Introduction to Fungi APS > Education > Introductory > Introduction to the Pathogen Groups Introduction to Fungi Carris, L. M., C. R. Little, and C. M. Stiles. 2012. Introduction to Fungi. The Plant Health Instructor. DOI:10.1094/PHI-I-2012-0426-01 Lori M. Carris, Christopher R. Little, and Carol M. Stiles Washington State University, Kansas State University, and Georgia Military College What is a fungus? A fungus is a eukaryote that digests food externally and absorbs nutrients directly through its cell walls. Most fungi reproduce by spores and have a body (thallus) composed of microscopic tubular cells called hyphae. Fungi are heterotrophs and, like animals, obtain their carbon and energy from other organisms. Some fungi obtain their nutrients from a living host (plant or animal) and are called biotrophs ; others obtain their nutrients from dead plants or animals and are called saprotrophs (saprophytes, saprobes).

Some fungi infect a living host, but kill host cells in order to obtain their nutrients; these are called necrotrophs . Fungi were once considered to be primitive members of the plant kingdom, just slightly more advanced than bacteria. We now know that fungi are not primitive at all. In fact, recent taxonomic treatments such as the Tree of Life Project show that fungi and animals both belong to the group Opisthokonta (Fig. 1). Fungi may not be our next of kin, but they are more closely related to animals than they are to plants. We also recognize that organisms traditionally studied as "fungi" belong to three very different unrelated groups: the true fungi in Kingdom Fungi (Eumycota), the Oomycetes, and the slime molds (Fig. 1). Figure 1 Let's briefly consider the major groups in Kingdom Fungi—they will be described in greater detail later. Open most introductory mycology books and you'll see that there are four main groups (phyla) of true fungi—Ascomycota, Basidiomycota, Chytridiomycota and Zygomycota (e. g., Alexopoulos et al.

1996; Webster and Weber 2007). Recent studies have provided support for the recognition of additional phyla, such as Glomeromycota, a group of fungi once placed in Zygomycota that form an association with the roots of most plants (Fig. 2). A group of parasitic organisms called Microsporidia that live inside the cells of animals are also now considered to belong in the fungal kingdom (Fig. 2). Hibbett et al. (2007) published a comprehensive classification of the Kingdom Fungi, the result of collaboration among many fungal taxonomists. This classification is used in the Dictionary of the Fungi (Kirk et al. 2008) and other fungal references and databases. However, the classification system will undergo additional changes as scientists use new methods to study the fungi. For example, Jones et al. (2011) described the "cryptomycota," a potentially new phylum of organisms within the Kingdom Fungi. Figure 2 How old are fungi? Fungi are an ancient group—not as old as bacteria, which fossil evidence suggests may be 3.

5 billion years old—but the earliest fungal fossils are from the Ordovician, 460 to 455 million years old (Redecker et al. 2000). Based on fossil evidence, the earliest vascular land plants didn't appear until approximately 425 million years ago, and some scientists believe that fungi may have played an essential role in the colonization of land by these early plants (Redeker et al. 2000). Mushrooms exquisitely preserved in amber from the Late Cretaceous (94 million years ago) tell us that there were mushroom-forming fungi remarkably similar to those that exist today when dinosaurs were roaming the planet (Hibbett et al. 2003). However, the fungal fossil record is incomplete and provides only a minimum time estimate for when different groups of fungi evolved. Molecular data suggest that fungi are much older than indicated by the fossil record, and may have arisen more than one billion years ago (Parfrey et al. 2011). How many fungi are there?

No one knows for sure how many species of fungi there are on our planet at this point in time, but what is known is that at least 99,000 species of fungi have been described, and new species are described at the rate of approximately 1200 per year (Blackwell 2011; Kirk et al. 2008). A conservative estimate of the total number of fungal species thought to exist is 1. 5 million (Hawksworth 2001). To come up with this figure, Hawksworth estimated the known numbers of plant and fungal species from countries in which both plants and fungi have been well-studied—Great Britain and Ireland, in this case—and determined there were six fungal species for every native plant species. The total number of plant species worldwide is approximately 250,000, and if the ratio of fungi to plants in Great Britain is typical of what occurs elsewhere, there should be at least 1. 5 million species of fungi (6 × 250,000; Hawksworth 2001). If 1.

5 million fungal species is a reasonable estimate, the vast majority of all extant fungi are yet to be named. Assuming a relatively constant rate at which new species are described, it will take more than 1100 years to catalog and describe all remaining fungi. However, many of these fungi are likely to become extinct before they are ever discovered given current rates of habitat and host loss. For example, up to 2% of tropical forests are destroyed globally each year (Purvis and Hector 2000). These habitats are exceedingly rich in fungal species (Hawksworth and Rossman 1997). For example, 15-25% of fungi collected in short-term studies in the tropics are new species (Kirk et al. 2008). Callan and Carris (2004) estimated that an 110,000 ha neotropical forest, such as in Costa Rica, could contain over 81,000 different species of plant parasitic fungi—almost as many as all the known species of fungi!

Consider that this estimate was based only on plant parasitic fungi, and did not take into account other ecological groups of fungi such as saprotrophs. What do fungi do? Fungi are involved in a wide range of activities—some fungi are decomposers, parasites or pathogens of other organisms, and others are beneficial partners in symbiosis with animals, plants or algae. Let's take a brief look at these various ecological groups. Fungi associated with animals Fungi have the ability to grow on and in both invertebrate and vertebrate animals. Many fungi can attack insects and nematodes, for example, and may play an important role in keeping populations of these animals under control. Insect-attacking fungi, called "entomopathogens," include a wide range of fungi in phyla Ascomycota, Zygomycota and Chytridiomycota. Some of the best-known and most spectacular entomopathogens belong in the Ascomycota genus Ophiocordyceps and related genera. These fungi infect and consume insects such as caterpillars and ants, and then form conspicuous stromata that emerge from their victim's body in a most dramatic manner (Fig. 3).

