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Introduction to

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APSnet > 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

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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

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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

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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 .

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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!

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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

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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

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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

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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

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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.

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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 µ m 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.

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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 Figure 17

Mitospores

Examples of mitospores are conidia ( sing. conidium), sporangiospores , and

zoospores, formed by members of the phyla Ascomycota, Zygomycota, and

Chytridiomycota, respectively. Another type of asexual propagule produced by fungi in

several different phyla is the chlamydospore .

Conidia

Conidia are formed from a modified hypha or a differentiated conidiogenous cell of the

fungus. Conidiogenous cells can be formed singly on hyphae, on the surface of

aggregated hyphal structures, or within different types of fruiting bodies. Fruiting

bodies inside which conidia are formed are pycnidia and acervuli. Sporodochia and

synnemata are examples of fruiting bodies on which conidia are formed. Conidia are

produced primarily by Ascomycota, although some Basidiomycota are capable of

producing them as well.

Sporangiospores

Sporangiospores are asexual propagules formed inside a globose or cylindrical

sporangium by a process involving cleavage of the cytoplasm. Sporangiospores are

thin-walled, one-celled, hyaline or pale-colored, and are usually globose or ellipsoid in

shape. One to 50,000 sporangiospores may be formed in a single sporangium. When

mature, sporangiospores are released by breakdown of the sporangial wall, or the

entire sporangium may be dispersed as a unit. Sporangiospores are produced by fungi

in phyla Chytridiomycota and Zygomycota, as well fungal-like Oomycetes (see section

"Fungal-like Organisms Studied by Plant Pathologists and Mycologists").

Zoospores

A zoospore is a microscopic, motile propagule, approx. 2 to 14 µm long and 2 to 6 µ m

in diameter that lacks a cell wall and is characterized by having one or more flagella .

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in diameter that lacks a cell wall and is characterized by having one or more flagella .

Flagella are ~ 0. 25 µm in diameter and up to 50 µm long. Zoospores are produced by

one group of true Fungi (Chytridiomycota), and by fungal-like organisms in Kingdom

Straminipila and some slime molds (see section "Fungal-like Organisms Studied by

Plant Pathologists and Mycologists"). Two types of flagella are known—the whiplash

flagellum, which is directed backward, and the tinsel flagellum, which is directed

forward. The tinsel flagellum is only present in members of Kingdom Straminipila and

does not occur in true fungi. The length of time zoospores are able to swim is

determined by their endogenous energy reserves—zoospores cannot obtain food from

external sources—and environmental conditions. Zoospores may exhibit chemotaxis—

movement in response to a chemical gradient, e. g., root exudates. At the end of its

motile phase, the zoospore undergoes a process called encystment in which it either

sheds or retracts the flagella and produces a cell wall. The encysted zoospore, called

a cyst, may germinate directly by the formation of a germ tube, or indirectly by the

emergence of another zoospore.

Zoospores are formed inside a sac-like structure called a zoosporangium by a process

involving mitosis and cytoplasmic cleavage—similar to the formation of

sporangiospores in sporangia. Depending upon the taxonomic group, zoospores

emerge from the zoosporangium through breakdown of the zoosporangial wall,

through a preformed opening in the wall covered with a cap called an operculum that

flips back, or by a gelatinous plug that dissolves.

Chlamydospores

Chamydospores are survival propagules formed from an existing hyphal cell or a

conidium that develops a thickened wall and cytoplasm packed with lipid reserves. The

thickened cell walls may be pigmented or hyaline, and chlamydospores develop singly

or in clusters, depending upon the fungus. Chlamydospores are passively dispersed,

in most instances when the mycelium breaks down. Chlamydospores are formed by

many different groups of fungi and are often found in aging cultures.

Sclerotia

Sclerotia (sing. sclerotium) are compact aggregations of hyphae differentiated into an

outer, pigmented rind, and an inner mass of hyaline cells called a medulla. Sclerotia

contain food reserves, and are a type of survival propagule produced by a number of

fungi in phyla Ascomycota and Basidiomycota; in some fungi, such as Rhizoctonia

solani, they are the only type of propagule produced, whereas in fungi such as

Claviceps purpurea and Sclerotinia sclerotiorum, they are overwintering structures that

can germinate directly, or give rise to structures in which the meiospores are formed.

Kingdom Fungi

The characteristics and diversity of the major phyla of true Fungi will be briefly

described. Selected representatives of the different phyla are introduced and, in many

instances, illustrated. A generalized life cycle also is presented for each phylum that

illustrates when plasmogamy (cell fusion), karyogamy (nuclear fusion) and meiosis

occur relative to each other, and the types of structures involved in these events. For

more detailed information on members of Kingdom Fungi, recommended reading is

provided at the end of this article.

Phylum Ascomycota is the largest group of fungi, with approximately 33,000

described species in three subphyla—Taphrinomycotina, Saccharomycotina, and

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Pezizomycotina. Members of this phylum reproduce sexually or meiotically (Fig. 18 –

general life cycle) via production of ascospores inside a sac-like structure called an

ascus (Fig. 19).

