spermogonium. The spermogonium produces small, uninucleate cells at its base, which are termed spermatia. These will generally not germinate but act rather as non-motile male gametes.

Inside the perithecium there may occur two kinds of elongated asci.

Most asci are initiated by crozier formation, and they are always formed at the base of the perithecium as a hymenium. Sterile hyphae will often project between the asci on the hymenium. They may vary in shape and length. Others may occur just under the ostiole and project into the opening. The basal structures are termed paraphyses and the apical ones periphyses. Pseudoparaphyses are sterile hyphae that are joined to the perithecial wall basally and apically.

Two stages that occur generally in the Pyrenomycetes are a perfect one that allows for a recombination of characteristics, and an imperfect one, which does not allow for character combination.

Two orders in the Pyrenomycetes, Huypoceales and Sphaeriales, constitute the "core" of the Pyrenomycetes. Previously the genera of both of these orders have been classified on the basis of the color of their ascocarps, and so they will be treated here:

Hypocreales = bright-colored ascocarps; soft, waxy, rarely brittle. Sphaeriales = black or brown ascocarps; leathery and brittle.

In the order Hypocreales the Genus Neocosmospora is a typical representative. This is an old order that some authorities have contended should be merged with the Sphaeriales. Generally the members of these two groups are basically much alike. They have been segregated primarily on the basis of what some feel are unimportant characters: principally differences in the color and texture of the ascocarps. The Sphaerailes as traditionally defined have ascocarps that are brown to black and carbonaceous to leathery in texture. Morphologically similar forms with softer, bright-colored fructifications have been assigned to the Hypocreales. In both orders the ascocarp is an ostiolate perithecium and in either the perithecia may be borne singly on the mycelium or they may be produced in dense clusters, seated upon or imbedded within a stroma. In the latter case the color and texture of the stroma are usually similar to that of the perithecial walls. Whatever the groupings are there is in the large assemblage of forms producing dark and bright-colored true perithecia the core group of the Pyrenomycetes. The perithecium is typically flask-shaped with a well-developed peridium. These are found grouped or distributed over the mycelium. Elongated asci are usually cylindrical, which at maturity contain 8 ascospores arranged linearly inside the asci. At maturity of the ascospores the neck of the perithecium becomes dark. There is a Cephalosporium Imperfect Stage. Here there are simple conidiophores and the conidia are suspended in a drop of slime at the apex of the conidiophore:

Perithecia may be borne singly on the mycelium or in a stroma.

The family Nectriaceae is distinguished by producing their perithecia superficially either on a well-developed stroma or without a stroma.

Species of the Genus Nectria are saprophytes on twigs and branches although some are also parasitic. The mycelium is distributed over the host bark and small patches of stroma form under the bark surface.

The bark ruptures, exposing orange, cushion-shaped stroma. The Fusarium Imperfect Stage forms on top of the stroma.

Perithecia develop on the stroma at the end of summer, with the youngest perithecia appearing at the apex. They completely cover the stroma.

Pseudoparaphyses occur in the perithecium. Asci contain 8, two-celled ascospores with a shape similar to Hypomyces.

The family Hypocreaceae has their perithecia either completely or partially buried in a well-developed stroma. But the existence of intermediate forms makes separation on this basis difficult.

The Genus Hypomyces contain species that are parasites of gill fungi (Agaricales). The host gills fail to develop and there is a distortion of tissue. A stroma completely covers the host with a thin, orange-colored layer. Perithecia are embedded in the stroma with their necks projecting. The ascospores are two-celled. The conidia that are produced by the Fusarium Imperfect Stage show much variation in size, shape, number of nuclei, etc.

The family Clavicipitaceae is characterized by asci that are elongated at maturity and they have a width that approaches that of the bacteria. A fascile, or bundle, of filamentous ascospores is produced. In many species crosswalls may come in so that each ascospore may contain up to 150 cells. Paraphyses disintegrate before the ascospores mature.