These fungi can also alter the insect's behavior. "Zombie-ant" fungi from Brazil infect insect brains, directing the victim to climb up plants and bite into the plant tissue in a "death grip" (Evans et al. 2011; Hughes et al. 2011). Paradoxically, humans have been using one of these entomopathogens, Ophiocordyceps sinensis, for thousands of years to treat a wide range of ailments. This fungus is an important component of traditional Asian medicine (Fig. 3) and is commonly called "winter worm, summer grass. " Figure 3 Entomopathogens such as Beauveria bassiana are so effective in killing insects that they are used as biological control agents for insect pests. Colony collapse disorder of honeybees has been associated with co-infection by a virus and a microsporidian fungus, Nosema ceranae (Bromenshenk et al. 2010). One group of fungi called Entomophthorales ("insect killers") includes a number of highly specialized entomopathogens.

A common example is Entomophthora musae, which is often observed forming a ring of white spores discharged around the body of a parasitized fly on panes of glass. Some fungi are specialized parasites of nematodes, rotifers, and other microscopic animals in the soil (Barron 1977). A common nematode predator is Arthrobotrys oligospora, a fungus that has evolved sticky networks of hyphae for trapping nematodes. Once the nematode is immobilized, the fungus invades and consumes its body. Fortunately, there are relatively few fungal pathogens of vertebrates—only 200-300 species—but some of these fungi can have devastating impacts. Consider the well-publicized frog killer, Batrachochytrium dendrobatidis, a member of phylum Chytridiomycota (Berger et al. 1998; Longcore et al. 1999). This fungus wasn't even known to scientists until 1996, when it was discovered associated with frogs that had died from a mysterious skin disease at the Smithsonian National Zoological Park in Washington, D. C.

The fungus doesn't invade the frog's body, but it is lethal, possibly because it disrupts electrolyte balance leading to cardiac arrest (Voyles et al. 2009)—infected frogs appear to die of a heart attack! The frog chytrid is implicated in the widespread decline of frog populations around the world. Fortunately, this is the only chytrid known to parasitize a vertebrate animal and it appears to infect only amphibians. Another devastating parasite of animals is Geomyces destructans, a cold-loving fungus that causes 'white-nose syndrome' in bats (Blehert et al., 2009). This fungus colonizes the skin on the muzzles, ears and wing membranes of some types of bats, and infected bats exhibit unusual behavior. The bat fungus is associated with declines in bat populations in the northeastern U. S. and has many wildlife biologists concerned. As of 2011, white-nose syndrome had been confirmed in 16 states and four Canadian provinces. In humans, there are several different types of fungal infections, or "mycoses.

" The most common are caused by dermatophytes, fungi that colonize dead keratinized tissue including skin, finger-, and toenails. Dermatophytes cause superficial infections such as 'ringworm' that are unsightly and difficult to treat, but rarely serious. Some fungi are members of the resident microflora in healthy people, but become pathogenic in people with predisposing conditions. For example, Candida species cause annoying yeast infections in the mucosal tissues of many healthy people, but can also cause diseases collectively called candidiasis in babies and immunocompromised individuals. Another group of fungi are inhaled as spores and initiate infection through the lungs. These fungi include Coccidioides immitis (coccidioidomycosis, commonly known as valley fever), and Histoplasma capsulatum (histoplasmosis). Opportunistic fungal pathogens are normally not associated with humans and other animals, but can cause serious infections in weakened or healthy individuals when inhaled or implanted in wounds.

Aspergillus fumigatus, one of the most important of these opportunists, produces small, airborne spores that are frequently inhaled; in some individuals the fungus starts growing invasively, causing a disease known as aspergillosis, especially in immunocompromised individuals. A remarkable discovery was that Pneumocystis carinii, the organism causing pneumonia-like symptoms in immunocompromised patients, is a fungus and not a protozoan as had been thought for decades. Why was this pathogen classified as a protozoan? It does not respond to the common drugs used to treat fungal infections, but does respond to anti-protozoan drugs. This unusual fungus emerged as one of the leading causes of death in AIDS patients in the late twentieth century. Fungi and plants The association of fungi and plants is ancient and involves many different fungi. Fungi are an important group of plant pathogens—most plant diseases are caused by fungi—but fewer than 10% of all known fungi can colonize living plants (Knogge, 1996). Plant pathogenic fungi represent a relatively small subset of those fungi that are associated with plants.

Most fungi are decomposers, utilizing the remains of plants and other organisms as their food source. Other types of associations that will be discussed here include the role of fungi as decomposers, as beneficial symbionts, and as cryptic plant colonizers called endophytes . Most fungi are associated with plants as saprotrophs and decomposers. These fungi break down organic matter of all kinds, including wood and other types of plant material. Wood is composed primarily of cellulose, hemicellulose, and lignin. Lignin is a complex polymer that is highly resistant to degradation, and it encrusts the more readily degradable cellulose and hemicellulose. Fungi are among the few organisms that can effectively break down wood, and fall into two main types—brown and white rot fungi. Brown rot fungi selectively degrade the cellulose and hemicellulose in wood, leaving behind the more recalcitrant lignin. The decayed wood is brown in color and tends to form cubical cracks due to the brittle nature of the remaining lignin (Fig. 4).

Only ~10% of the wood decay fungi cause brown rot, and most of these fungi (80%) occur on conifer wood. Brown rot residues make up 'humus' in temperate forest soils and are important for mycorrhizal formation (see the following paragraph for information on mycorrhizal fungi), moisture retention, and for sequestering carbon. Brown rot residues are highly resistant to decomposition and can remain in the soil for up to 300 years. White rot fungi are more common than brown rot fungi; these fungi degrade cellulose, hemicellulose, and lignin at approximately equal rates. The decayed wood is pale in color, light in weight, and has a stringy texture (Fig. 5). White rot fungi are the only organisms that can completely degrade lignin. Lignin is one of the most abundant organic polymers, accounting for 30% of the organic carbon on the planet—only cellulose is more abundant (Boerjan et al. 2003). Figure 4 Figure 5 An important group of fungi associated with plants is mycorrhizal fungi.

