Fungi cause about 100,000 diseases of plants, including about 70 percent of the major crop diseases, resulting in an economic loss of billions of dollars each year. These plant pathogens cause extensive disease to seeds, seedlings, mature plants, and aging plants, resulting in decreased growth and reproduction of crop plants. Fungi also attack forest trees and wooden structures.

A number of fungi cause diseases in humans and other vertebrates. In general, these fungus infections, or mycoses, develop slowly, recur more frequently than bacterial infections, and do not produce a lasting immunity in the body. A mycosis is classified in one of two groups, depending on the part of the body that is infected. A dermatomycosis is an infection of the skin, hair, or nails, such as ringworm or athlete’s foot. These infections rarely progress to the internal organs. Most respond well to medication, although treatment may take several weeks.

Fungi cause a number of human respiratory diseases. Coccidioidomycosis is caused by the yeast Coccidioides immitis. Although typically contracted by the inhalation of dust containing yeast spores, the fungus may also be introduced through the skin from infected soil. Initial symptoms may resemble the flu, with fever, cough, and possibly a skin rash, and the infection usually runs its course without specific treatment. In rare cases, the fungus penetrates internal tissues, such as the bones, joints, and brain, producing tumors that later form abscesses or ulcers. No treatment is available that can halt the course of this form of the disease.

Histoplasmosis is caused by the yeastlike fungus Histoplasma capsulatum, which grows in pigeon, bat, and chicken droppings. Contracted by the inhalation of dust from animal droppings, by ingestion, or through the skin. The fungus causing histoplasmosis lives as a parasite in certain tissue and blood cells of the infected person. An infection in the respiratory system is similar to tuberculosis small spots form in the lungs although these lesions heal on their own. A progressive form typically invades the bone marrow and is rapidly fatal.

Aspergillosis is an infection of the skin, nasal sinuses, and lungs or other internal organs caused by molds of the genus Aspergillus. The disease, contracted by the inhalation of spores, occurs most often among agricultural workers. Itching and pain are frequent symptoms, and if scratching is prolonged, the skin may thicken and become gray or black. A virulent type of pneumonia is caused by the yeastlike fungus Pneumocystis carinii, particularly prevalent in people with compromised immune systems, such as AIDS patients.

Mycotoxins are poisons produced by fungal growth in cereals, nuts, fruits, and vegetables. More than 100 species of fungi produce these toxins. The most common mycotoxin is aflatoxin, produced by Aspergillus flavus and Aspergillus parasiticus. Commonly found on corn, peanuts, and tree nuts, the toxin also can be transmitted to humans through the milk, meat, or eggs of animals fed contaminated grains. Aflatoxin is the most potent carcinogen, or potentially cancer-causing agent, yet discovered. Other mycotoxins include trichothecenes and zearalenone, compounds known to injure the intestines, bone marrow, lymph nodes, spleen, and thymus. They are produced by species of Fusarium that grow on grain, straw, or hay stored while damp. Occasionally, circumstances prevent the harvesting of grains during the autumn, and the grains lie dormant in the damp fields until they are harvested in the spring. These grains are especially vulnerable to trichothecenes and zearalenone contamination. A large outbreak of trichothecenes contamination occurred in Russia in early 1944 among hungry peasants who had been searching the winter fields for unharvested wheat and millet.

Mushrooms

Mushroom is a family of fungi with gills, but in popular usage any of the larger fleshy or woody fungi. The application of the term mushroom is  to edible species only and the term toadstool to those considered poisonous or otherwise objectionable has no scientific basis. For example, two poisonous fungi may be less closely related than are a poisonous species and an edible one.

Fly Agaric Mushroom. The brightly colored fly agaric mushroom is a deadly poisonous mushroom. Containing ibotenic acid and a number of other organic poisons, the fly agaric can cause severe damage to the central nervous system, blood vessels, kidneys, liver, and muscles. Symptoms, which may not become apparent for 8 to 12 hours or longer, include nausea, vomiting, and severe diarrhea and can lead to coma and death.
A systemic mycosis, which is an infection of the entire body, is typically more serious and can be fatal for individuals whose immune system has been weakened by diseases such as acquired immunodeficiency syndrome AIDS or cancer. Fungal infections are typically spread by spores that enter the body through inhalation or through an opening in the skin. Some infections are passed from animals to humans or between humans.

Life Cycle of a Mushroom. Most mushrooms undergo a complex reproductive cycle in which they produce reproductive spores that undergo sexual reproduction under proper environmental conditions. Mushrooms belong to the phylum Basidiomycetes, named for the tiny, club-shaped structures called basidia that these fungi use to produce spores. The basidia line the undersides of a mushroom cap on thin gills that radiate from the mushroom’s center.