Figure 18 Figure 19

Many species of Ascomycota also (or exclusively) produce spores through an asexual

or mitotic process; these spores, called conidia , exhibit a wide range of size, shape,

color and septation among the different fungi in which they are formed. Conidia and

ascospores are usually produced at different times of year, if ascospores are formed in

the lifecycle. The existence of many Ascomycota having sexual and asexual states

that are separated in time and space has long confused those new to mycology and

plant pathology. The asexual states of Ascomycota are especially important to the

plant pathologist because they are more commonly encountered than the sexual state,

and must be identified for control, quarantine, or other purposes. Fungi that reproduce

only via asexual means have been given various designations including

deuteromycetes, fungi imperfecti, mitosporic fungi, conidial fungi, and anamorphic

fungi.

Subphylum Taphrinomycotina includes fungi that, with one known exception, do not

form fruiting bodies—as examples, the fission yeast Schizosaccharomyces (Fig. 20),

plant parasites in the genera Protomyces and Taphrina (peach leaf curl; see Figs. 21

& 22), and Pneumocystis , a yeast-like fungus previously mentioned (in "Fungi

associated with animals") that causes pneumonia in animals including humans.

Figure 20 Figure 21 Figure 22

Subphylum Saccharomycotina contains approximately 1500 species of yeasts, most of

which live as saprotrophs in association with plants and animals, but also including a

small number of plant and animal pathogens (Suh et al. 2006). Asci are formed naked

(Fig. 23)—not enclosed in a fruiting body. Yeasts traditionally have been important in

the production of beer, wine, single cell protein and baker's yeast, but their role in

industry has expanded to the production of citric acid, fuel alcohol, and riboflavin

(Kurtzman and Sugiyama 2001). Saccharomyces cerevisiae (Fig. 15), the yeast used

in baking and brewing, is an important model organism for scientists studying a wide

range of genetic and physiological processes. In 1997, S. cerevisiae was the first

eukaryotic organism to have its genome completely sequenced.

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Figure 23

Subphylum Pezizomycotina is the largest group in the phylum, with more than 32,000

identified species that occupy a wide range of ecological niches, occurring as

saprotrophs, parasites and mutualists with plants, animals and other fungi. At least

40% of the species form lichens (Fig. 24). Three different types of asci occur in this

subphylum, prototunicate, unitunicate and bitunicate. Prototunicate asci release

ascospores by breakdown of the ascus wall, whereas in the unitunicate and bitunicate

asci, the ascospores are forcibly discharged. Bitunicate asci have an inner wall that

balloons out from the outer wall prior to ascospore discharge, and in unitunicate asci

the wall layers do not separate from each other. A wide range of fruiting bodies are

formed by members of subphylum Pezizomycotina, including cleistothecia ,

chasmothecia, apothecia, perithecia and pseudothecia. Stromata, hardened masses of

hyphae on or in which perithecia or pseudothecia are formed, occur in some members

of this subphylum.

Figure 24

Cleistothecia (sing. cleisothecium) (Fig. 25) lack a preformed opening and ascospores

are released by the breakdown of the ascomatal wall. Common fungi that produce

cleistothecia include the teleomorphic (sexual) states of Aspergillus and Penicillium

(Fig. 26). Species of Aspergillus are important in the production of fermented foods

and beverages, including soy sauce, miso and rice wine (sake). Some species of

Aspergillus infect animals, causing a disease known as aspergillosis, and others

produce mycotoxins. Aflatoxin is a potent carcinogen produced by A. flavus that

occurs in food made from cereals, corn and peanuts that have been colonized by the

fungus. The U. S. Food and Drug Administration established a strict limit of 20 parts

per billion on aflatoxin levels in food, and the U. S. spends $30-50 million annually on

aflatoxin testing (Robens & Cardwell 2003). Penicillium species are also used in food

production. For example, the blue veins in Roquefort and Gorgonzola cheeses are due

to the growth and sporulation of particular species of Penicillium (Fig. 27), as is the

white rind on the outside of Camembert cheese. The antibiotic penicillin, the "wonder

drug" of the 20th century, is produced by strains of P. chrysogenum (Raper, 1978).

Other species of Penicillium, such as P. digitatum and P. italicum cause the blue and

green molds commonly causing rots of citrus fruits.

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Figure 25 Figure 26 Figure 27

Chasmothecia (sing. chasmothecium) also lack a preformed opening, but ascospores

are released by a split (or "chasm") in the ascomatal wall (Fig. 28). The term is now

used to refer to the fruiting bodies of the powdery mildew fungi (Fig. 29) in order

Erysiphales.

Figure 28 Figure 29

Apothecia (sing. apothecium) are exposed, often cup-shaped fruiting bodies (Fig. 30),

but can take on a variety of forms, including those found in the morels (Morchella spp.

Fig. 31). Apothecia-forming fungi are also called "cup fungi" or discomycetes . Some

important groups of plant pathogens that form apothecia include species of Monilinia

(brown rot of peach; Figs. 32 & 33), and Sclerotinia.

Figure 30 Figure 31

Figure 32 Figure 33

Perithecia ( sing. perithecium) are enclosed ascomata with a preformed opening

(ostiole) through which ascospores are discharged (Fig. 34). Most fungi producing

perithecia also have unitunicate asci and are classified in Sordariomycetes , one of

the largest classes of Ascomycota with more than 3,000 described species (Zhang et

al. 2006). These fungi have also been called pyrenomycetes. Members of this group

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al. 2006). These fungi have also been called pyrenomycetes. Members of this group

are common in nearly all ecosystems, where they occur as saprotrophs, endophytes of

plants, or pathogens of plants, animals and other fungi. A large number of

economically important plant pathogens belong to Sordariomycetes, including those

that cause anthracnose diseases (Glomerella cingulata ), blasts (Magnaporthe oryzae ,

rice blast pathogen), blights (Cryphonectria parasitica , chestnut blight), ergot

(Claviceps purpurea ), and Fusarium head blight (scab) of small grains ( Gibberella

zeae).