Mycorrhiza means 'fungus root', and it refers to a mutually beneficial association (a type of symbiosis) between fungi and plant roots. There are seven major types of mycorrhizal associations, the most common of which is the arbuscular mycorrhizae, involving members of phylum Glomeromycota associated with roots of most major groups of plants. The vast majority (>80%) of vascular plants form mycorrhizae, as will be discussed later (under Glomeromycota). Another common type of association is ectomycorrhizae formed between forest trees and members of phyla Basidiomycota and Ascomycota. In this association, the fungus forms hyphae around host root cortical cells—the "Hartig net"— and a sheath of hyphae around the host roots called a "mantle. "Many of the ectomycorrhizal fungi are mushroom-forming species including highly prized edibles such as chanterelles (Cantharellus cibarius and related species), boletes (Boletus edulis and related species), and matsutake (Tricholoma magnivelare) (Fig. 6).

A valuable group of ectomycorrhizal fungi are truffles, members of phylum Ascomycota that form underground fruiting bodies. The French Périgord truffle, Tuber melanospora, and the Italian white truffle, Tuber magnatum, can bring phenomenal prices; for example, a 1. 5-kg Italian white truffle sold for $330,000 at an auction in 2007! Figure 6 Lichens are examples of a symbiotic association involving a fungus and green algae or less frequently Cyanobacteria. The lichen thallus is composed mostly of fungal hyphae, usually with the alga or cyanobacterium confined to discrete areas of the thallus. In lichens, reproductive structures of the fungus are often conspicuous, for example disc- or cup-like structures called apothecia (Fig. 7). The fungus obtains carbohydrates produced by photosynthesis from the algae or cyanobacteria, and in return provides its partner(s) with protection from desiccation and ultraviolet light. Lichens grow in a wide range of habitats on nearly every continent.

Think about an inhospitable place, and there's probably a lichen that grows there—on bare rocks, sidewalks, grave stones, the exoskeletons of some insects, and even on cars that remain for a long time in one place! Figure 7 Some fungi are hidden inside their plant hosts; these are endophytes , defined by their presence inside asymptomatic plants. All plants in natural ecosystems probably have some type of symbiotic association with endophytic fungi (Rodriguez et al. 2009). Endophytic fungi have been shown to confer stress tolerance to their host plant, for example, to disease, herbivory, drought, heat, salt and metals. The clavicipitaceous endophytes in the genus Neotyphodium (phylum Ascomycota) are among the best studied. These fungi produce alkaloid compounds that protect the grass host from insects that would otherwise feed on them; endophyte-infected turfgrass seed is sold commercially for seeding lawns and other types of grassy recreational areas. Unfortunately, livestock such as sheep, cattle, llamas and horses also are negatively affected by toxins produced by endophytes when they eat infected grass.

'Ryegrass staggers' occurs when animals graze on perennial ryegrass (Lolium perenne) that is colonized by Neotyphodium lolii. Afflicted animals develop symptoms including tremors and jerky or uncoordinated movements. Let's now consider the role of fungi as plant pathogens. There are thousands of species of plant pathogenic fungi that collectively are responsible for 70% of all known plant diseases. Plant pathogenic fungi are parasites, but not all plant parasitic fungi are pathogens. What is the difference between a parasite and a pathogen? Plant parasitic fungi obtain nutrients from a living plant host, but the plant host doesn't necessarily exhibit any symptoms. In this sense, endophytic fungi discussed in the preceding paragraph are plant parasites because they live in intimate association with plants and depend on them for nutrition. Plant pathogenic fungi are parasites and cause disease characterized by symptoms. Biotrophic fungal pathogens obtain nutrients from living host tissues, often via specialized cells called haustoria that form inside host cells (Fig. 8). Necrotrophic pathogens obtain nutrients from dead host tissue, which they kill through the production of toxins or enzymes.

Most biotrophic fungi have fairly narrow host ranges—they are specialized on a limited number of plant hosts. Necrotrophic fungi can be either generalists, growing on a wide range of host species, or specialized on a restricted range of hosts. Some plant pathogenic fungi change the way that their hosts grow, either by affecting the level of growth regulators produced by the plant, or by producing growth regulators themselves. Examples of changes in plant growth caused by plant pathogenic fungi include cankers , galls , witches' broom , leaf curl and stunting . Figure 8 We can further divide plant pathogenic fungi by the stage of the plant host that is attacked, for example, seeds, seedlings, or adult plants, and by what part of the plant is affected—roots, leaves, shoots, stems, woody tissues, fruits or flowers. A group of fungi including species of Fusarium, Rhizoctonia and Sclerotium cause seed rot and infect plants at the seedling stage. These pathogens can attack a wide range of plants. Often, seedling pathogens cause damping-off symptoms because they occur in wet soils. Many of the same fungi that kill seedlings can also infect the roots of mature plants and cause root and crown rot diseases.

Infection often occurs through wounds, and results in lesions or death of part or all of the root system and crown. Some common root rots of trees are caused by members of phylum Basidiomycota in the genera Armillaria and Heterobasidion. Armillaria spp. produce shoe-string-like bundles of hyphae called rhizomorphs that allow the fungus to grow from one tree to another. Species of Heterobasidion survive as saprotrophs in dead tree stumps and roots, but can also infect living hosts through root contact. These fungi cause decay in the roots and crown; infected trees become weakened and die, or may blow over in high winds. Wood rot fungi, most of which are also members of Basidiomycota, infect trees through wounds, branch stubs and roots, and decay the inner heartwood of living trees. Extensive decay weakens the tree, and reduces the quality of wood in trees harvested for timber (see the discussion of "white rot" and "brown rot" fungi above).