The Genus Cordyceps has over 200 species most of them parasitic on insects. Coremia are usually found in the Imperfect Stage (Isaria Stage). Erect, clavate or stalked perithecial stromata also grow out from the mummified insect. When parasitic on Elaphomyces an easy to locate the hypogenal ascocarps is to view the brightly colored stromata that appear on the ground surface.

Perithecia occur on the periphery of the stroma.

The larvae of the June beetle are attacked and mummification ensues.

Perithecia develop on the periphery of an apical club sent up from the host, but not all stroma are club shaped here.

In the Isaria Imperfect Stage a pyramid of hyphae with condiiophores branch off.

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Claviceps purpurea causes Ergot (or Spur) Disease of rye, wheat and wild grasses. A mycelium becomes established in the ovary of the host and may actually replace the ovary. This mycelium will form a highly convoluted stroma that will be slightly projected out of the ovary tissue. Conidia are borne on this "mycelium" and they are mixed with a sticky, sweet, nectar-like secretion, which is attractive to insect vectors.

The mycelial mat hardens into a stroma, which takes a spur-like form or a "pseudoparenchymatous sclerotium." This is slightly longer than the grain pieces that it assimilates.

These drop to the ground in autumn and form the overwintering stage. In the spring these sclerotia will germinate and send out tubes that form a stroma at their apices, which in turn bears the perithecia. The ascospores may be one-or two-celled.

With regards to the spur produced as the overwinter stage, a toxic alkaloid is produced within, which causes a disease of humans and animals called Ergotism. It causes constriction of the blood vessels and results in a dropping-off of the extremities. Spurs also produce the drug called Ergotin, which has been used in medicine to contract the uterine muscles after childbirth.

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The Genus Epichloe causes "Choke Disease." Some species are also parasitic on grasses. A mycelium forms a stroma, which extends around the stem. The perithecia are embedded in the stroma with their necks extruded. In the Tubercularia Imperfect Stage conidia are densely packed on a stroma.

The Genus Gibberella has pink ascospores and a Fusarium Imperfect Stage. The mycelium may or may not produce a stroma. Dark blue perithecia are produced singly and scattered over the stroma or mycelium. Gibberella zeae and closely related species are important pathogens on cereals, inciting various rots. Gibberella fugihuroi was found to cause rice plants to grow up to twice the size of normal plants. This took place without a distortion of tissues. It is believed that this enlargement is due to both hyperplasia and hypertrophy of the host tissue, but occurring evenly and without distortion. The research was developed in Japan but halted during World War II. A mixture of compounds is involved, and it is very effective at 1 ppm. on some host species. Certain genetic deficiency may be masked by applying the chemicals, as in the case of dwarf peas returning to normal size and bush beans to pole beans.

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Please refer to the following plates for characteristic structures and Life Cycles in the Hypocreales:

Ascomycota: Euascomycetes: Hypocreales

Plate 135 = Life Cycle -- Nectria cinnabarina.

Plate 136 = Life Cycle -- Claviceps purpurea.

Plate 137 = Structures of Claviceps purpurea.

Plate 202 = Life Cycle -- Pyrenomycetes: Hypocreales: Nectria sinnabarina

Plate 203 = Life Cycle -- Pyrenomycetes: Hypocreales: Claviceps purpurea

Plate 204 = Diagnostic Characters: Pyrenomycetes: Hypocreales: Claviceps, Cordyceps, Epichloe, Gibberella, Hypomyces,

Nectria, Neocosmospora

Plate 207 = Example Structures -- Pyrenomycetes: Hypocreales: Hypomyces, Nectria, Neocosmospora

Plate 208 = Example Structures -- Pyrenomycetes: Hypocreales: Claviceps purpurea, Gibberella zeae, Cordyceps

agariciformis, C. ophloglossoides & Epichloe typhina

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Bibliography

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True Fungi (Eumycophyta1

Ascomycota (Ascomycetes, Ascomycotina) -- Sac fungi

True Fungi (Eumycophyta¹