Figure 34

Pseudothecia (sing. pseudothecium) look similar to perithecia, but they differ in

development. Asci form in locules (openings) inside vegetative fungal tissue called

ascostroma; this group has been called loculoascomycetes, but is now placed in class

Dothideomycetes. Other characteristics of Dothideomycetes include the formation of

bitunicate asci, and many members of this group produce darkly pigmented,

multiseptate asospores or conidia. Similar to the Sordariomycetes, members of

Dothideomycetes occur in a wide range of habitats as saprotrophs and associate with

plants as pathogens, endophytes and growing on the surface of plants as epiphytes

(Schoch et al. 2006). Examples of well-known plant pathogens belonging to this group

include Venturia inaequalis (apple scab; Fig. 35) and Mycosphaerella fijiensis (Black

Sigatoka disease of banana); some of the common asexually reproducing fungi that

belong in Dothideomycetes are Alternaria (Fig. 36), Cladosporium (Fig. 37), Phoma,

and Stemphylium .

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Figure 35 Figure 36 Figure 37

Most of the lichen-forming members of Ascomycota belong in class Lecanoromycetes.

This is the largest class of fungi, with over 13,500 described species (Miadlikowska et

al. 2006). Most of the members of this class produce apothecial fruiting bodies (Figs.

7, 24, 38, 39). The majority of lichenized fungi form a symbiotic association with green

algae, and approximately 10% are associated with cyanobacteria. The lichen thallus

produces a wide range of secondary metabolites that are of biological and ecological

importance (Miadlikowska et al. 2006). The lichen thallus is able to grow under a range

of adverse conditions and some can survive for hundreds of years. Lichens are found

in a wide range of habitats from the Arctic to Antarctic, including some species that

can grow in aquatic and marine environments (Webster and Weber 2007).

Figure 38 Figure 39

Phylum Basidiomycota

Phylum Basidiomycota represents the second largest phylum of fungi, with nearly

30,000 described species. Members of phylum Basidiomycota produce basidiospores

on a typically club-shaped structure called a basidium (Fig. 17). Characteristic of the

mycelium of many members of Basidiomycota is the presence of clamp connections

(Figs. 40 & 41) and dolipore septa.

Figure 40 Figure 41

Three main lineages are recognized in phylum Basidiomycota: subphyla

Ustilaginomycotina, Pucciniomycotina, and Agaricomycotina (Blackwell et al. 2006).

Ustilaginomycotina and Pucciniomycotina are composed mostly of plant parasitic

species, known as smut and rust fungi, respectively, characterized by a state that

produces thick-walled teliospores (Figs. 42 & 43). The most extensively studied

members of Ustilaginomycotina are species of Tilletia and Ustilago . Ustilago maydis,

which causes corn smut (Fig. 44), produces conspicuous tumor-like growths on

infected host plants. These structures eventually become filled with dark teliospores

and are considered a delicacy in Mexico called "cuitlacoche. " The most economically

important species of Tilletia are the wheat bunts ( T. caries and T. laevis —common

bunt; T. contraversa—dwarf bunt; and T. indica—Karnal bunt), plant pathogens that

convert host ovaries into masses of dark, thick-walled teliospores that smell like rotting

fish (Fig. 45).

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Figure 42 Figure 43

Figure 44 Figure 45

Subphylum Pucciniomycotina include the group of plant parasites called rust fungi.

The rust fungi are remarkable in having as many as five distinct types of spores in a

single life cycle (spermatia, aeciospores, urediniospores, teliospores, and

basidiospores) (Fig. 46 – general life cycle). Rust fungi that produce all five spore

states are macrocyclic, those that do not form uredinospores are demicyclic, and those

that do not form urediniospores and aeciospores are microcyclic . Rust fungi may

complete the life cycle on one host (autoecious rusts) or require two unrelated

alternate hosts for completion of the life cycle (heteroecious rusts). The most widely

cited example of a macrocyclic, heteroecious rust is Puccinia graminis (black stem

rust), which forms two spore states (uredinia and telia ) on cultivated wheat (Fig. 47)

and two different spore states (spermagonia and aecia ) on barberry leaves (Fig. 48).

The fifth state, the probasidium producing basidiospores, is formed upon germination

of the teliospores (Fig. 46).

Figure 46 Figure 47 Figure 48

Subphylum Agaricomycotina, previously known as the Hymenomycetes, includes the

morphologically diverse group of fungi that produce basidia in various types of fruiting

bodies (Fig. 49 – general life cycle). This group includes the fungi commonly known as

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mushrooms (Fig. 50), puffballs (Fig. 51), shelf fungi (Fig. 52), stinkhorns (Fig. 53), jelly

fungi (Figs. 54 & 55) and bird's nest fungi (Fig. 56). Many species are saprotrophic,

utilizing dead plant material including woody substrates. Some of these saprotrophic

species are cultivated for food, for example, the common button mushroom (Agaricus

bisporus), oyster mushrooms (Pleurotus ostreatus), and shiitake (Lentinula edodes).