Vascular wilt pathogens kill their host by infecting through the roots or through wounds and growing into the xylem, where they produce small spores that get carried upward until they are trapped at the perforated ends of the xylem vessels. The spores germinate and grow through the pores. The fungus is transported throughout the plant in this manner. The first symptom of vascular wilt is a loss of turgidity in the plant leaves, often on one side of the plant or a single branch. If the stems of infected plants are cut open, vascular discoloration is evident. Among the important vascular wilt fungi are Fusarium oxysporum , Verticillium albo-atrum and V. dahliae . One of the most famous vascular wilts is Panama disease of bananas, caused by Fusarium oxysporum forma specialis (f. sp. ) cubense. This fungus nearly wiped out banana production in Latin America in the early twentieth century. Most bananas that were being grown for export were a single cultivar, 'Gros Michel', which turned out to be highly susceptible to Panama disease. There is no effective method for controlling Panama disease and it rapidly spread throughout banana plantations around the world.

The banana industry was saved by the discovery of the cultivar 'Cavendish' that is resistant to the strain of Panama disease that killed 'Gros Michel'. 'Cavendish' is now the banana that Americans and Europeans consume, but a new strain of the Panama disease pathogen began killing 'Cavendish' in Malaysia in 1985 and scientists are concerned that this strain will begin to spread (Ploetz, 2005). Leaf spot pathogens infect through natural plant openings such as stomates or by penetrating directly through the host cuticle and epidermal cell wall. In order to penetrate directly, fungi produce hydrolytic enzymes—cutinases, cellulases, pectinases and proteases—for breaking down the host tissue. Alternatively, some fungi form specialized structures called appressoria (sing. appressorium) at the end of germ tubes . Turgor pressure builds up in the appressorium, and in combination with an infection peg , mechanical force is exerted to breach the host cell walls. Once inside the plant leaf, the fungus must obtain nutrients from the cells, and this is often accomplished by killing host cells ( necrotrophs ).

Death of host cells is evident as an area of dead cells called a lesion (Fig. 9). Figure 9 Many leaf-spotting fungi produce toxins that kill host cells and this often produces a lesion surrounded by a yellow halo (Fig. 10). If enough of the leaf surface is killed, or if the infected leaves drop prematurely, the plant's ability to produce photosynthates is severely impaired. Figure 10 Returning to bananas, another devastating disease of this host is black leaf streak, or black Sigatoka , caused by Mycosphaerella fijiensis. Unlike the root-infecting pathogen that causes Panama disease, the black Sigatoka pathogen can be controlled by applications of a protective fungicide to banana leaves. Effective control of black Sigatoka requires multiple fungicide applications and control of this disease accounts for up to 25% of the total production cost for bananas (Ploetz, 2001). American chestnut trees were once a prominent hardwood tree in the eastern U. S., but have largely been eliminated by the chestnut blight pathogen, Cryphonectria parasitica, an example of a canker-causing fungal pathogen.

Cankers develop when the pathogen kills the phloem and vascular cambium in a woody host. If the canker encircles the trunk or branch of a tree, that plant part will die. The canker-causing fungus can often be identified based on the fruiting bodies that form in the canker. In contrast to cankers, galls result from abnormal growth of a plant, usually due to an increase in cell size and cell division. Although galls are often associated with insect pests, some fungal pathogens induce galls; two common examples are the black knot pathogen , Apiosporina morbosa on Prunus spp. (Fig. 11), and species of Gymnosporangium, which induce the formation of galls on their coniferous hosts (Fig. 12). Figure 11 Figure 12 Gymnosporangium is a type of rust fungus. Rust fungi are biotrophic pathogens—they infect, grow, and sporulate in living plant tissue. Even though biotrophs require living host tissue for their growth and reproduction, they can be devastating pathogens by reducing the photosynthetic surface and increasing water loss in the host plant.

Rust fungi attack a wide range of plants, and often require two, unrelated hosts in order to complete their life cycles. Rust fungi are so-named because of the abundant orange spores that are formed on plants that are infected by these fungi; infected plants often look as though they are rusting. One historically important rust fungus is black stem rust of wheat, a disease that was well known to the ancient Romans. Black stem rust , caused by Puccinia graminis f. sp. tritici, infects wheat and barberry (Berberis species). Since the barberry host is required for the pathogen to complete its life cycle, early control measures in the United States and Canada were aimed at eliminating this host, not the economically important host, wheat. We now know that this method of eradication was of limited success because rust spores can be carried long distances—for example, from northern Mexico to the U. S. -Canada border—by wind currents via the " Puccinia pathway " (Nagarajan and Singh 1990). Morphological characteristics of fungi:mycelium and hyphae Now let's take a closer look at fungi and the types of structures that they form.

A key characteristic of fungi that has contributed to their successful exploitation of diverse ecological niches is the formation of a filamentous thallus called the mycelium . A mycelium is composed of branching, microscopic tubular cells called hyphae (Fig. 13) that grow through and across substrates or food sources, secreting enzymes that break down complex substrates into simple compounds that can be absorbed back through the cell wall. The fungal cell wall in the Kingdom Fungi is composed of chitin and glucans (in Ascomycota, Basidiomycota and Chytridiomycota) as well as chitosan and other components (in Zygomycota) (Kirk et al., 2008) Figure 13 Hyphae can have cross walls called septa , or lack cross walls (nonseptate; aseptate; coenocytic). The type of hyphae— septate or aseptate —is characteristic of specific groups of fungi. In fungi that form septate hyphae, there are perforations at the septa, called septal pores, which allow the movement of cytoplasm and organelles from one compartment to the next.

The type and complexity of the septal pore is characteristic of specific groups of fungi. Hyphae grow from a germinating spore or other type of propagule, and these are described in more detail in the section "Fungal Reproduction. "Hyphae elongate almost exclusively at the tips, growing outwards from the point of establishment. As a result of apical growth, hyphae are relatively uniform in diameter, and mycelium that grows in an unimpeded manner forms a circular colony on solid substrates that support fungal growth; agar, a gelatinous material derived from seaweed, amended with different types of nutrients is commonly used to grow fungi in culture (Fig. 14). Figure 14 Some fungi grow exclusively or mostly as yeasts , defined as single-celled fungi that reproduce by budding or fission. In contrast to apical growth that is characteristic of hyphae, yeasts exhibit wall growth over the entire cell surface, often resulting in a nearly spherical cell (Fig. 15). There are also fungi that can switch between mycelial growth and yeast-like growth, dependent upon the environmental conditions.

The ability to grow in different forms is called dimorphism, and is exhibited by some members of phyla Ascomycota, Basidiomycota and Zygomycota. Figure 15 Inside the fungal cell Most of the organelles present in fungal cells are similar to those of other eukaryotes. Fungal nuclei are usually small (< 2 µm diameter), and can compress and/or stretch to move through septal pores and into developing spores. Fungi have been found to possess between 6 and 21 chromosomes coding for 6,000 to nearly 18,000 genes. Genome sizes range from 8. 5 megabase pairs (Mb) to just over 400 Mb in filamentous fungi (Zolan 1995; Spanu et al. 2010; Duplessis et al. 2011), making fungal genomes among the smallest of eukaryotic organisms on average—approximately 1% the size of mammalian genomes and only 1. 3 times the size of the largest known bacterial genome (Stover et al. 2000).

Many fungi (Ascomycota) have a life cycle that is predominantly haploid, while others (Basidiomycota) have a long dikaryotic phase. Fungal reproduction Fungi frequently reproduce by the formation of spores. A spore is a survival or dispersal unit, consisting of one or a few cells, that is capable of germinating to produce a new hypha. Unlike plant seeds, fungal spores lack an embryo, but contain food reserves needed for germination. Many fungi produce more than one type of spore as part of their life cycles. Fungal spores may be formed via an asexual process involving only mitosis (mitospores), or via a sexual process involving meiosis (meiospores). The manner in which meiospores are formed reflects the evolutionary history and thus the classification for the major groups (phyla) of fungi. Many fungi produce spores inside or upon a fruiting body. Many people are familiar with the mushroom, a type of fruiting body produced by some Basidiomycota. You may recognize other fungal fruiting bodies such as puffballs, or shelf fungi.

These are examples of large, conspicuous fruiting bodies, but there is an even greater diversity of microscopic fruiting bodies produced by various fungi. What all fruiting bodies have in common is that they produce spores and provide a mechanism for dispersing those spores. Fruiting bodies will be discussed in more detail within the fungal groups. Teleomorph and anamorph Many fungi are able to reproduce by both sexual and asexual processes. Sexual and asexual reproduction may require different sets of conditions (e. g., nutrients, temperature, light, moisture). In some fungi, two sexually compatible strains must conjugate (mate) in order for sexual reproduction to occur. The terms ' anamorph ' and ' teleomorph ' are used to convey the asexual and sexual reproduction morphological types, respectively, in a particular fungus. The concept of anamorph and teleomorph is a confusing one for many students, as we are not accustomed to thinking about organisms with such reproductive flexibility. For a more thorough discussion of anamorph and teleomorph, refer to Alexopoulos et al. (1996), Kendrick (2000), or Webster and Weber (2007).

Meiospores Examples of meiospores—spores that are the products of meiosis—include ascospores (see Ascomycota) and basidiospores (see Basidiomycota). Ascospores are formed inside a sac-like structure called an ascus (Fig. 16). An ascus starts out as a sac of cytoplasm and nuclei, and by a process called "free cell formation" (Kirk et al. 2008) a cell wall forms de novo around each nucleus and surrounding cytoplasm to form ascospores (typically eight per ascus). Ascospores vary in size, shape, color, septation, and ornamentation among taxa. Basidiospores are formed on a basidium (Fig. 17) and are typically one-celled with one or two haploid nuclei. Basidiospores vary in size, color and ornamentation depending upon the taxonomic group. More information on dispersal of ascospores and basidiospores can be found below.

Figure 16 Patterns of Subsistence: Foraging aquatic (concentrating on fish and/or marine mammal hunting usually from boats) Equestrian and aquatic foragers have specialized subsistence patterns. They focus their food getting efforts on a limited range of species. This allows them to become very efficient at harvesting them. As a result, they usually have abundant food supplies and larger communities than pedestrian foragers. However, there is a downside to a specialized subsistence pattern. It can be a precarious way of life in a changing environment. An epidemic or other catastrophic event can rapidly reduce the numbers of animals that are the focus of the hunt. In contrast, the diversified subsistence approach of most pedestrian foragers has the advantage of relative economic security if there are fluctuations in the weather, water supply, or periodic die-offs of the food sources. The disadvantage of a diversified subsistence pattern is that the total amount of food calories acquired is often less and the amount of time required to secure them is greater. Pedestrian Foraging The general foraging way of life described above mostly fits the pedestrian pattern. This has been the commonest form of hunting and gathering. At one time, pedestrian foragers lived on all continents except Antarctica.

Prior to the invention of agriculture around 10,000 years ago, almost all people lived in this way. They occupied the rich fertile valleys, hills, and grasslands. By the early 20th century, however, this subsistence pattern survived only in marginal areas that farmers and herders were not able to or interested in exploiting. The most well known pedestrian foragers were the Australian Aborigines, the San speakers of Southwest Africa, the pygmies of West Central Africa, most California Indians, and the Paiutes of the Great Basin in Western North America. Other pedestrian foragers lived in marginal areas of Asia, Africa, and the Americas. Surviving pedestrian foragers have been heavily impacted by large-scale agricultural societies. Few if any of these foragers are still following their old subsistence pattern exclusively. Pedestrian foragers during the late 19th century The pedestrian hunting and gathering way of life was highly mobile. Most of these societies moved their camps several times a year and had temporary dwellings. The number of people living in a camp also often varied throughout the year depending on the local food supply. Material possessions were generally few and light in weight so that they could be transported easily.