Other members of this group are important ectomycorrhizal fungi, forming mutualistic

associations with the roots of a wide range of trees. Some fruiting bodies produced by

ectomycorrhizae are considered choice edibles, for example, chanterelles

(Cantharellus cibarius and other species), porcini ( Boletus edulis ), and the American

matsutake (Tricholoma magnivelare) (Fig. 6). A few members of this group are

economically important plant parasites, e. g., species of Armillaria and Rhizoctonia .

Figure 49 Figure 50

Figure 51 Figure 52

Figure 53 Figure 54

Figure 55 Figure 56

Phylum Glomeromycota

The arbuscular mycorrhizal (AM) fungi, long considered to belong in the Zygomycota,

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are now recognized as comprising a distinct phylum, Glomeromycota (Shüβ ler et al.

2001). This is an ancient group of fungi, recognizable in the fossil record dating back

at least 400 million years. The AM fungi form obligate, mutualistic associations, called

endomycorrhizae, with the roots of most (~80%) vascular plants. Only a small number

(~160) of species is recognized in the phylum. One of the most distinctive features of

these fungi is the highly branched arbuscules formed inside the cortical cells of host

roots; arbuscules are the point of exchange between fungus and plant, where

carbohydrates produced by the plant are acquired by the fungus, and where nitrogen,

phosphorous, and other minerals acquired by the mycelium of the fungus are

transferred to the plant. The fungus acquires as much as 20-40% of the photosynthate

produced by the plant. Some AM fungi also produce storage structures inside plant

roots called vesicles. Endomycorrhizal fungi produce an extensive network of hyphae

outside the roots (extraradical hyphae). The extraradical hyphae act like an extension

of the plant roots, increasing the plant's access to water and soil minerals, particularly

phosphorous and nitrogen. The fungus is also able to access phosphate not otherwise

available to plants, for example from organic matter by production of acid

phosphatases. Reproduction of AM fungi is by thick-walled spores ranging in size from

40–800 µm in diameter, each of which can contain hundreds or thousands of nuclei.

Spores may be formed singly or in clusters, and the mycelium of AM fungi is

coenocytic. Sexual reproduction is not known to occur in this phylum.

Phylum Chytridiomycota

"Chytrids" are a small group of fungi with approximately 900 identified species

occurring in a wide range of aquatic and terrestrial habitats around the world. The

feature that is shared by all members of this phylum is the formation of zoospores with

one posteriorly directed, whiplash flagellum . A few chytrids are economically important

plant pathogens, e. g., Synchytrium endobioticum , which causes the black wart

disease of potato, others are vectors of plant viruses (Olpidium), but most are

saprotrophs using substrates such as cellulose, chitin, and keratin as a food source.

As previously noted, the frog chytrid, Batrachochytrium dendrobatidis , has been

implicated as a major factor in population declines of frogs and other amphibians

around the world (Berger et al. 1998; Wake and Vredenburg 2008).

Phylum Zygomycota

This phylum contains approximately 900 identified species divided amongst two

ecologically distinct classes, Zygomycetes and Trichomycetes (White et al. 2006).

Hibbett et al. (2007) indicate the phylum is polyphyletic and further work is needed to

clarify relationships of fungi traditionally considered in Zygomycota. The most

commonly encountered Zygomycetes are members of orders Mortierellales and

Mucorales. Many members of these two orders are saprotrophs with rapidly growing,

coenocytic mycelium. The sexual reproductive state is the zygospore (Fig. 57 –

general life cycle), but many of these fungi produce a large number of readily

dispersed asexual spores called sporangiospores . Members of order Mucorales,

commonly called mucoraceous fungi, are common in soil, dung, plant material, and

other types of organic matter. Some mucoraceous fungi are plant or animal

pathogens, and others are used in the production of Asian foods such as tempeh.

Species of Mucor and Rhizopu s (Fig. 58) are commonly isolated from decaying

organic matter and can cause decay diseases of fleshy fruits, vegetables, and

sunflower peduncles.

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Figure 57 Figure 58

Species of Pilobolus (Figs. 59 & 60) are among the first fungi observed growing on

herbivore dung incubated in moist chambers. Other Zygomycetes are associated with

animals. For example, some species of Rhizopus and Mucor cause zygomycosis in

immunocompromised humans. Entomophthorales, as the name suggests, include

parasites of insects and other animals. Members of class Trichomycetes live in the

guts of insects, millipedes, and crustaceans, but cause little or no harm to their hosts.

Figure 59 Figure 60

Fungal-like Organisms Studied by Plant Pathologists and

Mycologists

Oomycetes

Oomycetes are fungal-like organisms that form zoospores with two flagella—a

whiplash flagellum that is directed backwards and propels the zoospore, and a tinsel

flagellum adorned with hairs that is directed forward, pulling the zoospore. The cell

walls of Oomycetes contain cellulose, rather than chitin, and glucans. (Kirk et al.,

2008). Another characteristic of Oomycetes is the formation of an oospore, a thick-

walled, resistant propagule which is the result of sexual reproduction. Oomycetes

belong in Kingdom Straminipila, also known as Chromista. In addition to Oomycetes,

this kingdom includes diatoms, golden and brown algae, a type of algae called

cryptomonads, and two other groups of organisms studied by mycologists in phyla

Labyrinthulomycota and Hyphochytriomycota. The tinsel type flagellum is a

characteristic of all members of Kingdom Straminipila, hence it is also called a

straminipilous flagellum.