Subsistence tools included such things as simple digging sticks, baskets, spears, and bows and arrows that could be easily replaced when needed. This settlement flexibility is an efficient way of responding to changing environmental opportunities. Equestrian foragers Equestrian foragers have evolved in only two areas of the world--the Great Plains of North America and the sparse grasslands of Southern Argentina. In both cases, pedestrian foragers acquired horses from Spanish settlers in the early 17th century. Over several generations, horse breeding and riding skills were honed. This resulted in a revolutionary change in these Native American societies. The horse became the principle mode of transportation and dramatically increased hunting success in the pursuit of big animals. These societies became larger, more mobile, and were now able to travel over larger areas throughout the year. Horses allowed them to effectively follow the seasonal migrations of large herbivores over hundreds of miles. In North America the prey of choice for the Sioux , Crow , and others was the bison or buffalo. In South America, the guanaco was hunted by the Tehuelche . Guanacos are camelids related to the domesticated llamas, alpacas, and vicu�as.

North American bison South American guanaco The equestrian foraging societies became male dominated and warlike. This was particularly true on the Great Plains of North America after the widespread acquisition of guns in the 19th century. Raiding other societies to steal horses and defeat enemy warriors was the principle path to respect and success for men. Their societies acquired hierarchical , almost military-like forms of political organization with chiefs at the top. In contrast, pedestrian foragers are comparatively peaceful and have very democratic political systems that usually lack fulltime leaders. North American equestrian foragers of the Great Plains in the late 19th century The combination of aggressiveness and high mobility made it very difficult for the U.S. and Argentine armies to subdue the equestrian foragers during the last half of the 19th century. In both countries, it required a number of army regiments with rapid fire guns and light artillery. In the case of the North American Plains Indians, decimation of the bison herds at that time by non-Indian hunters was the most important factor in subduing them because it destroyed their traditional subsistence base.

All equestrian foragers were forced to move from their homelands to relatively small isolated reservations. Today, their survivors are mostly living in poverty with little hope for their future. Aquatic Foraging Aquatic foragers of the Americas Aquatic foragers focus their subsistence activities on fish, mollusks, crustaceans, and/or marine mammals. The most well known aquatic foragers lived on the Northwest Coast of North America from the Klamath River of California to the Aleutian Islands of Alaska. These societies specialized mostly in salmon fishing along the rivers and hunting seals and whales off the coast. Other aquatic foragers occupied the coastal regions of the American subarctic, the Channel Islands of Southern California, the southern tip of South America, the coastal areas of Eastern Siberia, and a few other regions of the world. The exploitation of fish and other marine resources is usually a far more reliable and productive form of foraging than the diversified hunting and gathering of most foragers who live away from the coasts and major rivers. Among the Northwest Coast Indians, settlements were usually permanent.

On Vancouver Island in Western Canada, the Kwakiutl and some of their neighbors built split plank wood homes and large community halls for ritual performances, dances, and feasts. They had the leisure time to invest considerable effort in creating elaborate clothing, masks, and large wood carvings (e.g., totem poles and boxes). They also had relatively complex social stratification for foraging peoples. Politically powerful men had titles and privileges reminiscent of Medieval European lords. Aquatic foragers of the Northwest Coast of North America in the late 19th century Today, many aquatic foraging societies continue exploiting marine resources but as part of the international economy. They fish commercially with well equipped modern boats and sell most of their catch to buy things that they do not produce themselves. Apparently, no aquatic foragers still solely practice their traditional subsistence patterns. Misconceptions about Foraging The most common image of the hunting and gathering way of life until recently was that of an uncertain existence in a harsh environment and a short miserable life. This view was more of an ethnocentric projection than an accurate description of the lives of foragers. It was based on our tendency to pigeonhole human societies in terms of an outdated evolutionary framework.

It was assumed that if the technology is simple then life must be grim. It was a vestige of the Victorian belief that our "civilized" lives are not only more efficient but also more comfortable than those of "primitive" peoples. Ethnographic research carried out over the last half century has largely demolished this myth that foragers in the past had to struggle for existence. In fact, they usually had a food supply that was adequate and reliable. Most of them only had to expend minimal labor to provide for their basic needs. What is particularly surprising to people in industrialized nations is that foragers often lived well into old age with few signs of anxiety and insecurity. Outside of the subarctic , most foragers subsisted mainly on nuts, vegetables, fruits, and even protein rich insects. Hunting large animals was relatively unreliable. Among the San speakers of the Kalahari Desert in Namibia and Botswana, 60-80% of the diet came from non-meat sources, especially nuts and roots. Since women provided most of the vegetable foods, they were responsible for the majority of the calories that were consumed. Men mostly provided the most desirable food, which was meat. The San way of life was remarkably efficient.

While they had few days that were free of subsistence activities, the ratio of labor expenditure to production was low. The ethnographer Richard Lee discovered that adult San spent only about 2� days of 6 hours each week hunting and gathering. Young people did not fully join the work force until around 20 years old. The 60% of the society that were healthy adults, provided the food for everyone by working only 15 hours a week. Foragers have rightly been referred to by Richard Lee as the most leisured people. In the United States today, less than 1% of the population produces all of the food for the entire society. Given this remarkable efficiency, it is worth asking why the rest of us work 40-50 hours a week, often with considerable psychological stress. Foragers often have long lives compared to people in the industrial societies of the 19th and early 20th centuries. Richard Lee discovered that 10% of the San were over 60 years of age and the elderly, blind, senile, and crippled were supported by their families.

In the United States today, 10-15% of the people are over 60 and life expectancy is in the low to mid 70's. However, living to an advanced age in the Western World is a very recent trend primarily resulting from modern medicine. In 1900, the average life expectancy in the United States was only 50. At the time of the American Revolution, it was 35. By comparison, the San lived relatively long, healthy lives. Note: The most well known San speakers of southern Africa are the !Kung or Ju/'hoansi . They maintained their traditional foraging way of life into the 1960's. Since then, they have been increasingly forced into smaller territories that are not adequate to support their population by foraging. Today, most of them are poor subsistence farmers in Namibia. In 2002, the government of Botswana drove their surviving San foragers out of the Central Kalahari Desert because they continued to hunt in what was declared a game reserve. In 2004, a delegation of these San went to Washington D.C., the World Bank, and the United Nations to plead for help.

They were hoping that world public opinion would force Botswana to allow them to return to what has been their homeland probably for the last 20,000 years. Few have paid heed to their appeal. Soil Management Nitrogen gas N2 Although 78% of our atmosphere is nitrogen gas, this form of nitrogen must be transformed to usable forms before it is available for plant uptake. Ammonia NH3 Ammonia is a gas. Ammonium can escape from the surface of the soil under certain conditions and is harmful to plants in high quantities. Ammonium is the basic building block of commercial nitrogen fertilizers. Ammonium NH4+ Soil particles attract and retain ammonium on cation exchange complexes. This form may be directly taken up by plants. Nitrate NO3- Nitrate is the second form of nitrogen which is available for plant uptake. In most soils, nitrate is highly mobile. However, in the highly weathered soils of Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes less mobile. Nitrite NO2- Nitrite is an intermediate product in the conversion of ammonium to nitrate (nitrification).

It is usually present in low quantities, but is toxic to plants. Organic Nitrogen Various compounds Organic nitrogen must be converted to ammonium before it is used by plants. This conversion occurs with time and is known as mineralization. Though complex, the nitrogen cycle: Helps us to understand the complex relationships that exist between the many forms of nitrogen Provides us with insight pertaining to the availability of ammonium and nitrate, which are the only nitrogen forms usable by plants To understand the many ways in which N may be lost from the soil In this section, we will discuss eight major transformations of nitrogen in the soil: nitrogen fixation, mineralization, immobilization, nitrification, denitrification, volatilization, and leaching. Nitrogen fixation Although atmospheric nitrogen gas(N2) makes up approximately 78% of the air, it cannot be directly used by plants. Instead, atmospheric N2 only becomes available to plants through three unique processes. The final product of each of these processes is ammonium, which is then available for plant uptake.

The three processes which convert atmospheric nitrogen to ammonium biological nitrogen fixation Nitrogen is deposited onto the earth’s surface by: Rain In the form of ammonium, nitrate, nitrite Finely divided organic N swept along the earth’s surface Lightning Responsible for approximately 10-20 % of soil nitrate (114, Fertilizers) Industrial wastes In Hawaii, as compared to the major industrial regions of the Mainland, industrial wastes do not significantly contribute to atmospheric N. Table 7. This table presents estimates of the different sources of atmospherically fixed nitrogen that was deposited onto the earth in the latter half of the twentieth century. Biological sources account for around 20% of total nitrogen deposition. Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major commercial and environmental issues, IFDC Muscle Shoals Alabama. Nitrogen Mineralization in Soils When absorbed by plants, ammonium and nitrate are incorporated into plant cells as organic, or living, forms of nitrogen. When plants die, microorganisms break down, or decompose, dead plant cells.

During the decomposition about plant matter, organic nitrogen is once again converted to inorganic ammonium and released into the soil. The process that converts organic N to ammonium is called mineralization and plays a significant role in the management of nitrogen. To calculate the amount of nitrogen which can potentially be mineralized from your organic fertilizer source, click on the link below: Conditions affecting N mineralization The amount of ammonium that is released to the soil through mineralization depends on several factors: Quantity of Organic Nitrogen: The amount of organic nitrogen originally present in the organic matter determines the amount of N that can ultimately be mineralized. Temperature: The optimal range for mineralization to occur is between 77-95 degrees Fahrenheit. Oxygen: Microorganisms need oxygen and since microorganisms mediate mineralization, sufficient oxygen must be available in the soil. Moisture content: Ideally, water should fill 15 – 70 % of pore space for maximum mineralization. This roughly corresponds to field capacity. Ratio of carbon to nitrogen (C:N): The C:N ratio is a term used to describe the relative amount of total carbon in comparison the amount of total nitrogen present in the soil and/or organic matter.

This ratio is very important in determining the rate of mineralization that should occur for a given type of organic matter. Since the microorganisms living in the soil need both carbon and nitrogen, net mineralization occurs when C:N ratio is less than 20:1. This means for every two parts of carbon, there should be 1 part nitrogen for net mineralization. If you are applying organic amendments to your soil, it is important to become familiar with the C:N ratio to ensure N availability. After mineralization of N occurs, ammonium can be: Taken up by the plants Consumed by other organisms Volatilized Immobilization Immobilization is process that converts inorganic nitrogen to organic nitrogen. It is the reverse reaction of mineralization. Immobilization occurs when decomposing organic matter contains low amounts of nitrogen. Thus, immobilization occurs if the source of organic matter has a high C:N ratio. Microorganisms, who also need nitrogen to live, scavenge the soil for nitrogen when plant residues contain inadequate amounts of nitrogen. As inorganic ammonium and nitrate are incorporated into the cells of living microorganisms, the total N levels in the soil are reduced. Immobilization can ultimately result in nitrogen deficiencies.

When nitrogen is immobilized in the soil, there may be little nitrogen available for crop growth. As a result, plants can suffer from nitrogen deficiency and develop a yellow coloration. This is the reason why organic materials with a high C:N ratios, such as grass clippings and grain stover, are usually composted before they are incorporated into the soil or planting. This allows time for soil microorganisms to decompose the materials and begin to release nitrogen and other nutrients back into the soil Mineralization or Immobilization? The processes of mineralization and immobilization are constantly occurring simultaneously. As organic matter decomposes, inorganic nitrogen will be released into the soil. As both plants and microorganisms grow, they utilize the nitrogen in the soil. Once plants and microorganisms die, they decompose and release inorganic nitrogen to the soil through mineralization. Even though mineralization and immobilization are both occurring, we can determine which process, mineralization or immobilization, predominates. When mineralization occurs at a greater rate, we say that there is net mineralization. Likewise, when immobilization occurs to a greater extent, there is net immobilization. What determines whether there is net mineralization or net immobilization?