Oomycetes include some of our most devastating plant pathogens. These fungal-like

organisms have changed the course of history. Consider 19th century Ireland; life was

hard for the millions of Irish in the 1840s who relied almost entirely upon the "lumper"

potatoes they grew on leased quarter-acre plots for food and rent. It's said that the

stomachs of these "cottiers" were distended from eating up to fourteen pounds of

potatoes each day (Large 1940). Then, in 1845, the potatoes began to rot from a

malady known as "Potato Murrain," what we now call late blight of potato . Without

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malady known as "Potato Murrain," what we now call late blight of potato . Without

potatoes, 4. 5 million Irish faced starvation. Over the next 15 years, one million Irish

died from the famine, and one and a half times that number fled Ireland. Late blight is

caused by Phytophthora infestans, and this oomycete continues to be a major

pathogen in potato production, although we now have the ability to control it through

the application of fungicides.

A number of other important plant pathogens are found among the Oomycetes, but

only a few will be mentioned here. Phytophthora ramorum causes sudden oak death

and ramorum blight. Pythium species cause damping-off diseases under extremely

wet conditions. Seedlings are particularly vulnerable to attack by damping-off

pathogens because their tissues are soft and easily invaded. Seedlings may be killed

before or after they emerge from the soil. Downy mildews (Peronosporales) are

biotrophic Oomycetes that are characterized by the formation of white, downy

sporangiophores on the surface of infected hosts. The white rusts ( Albugo spp. )

produce chains of sporangia that erupt through the cuticle of infected hosts. Albugo

species parasitize crucifers and produce blister-like pustules filled with sporangia,

which will germinate to produce motile zoospores. White rusts can also cause infected

stems to grow in a contorted or twisted manner.

Members of Saprolegniales are called water molds. Many of these produce fast-

growing, robust hyphae on organic matter in aquatic environments, but some species

of Saprolegnia are parasitic on fish and fish eggs. For more information on

Oomycetes, see Introduction to Oomycetes (Fry & Grünwald 2010).

Other groups

Labyrinthulids (phylum Labyrinthulomycota), a small group of Straminipila, include

organisms that cause rapid blight of turf grass and the wasting disease of eelgrass.

Labyrinthulids have a unique manner of movement—their microscopic, football-shaped

cells produce an ectoplasmic net through which the cells glide. The slow, gliding

movement of the cells within the ectoplasmic net can be observed under the

microscope.

Hyphochytrids (phylum Hyphochytridomycota, Kingdom Straminipila) are similar to

chytrids in appearance, as their name suggests, and produce zoospores with a single

anterior tinsel flagellum. Hyphochytrids are one of the smallest groups of fungal-like

organisms, both in size and in number of species with only 23 known. Some

hyphochytrids are known to parasitize algae, spores of AM fungi, and oospores of

Oomycetes.

Slime molds

Slime molds are organisms that have a trophic (feeding) stage in their life cycle that

lacks a cell wall, either uninucleate (amoeba) or multinucleate (plasmodium). The lack

of a cell wall facilitates engulfment of food, in contrast to true fungi that must absorb

their nutrients through a cell wall. The slime molds are now included in the Amoebozoa

(Adl et al. 2005).

Four groups of slime molds are recognized—plasmodial slime molds (Myxomycota),

cellular slime molds (Dictyosteliomycota and Acrasiomycota) and endoparasitic slime

molds (Plasmodiophoromycota). We will briefly cover plasmodial slime molds and

endoparasitic slime molds. For information on cellular slime molds, refer to one of the

introductory mycology texts listed below (Recommended Further Reading). The

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introductory mycology texts listed below (Recommended Further Reading). The

plasmodial slime molds are most commonly found in temperate forests, where they

occur on plant litter, tree bark, and other types of plant material. They produce a

multinucleate trophic stage lacking a cell wall called a plasmodium (Fig. 61) that

moves over and through decaying organic matter, engulfing bacteria, fungi, and other

microorganisms (Frederick 1990). The most conspicuous stage of the plasmodial

slime mold is the fruiting structures, called sporophores (Alexopoulos et al. 1996),

which are often brightly colored and visible to the naked eye (Fig. 62). One of the most

common slime molds in temperate regions is Fuligo septica. The sporophores of this

slime mold are often found in ornamental bark and mulch, and look more like an

animal's vomit than the fruiting structure of a living organism, thus earning the

nickname: "dog vomit slime" (Fig. 63). The presence of slime molds in landscaping

(Fig. 64) occasionally prompts calls to plant disease clinics, however, none of the

plasmodial slime molds are known to be plant or animal parasites, and are of no

known economic importance except as model organisms for research.

Figure 61 Figure 62

Figure 63 Figure 64

Members of phylum Plasmodiophoromycota are biotrophic parasites that produce

their plasmodial stage inside the cells of plants, algae, diatoms, and Oomycetes.

Several members of this phylum are economically important plant parasites, including

Plasmodiophora brassicae, which causes clubroot of crucifers (Fig. 65) and

Spongospora subterranea, which causes powdery scab of potato (Fig. 66). Polymyxa

graminis is a vector for soilborne wheat mosaic virus, an economically important

disease of wheat. Members of this phylum produce cysts inside host cells; the cysts

are released when the plant tissue breaks down, and germinate to release a zoospore

that infects the host by injecting its cytoplasm into a host cell. Infected host tissue may

become greatly swollen as in clubroot of crucifers.