The answer is the C:N ratio of the decomposable organic matter. The C:N ratio is a characteristic of all organic matter, which includes: Crop residues Soil organic matter (including humus) Soil microorganisms (remember microorganism have both carbon and nitrogen) Rule of thumb When the C:N ratio of decomposing organic residues is between 20:1 and 30:1, mineralization and immobilization occur at fairly equal rates. Net mineralization occurs at C:N ratios less than 20:1. Net immobilization occurs at C:N ratios greater than above 30:1. Most well decomposed organic matter in soils have a C:N ration near 10:1 Management of organic residues If your program involves the addition of organic residues, it is important to know its C:N ratio. This knowledge allows you to predict whether net mineralization or net immobilization will occur. If the residue has a wide C:N ratio range, it may be necessary to apply additional amounts of nitrogen to your soil or choose a residue with a narrower range. Nitrification In most aerobic soils under optimal soil conditions, ammonium is rapidly converted to nitrate by soil bacteria through a process known as nitrification.

Nitrification involves two steps: First, ammonium is converted to nitrite Then, nitrite is converted to nitrate. As you can see from the outline of steps above, the intermediate product of nitrification is nitrite. If conditions are unfavorable to undergo the second step of nitrification, nitrite can leach into the ground water and pose as a health risk. The process of nitrification produces hydrogen ions. When large quantities of ammonium-containing fertilizers are applied to soil over time, this process can acidify the soil. See figure below for a simplified presentation of the nitrification process. Figure 10. Basic process that causes soil acidity by ammonium fertilizers. Source: Singleton, P. Nutrient Management Concepts: pH & Nutrient Formulation, University of Hawaii Cooperative Extension Service, Hilo Jul 25 2006. Factors affecting nitrification There are many factors that affect nitrification. Since nitrification is mediated by microorganisms, environmental factors that affect biological life will also influence nitrification.

In general, the optimal conditions for most plant growth are also the optimal conditions for nitrification: Presence of ammonium in the soil: In order for nitrification to occur, there must be a source of ammonium in the soil. Sources include mineralized ammonium or additions of ammonium-containing synthetic fertilizers Presence of microorganisms: Microorganisms that carry out nitrification must be present in the soil. Soil pH: The optimal pH for nitrification is 8.5, but it may occur over a fairly wide pH range. However, acidity (less than 5.5) has a detrimental effect on the nitrifying bacteria, thus reducing nitrification. Soil moisture: Nitrification is optimal at the field capacity of the soil. Nitrification is reduced at moisture levels greater and below field capacity. Field capacity is the amount of water that remains in the soil after free drainage in a saturated soil ceases. Field capacity is also the optimal soil moisture for most plant growth. Soil aeration: Nitrification requires oxygen. Any management factor that improves soil aeration, such as adding organic matter, will help optimize nitrification. Soil temperature: Nitrifying bacteria are sensitive to temperature.

The optimal temperature range for nitrification is between 77 and 95 degrees Fahrenheit. However, nitrification can occur between 41 and 95 degrees Fahrenheit. Environmental Considerations Nitrate is generally a very mobile in most soils. Excessive amounts of nitrate that are not taken up by plants is subject to leaching. Nitrate leaching can have an adverse effect on the environment. Nitrate, which moves through the soil profile during periods of rain, pollutes ground water reserves. Surface runoff of nitrate is a source of eutrophication, or algal blooms, in lakes and estuaries. To learn more about the possible negative effects associated with nitrate/nitrite and other elemental leaching on our water systems, click on the links below: Losses of ammonium due to leaching are minimal In contrast, nitrate is not retained by cation exchange capacity. Nitrate is highly immobile Losses of nitrate due to leaching can potentially be high Not only is nitrate leaching an economic loss to the farmer, it is also an environmental concern.

The conditions that lead to nitrate leaching follow: High rainfall intensity and distribution Highly irrigated fields Coarsely textured soils Nitrate Leaching in Maui soils In Hawaii, there are soils that can have a high anion exchange capacity under acidic conditions, which reduces nitrate leaching. This phenomenon primarily occurs in the acidic subsurface soil layers that have the greatest anion exchange capacity. As a result, researchers have found that subsurface soil layers in Hawaii can retain nitrate and may prevent it from leaching into the ground water. Acidification: Management of Nitrogen Fertilizers The fertilizer formulation can alter the pH of soil. Generally, fertilizers with high proportions of total nitrogen and are derived from ammonium sources (such as urea, ammonium sulfate, ammonium phosphate or ammonium nitrate) can acidify soils with repeated applications. Most fertilizers provide the “Lime Equivalent” on the bag’s label. The lime equivalent is the amount of limestone (calcium carbonate) it takes to neutralize the acidifying effects of using one ton of a particular fertilizer. In contrast, other fertilizers can increase soil pH. These fertilizers are usually low in ammonium, but high in nitrate.

Additionally, these fertilizers sometimes contain calcium from calcium nitrate. The lime equivalent is also given for these fertilizers, but it indicates the equivalent liming effect rather than the lime needed to offset acidity. The lime equivalent is only an estimate. The actual acidifying effect of the fertilizer is influenced, to some degree, by soil conditions that affect the transformation of the ammonium to nitrate and also by how much ammonium the plant assimilates before this transformation occurs. If growers know the history of their fertilizer applications over time, they can use the lime equivalent to predict when lime additions are likely needed. See the table below for some fertilizer formulations and note the relationship between the percentage of nitrogen in the fertilizer that is derived from ammonium and the lime equivalent needed to counter this acidifying effect. Table 8. Fertilizer composition can change the pH of soil Fertilizer

Question: Graminoids are more commonly known as what plant form?
Answer: Grasses