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Figure 65 Figure 66

A Brief Summary

As we have seen, a fungus is a eukaryotic organism that absorbs nutrients through its

cell walls and generally reproduces by sporesTrue fungi belong to Kingdom Fungi, and

other fungal-like organisms are placed in phyla outside the Kingdom Fungi. Most fungi

consist of a hyphal thallus that allows these organisms to colonize and exploit many

different substrates and fill various ecological niches, as parasites, pathogens,

mutualists, saprotrophs and decomposers. Fungi and fungal-like organisms survive

and reproduce via a huge diversity of spore types, characteristic of each taxonomic

group. This introduction has provided some basic information on reproduction, nutrient

acquisition, and roles in the ecosystem, but much more information is available (see

Recommended Further Reading and the References). Fungi are fascinating in and of

themselves, but they are also critically important to humans in both detrimental and

beneficial ways.

Recommended Further Reading

For more information on fungi, popularized accounts by Hudler (1998), Money (2002,

2007), and Moore (2001) are good sources of information written in an engaging

manner. Introductory mycology books by Alexopoulos et al. (1996), Deacon (2006),

Kendrick (2000), Moore et al. (2011), and Webster and Weber (2007) provide more

detailed information on fungi than can be included in this brief introduction. Tales of

fungi, folklore, and human affairs can be found in Dugan (2008) and Findlay (1982),

and an engaging book on the impact of fungal plant pathogens by Money (2007). A

number of books on hallucinogenic/psychoactive mushrooms have been written, some

describing the history of these intriguing mushrooms. Gordon Wasson's (1968) book

"Soma" is one of the first ethnomycological treatments of hallucinogenic mushrooms,

and a recent book on the history of magic mushrooms by Letcher (2007) relays the

story of their use from ancient Aztecs to contemporary society.

Acknowledgments

We thank APS Press for allowing us to use images from "Fundamental Fungi. "All

other images used with permission. We thank a number of colleagues who have kindly

provided us with images. PPNS No. 0572, Department of Plant Pathology, College of

Agriculture, Human, and Natural Resource Sciences, Agricultural Research Center,

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Agriculture, Human, and Natural Resource Sciences, Agricultural Research Center,

Project No WNP0837, Washington State University, Pullman, WA99164-6430, USA.

This paper is Contribution No. 11-288-J from the Kansas Agricultural Experiment

Station, Manhattan.

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Reviews 59:686-698.

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... Though both of the species have strong capabilities to degrade the hydrocarbon but fungal strains got less attention (8). Fungal strain can grow in the soil in the hyphal form which help them to reach and penetrate deeply into the contaminated soil (9). besides several beneficial properties associated with the fungal strains there are also some limitation in the application of the fungi, which should be kept in mind, such as fungal strains degradation can be a very slow process, due to the reason that they need longer period to adapted to environment. ...

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Human and industrial activities produce and discharge wastes containing heavy metals into the water resources making them polluted, threatening human health and the ecosystem. Biosorption, the process of passive cation binding by dead or living biomass, represents a potentially cost-effective way of eliminating toxic heavy metals from industrial wastewater. The abilities of microorganisms to remove metal ions in solution have been extensively studied; in particular, live and dead fungi have been recognized as a promising class of low-cost adsorbents for the removal of heavy metal ions. The biosorption behavior of fungal biomass is getting attention due to its several advantages; hence, it needs to be explored further to take its maximum advantage on wastewater treatment. This review discusses the live and dead fungi characteristics of sorption, factors influencing heavy metal removal, and the biosorption capacities for heavy metal ions removal and also discusses the biosorption mechanisms.

... Há relevantes estudos sobre fitotelmata de Bromeliaceae e a dinâmica com diversos tipos de organismos. Estes incluem microrganismos procarióticos e eucarióticos, bem como diferentes grupos de invertebrados e vertebrados (MONTERO et al., 2010;CARRIS, 2012;DIAS et al., 2014;PAULA JÚNIOR et al., 2017;LADINO et al., 2019). ...

Bromélias apresentam alto potencial ecológico, sobretudo pela capacidade no acúmulo de água e matéria orgânica no centro de bainhas foliares alargadas e sobrepostas em forma de espiral, na qual possibilitam formarem habitat para uma riqueza de espécies, incluindo os microrganismos. Este trabalho objetivou quantificar e identificar microfungos, bem como analisar os principais índices ecológicos (constância; riqueza de espécies; índice de similaridade; índice de variabilidade) em fitotelmata das bromélias Vriesea procera e Aechmea alba, localizadas em fragmento de Mata Atlântica do Extremo Sul, Teixeira de Freitas-BA. As coletas foram realizadas entre janeiro a outubro de 2019 em fragmento florestal remanescente particular (Fazenda Sayonara). O processamento, análises e identificação das amostras foram realizados no Laboratório de Biologia dos Fungos da Universidade do Estado da Bahia (UNEB), Campus X. As colônias de microfungos foram quantificados e identificados a partir da água acumulada no fitotelmo das duas espécies de bromélias. Foram identificados 29 espécimes de fungos filamentosos, distribuídos em oito gêneros: Penicillium, Aspergillus, Fusarium, Talaromyces, Exophiala, Mucor, Cladosporium e Aphanocladium. As análises ecológicas permitiram observar a presença de 55,17% de microfungos nas bromélias V. procera estudadas e 65,52% de microfungos nas bromélias A. alba. A riqueza de espécies teve pouca diferença entre as duas espécies de bromélias. O índice de diversidade de espécies de microfungos foi semelhante entre as duas plantas. O coeficiente de similaridade de espécies de microfungos entre as duas bromélias foi relativamente baixo. Palavras-chave: Microfungos; Bromeliaceae; Fitotelmo. Microfungi in bromeliad phytotelmata from a fragment of Atlantic Forest of the Extreme South of Bahia Abstract Bromeliads have high ecological potential, mainly due to the capacity to accumulate water and organic matter in the center of the enlarged and spiral-shaped leaf sheaths, in which it is possible to create a perfect habitat for a wealth of species, including microorganisms. This study aimed to quantify and identify microfungi, as well as to analyze the main ecological indices (constancy; species richness; similarity index; variability index) in phytotelmata of the bromeliads Vriesea procera and Aechmea alba, located in a fragment of Atlantic Forest of the Extreme South, Teixeira de Freitas-BA. The collections were carried out between January and October 2019 in a private remaining forest fragment (Sayonara Farm). The processing, analysis and identification of the samples were carried out in the Fungal Biology Laboratory of the State University of Bahia (UNEB), Campus X. Microfungal colonies were quantified and identified from the water accumulated in the phytotelma of the two bromeliad species. Twenty-nine specimens of filamentous fungi were identified, distributed in eight genera: Penicillium, Aspergillus, Fusarium, Talaromyces, Exophiala, Mucor, Cladosporium, and Aphanocladium. The ecological analyses allowed to observe the presence of 55.17 % of microfungi in the bromeliads V. procera and 65.52 % of microfungi in bromeliads A. alba. Species richness showed little difference between the two bromeliad species. The diversity index of microfungal species was similar between the two plants. The similarity coefficient of microfungal species between the two bromeliads was relatively low.

Fertility is the simplest yet most sophisticated word to describe a well-cultivated soil. Simply because it can, in general, make the most of the product available to everyone and complex because many aspects of its sustainable management are still unknown, even to experts in the field of soil science. In fact, fertility is a reflection of the intrinsic complexity of the plant's soil ecosystem, because of one of the characteristics of the various components of this vital system, as well as the numerous interactions between them. These components are affected and, as a result, they provide the sum of their effects in the capacity to support plant growth and crop production. Therefore, maintaining this capability at optimum crop production level requires steady, comprehensive management, and is aware of all the physical, chemical, and biological aspects that affect not only the quantity of production but also the quality and health of the soil and environmental resources. Microbial fertilizers and soil microorganisms play an important role in controlling plant diseases, eliminating plant pests, and converting part of the minerals to a usable form for plants. Chemical fertilizers are essential components of biocontrol and plant growth factors. The use of plant food-producing bacteria and the application of proper soil fertility and plant nutrition in addition to protecting the environment and human health also avoid the unnecessary and wasteful use of chemical fertilizers.

Bioprospecting is a new way of getting natural compounds in a sustainable and innovative manner. There are various biological sources for natural compounds, but endophytic fungi from medicinal plants are considered most favorable be- cause of their diversity in species and power in producing secondary metabo- lites. Countless drugs are being provided by medicinal plants and endophytes in the form of secondary metabolites, which are selected for important therapeutic alternatives for numerous diseases like antimicrobial, antifungal, antimalarial, antioxidant, anticancer, insecticidal, and pesticidal.

Captive and wild frogs from North and Central America and Australia recently have died with epidermal infections by chytridiomycete fungi. We isolated a chytridiomycete into pure culture from a captive, blue poison dart frog that died at the National Zoological Park in Washington, D.C. Using this isolate, we photographed developmental stages on nutrient agar, examined zoospores with transmission electron microscopy, and inoculated test frogs. This inoperculate chytrid develops either monocentrically or colonially and has thread-like rhizoids that arise from single or multiple areas on the developing zoosporangium. The taxonomically important features of the kinetosomal region of the zoospore indicate that this chytrid is a member of the Chytridiales but differs from other chytrids studied with transmission electron microscopy. Its microtubule root, which begins at kinetosome triplets 9-1 and extends parallel to the kinetosome into the aggregation of ribosomes, is distinctive. Histologic examination of test frogs revealed that the pure culture infected the skin of test frogs, whereas the skin of control frogs remained free of infection. The fungus is described as Batrachochytrium dendrobatidis gen. et sp. nov.

• David Hawksworth

The number of known species of fungi is estimated as at least 74 K, but could be as much as 120 K with allowances for 'orphaned' species. Yet in 1990 the magnitude of fungal diversity was estimated 'conservatively' at 1.5 M species. This figure has been widely accepted as a working hypothesis, but subsequent estimates have ranged from 500 K to 9.9 M and the bases of these suggestions are analyzed. Additional data pertinent to the estimation of the number of fungal species on Earth that has become available since 1990 is discussed. Site inventories demonstrate the need for long-term (20 yr plus) intensive studies to determine the number of species in a site. Fresh data sets on fungus:plant ratios and degrees of host specificity, especially from well-studied hosts in the tropics, are consistent with earlier estimates. The extent of novelty discovered in recent monographic generic revisions and studies of species in particular habitats varies from 0-96%. Allowances for cryptic species, now known to be widespread by incompatibility and molecular studies, could on their own justify an upward revision by a factor of at least five. To enable confidence in any overall estimate to be increased, more detailed studies, especially on particular sites in the tropics, are needed. The consensus of tropical and molecular mycologists in particular is that an increased estimate could be justified. However, it is prudent to retain 1.5 M as the current working hypothesis for the number of fungi on Earth while additional data to test it further accumulates.

Epidermal changes caused by a chytridiomycete fungus (Chytridiomycota; Chytridiales) were found in sick and dead adult anurans collected from montane rain forests in Queensland (Australia) and Panama during mass mortality events associated with significant population declines. We also have found this new disease associated with morbidity and mortality in wild and captive anurans from additional locations in Australia and Central America. This is the first report of parasitism of a vertebrate by a member of the phylum Chytridiomycota. Experimental data support the conclusion that cutaneous chytridiomycosis is a fatal disease of anurans, and we hypothesize that it is the proximate cause of these recent amphibian declines.

• Jane Robens
• Kitty Cardwell

Mycotoxin losses and costs of mycotoxin management are overlapping areas of concern. Costs of mycotoxin management include research production practices, testing and research necessary to try to prevent the toxins from appearing in food and feed products of affected commodities. Mycotoxin losses result from (American Association of Veterinary Laboratory Diagnosticians (AAVLD), 1993) lowered animal production and any human toxicity attributable to the presence of the toxin, (CAST (Council for Agricultural Science and Technology), 1989) the presence of the toxin in the affected commodity which lowers its market value, as well as (Hawk, 1998) secondary effects on agriculture production and agricultural communities.

• Meredith Blackwell

Fungi are major decomposers in certain ecosystems and essential associates of many organisms. They provide enzymes and drugs and serve as experimental organisms. In 1991, a landmark paper estimated that there are 1.5 million fungi on the Earth. Because only 70000 fungi had been described at that time, the estimate has been the impetus to search for previously unknown fungi. Fungal habitats include soil, water, and organisms that may harbor large numbers of understudied fungi, estimated to outnumber plants by at least 6 to 1. More recent estimates based on high-throughput sequencing methods suggest that as many as 5.1 million fungal species exist. Technological advances make it possible to apply molecular methods to develop a stable classification and to discover and identify fungal taxa. Molecular methods have dramatically increased our knowledge of Fungi in less than 20 years, revealing a monophyletic kingdom and increased diversity among early-diverging lineages. Mycologists are making significant advances in species discovery, but many fungi remain to be discovered. Fungi are essential to the survival of many groups of organisms with which they form associations. They also attract attention as predators of invertebrate animals, pathogens of potatoes and rice and humans and bats, killers of frogs and crayfish, producers of secondary metabolites to lower cholesterol, and subjects of prize-winning research. Molecular tools in use and under development can be used to discover the world's unknown fungi in less than 1000 years predicted at current new species acquisition rates.

• Randy Ploetz

This review summarizes how and where the banana export trades began, the early history of Panama disease, and the important effect that the disease had on the development of this industry. This is a story of political and social upheaval, and of the first large agriculture ventures in the tropics. It focuses on the trades in the Americas before they converted to the resistant Cavendish cultivars. Accepted for publication 17 October 2005. Published 21 December 2005.

The ecologically and economically important arbuscular mycorrhizal (AM) fungi, crucial in the ecology and physiology of land plants, and the endocytobiotic fungus, Geosiphon pyriformis, are phylogenetically analysed by their small subunit (SSU) rRNA gene sequences. They can, from molecular, morphological and ecological characteristics, unequivocally be separated from all other major fungal groups in a monophyletic clade. Consequently they are removed from the polyphyletic Zygomycota, and placed into a new monophyletic phylum, the Glomeromycota. The recognition of this monophyletic group, which probably diverged from the same common ancestor as the Ascomycota and Basidiomycota, gives these fungi their proper status, and provides a basis for a new and natural systematics of these fascinating, yet largely hidden organisms, with three new orders (Archaeosporales, Paraglomerales, Diversisporales) described herein, Additionally, several clades resolve at family level; their formal description is in progress

Although macroscopic plants, animals, and fungi are the most familiar eukaryotes, the bulk of eukaryotic diversity is microbial. Elucidating the timing of diversification among the more than 70 lineages is key to understanding the evolution of eukaryotes. Here, we use taxon-rich multigene data combined with diverse fossils and a relaxed molecular clock framework to estimate the timing of the last common ancestor of extant eukaryotes and the divergence of major clades. Overall, these analyses suggest that the last common ancestor lived between 1866 and 1679 Ma, consistent with the earliest microfossils interpreted with confidence as eukaryotic. During this interval, the Earth's surface differed markedly from today; for example, the oceans were incompletely ventilated, with ferruginous and, after about 1800 Ma, sulfidic water masses commonly lying beneath moderately oxygenated surface waters. Our time estimates also indicate that the major clades of eukaryotes diverged before 1000 Ma, with most or all probably diverging before 1200 Ma. Fossils, however, suggest that diversity within major extant clades expanded later, beginning about 800 Ma, when the oceans began their transition to a more modern chemical state. In combination, paleontological and molecular approaches indicate that long stems preceded diversification in the major eukaryotic lineages.

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Source: https://www.researchgate.net/publication/230888186_Introduction_to_Fungi