In the reproductive cycle of mushrooms, the mycelium contain hyphae of two mating types, sometimes called plus and minus strains, with no obvious anatomical differences distinguishing them. If plus and minus strains of hyphae fuse, sexual reproduction begins. Initially the nuclei of the two hyphae remain separate, producing an intermediate stage called the dikaryon, meaning “two nuclei.” The dikaryon stage can last from weeks to years, depending upon the species. The two nuclei in the dikaryon eventually fuse to produce a diploid cell—that is, a cell that contains one nucleus with two sets of chromosomes. This cell immediately undergoes meiosis, a type of nuclear cell division that produces offspring with half the genetic material as the parents. Meiosis usually produces four genetically unique haploid spores and the reproductive cycle begins again. This population of genetically different spores has a better chance of surviving environmental changes, such as disease or temperature changes, that may wipe out an entire population of genetically identical spores.

Of the thousands of species of mushrooms known throughout the world, the great majority are tough, woody, bitter, tasteless, or of such rare occurrence that they are of no interest as food. A few species produce death or serious illness when eaten. No simple rule exists for distinguishing edible and poisonous mushrooms, but the characteristics of the more common edible species can be readily learned, and collecting activities should be confined to such species. Morels, puffballs, and other species described below are not ordinarily confused with dangerous types; whenever doubt arises, the only safe procedure is to discard all suspicious mushrooms. Fresh commercially grown mushrooms can always be eaten with safety.

Yeasts

Yeast is any of a number of microscopic, one-celled fungi important for their ability to ferment carbohydrates in various substances. Yeasts in general are widespread in nature, occurring in the soil and on plants. Most cultivated yeasts belong to the genus Saccharomyces, those known as brewer’s yeasts are strains of S. cerevisiae.

Yeasts have been used since prehistoric times in the making of breads and wines, but their cultivation and use in large quantities was put on a scientific basis by the work of the French microbiologist Louis Pasteur in the 19th century. Today they are used industrially in a wide range of fermentation processes; medicinally, as a source of B-complex vitamins and thiamine and as a stage in the production of various antibiotics and steroid hormones; and as feed and foodstuffs. Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells, or the yeast may be pressed into cakes with some starchy material.

Molds

Mold is the fuzzy, cobweblike growth produced on organic matter by several types of fungi. Mold and mildew are commonly used interchangeably, although mold is often applied to black, blue, green, and red fungal growths, and mildew to whitish growths.

Black bread mold, Aspergillus niger, one of the most familiar molds, begins as a microscopic, airborne spore that germinates on contact with the moist surface of nonliving organic matter. It spreads rapidly, forming the mycelium (fungal body), which is made up of a fine network of filaments. The mycelium produces other clusters of rootlike hyphae, called rhizoids, which penetrate the organic material, secreting enzymes and absorbing water and the digested sugars and starches. Other clusters of hyphae called sporangiophores then reach upward, forming sporangia, which bear the particular color of the mold species. Upon ripening, the sporangia break open and the windborne spores land elsewhere to reproduce asexually. Some molds also reproduce sexually through conjugation of gamete cells by the joining of two specialized hyphae. The resulting zygote matures into a zygospore that germinates after a dormant period.

Lichens

Lichen is a living partnership of a fungus and an alga. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthesis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.

Most fungi are composed of threadlike tubular filaments called hyphae. Each individual hypha is surrounded by a fairly rigid wall usually made of chitin—the same material that forms the exoskeletons of insects. Hyphae that are partitioned by dividing cross walls are called septate hyphae, and hyphae without cross walls are called nonseptate hyphae. Fungal cells contain cytoplasm, which is a mixture of internal fluids and nutrients. Cytoplasm flows freely within the hyphae, providing nutrients wherever they are needed.

Hyphae grow by elongation at the tips and by branching to form an interwoven mat known as the mycelium. As the mycelium develops, it may produce large fruiting bodies or other structures that contain reproductive spores. Fruiting bodies are often the most visible structure of a fungus, usually growing above the soil or other surfaces so that the spores can be dispersed by air currents or other mechanisms. In contrast, the mycelium is usually hidden beneath the surface of the plant, animal, or other material it is decomposing. For example, a mushroom mycelium is typically buried beneath the soil surface, while its fruiting body, the familiar umbrella-shaped structure, sprouts from the ground.

Reproduction of Fungi

The wide variety of fungi demonstrate many reproductive methods. In general, most fungi reproduce by making tiny spores. Fungi typically produce large numbers of spores. A giant puffball, for example, produces an estimated 7 trillion spores.

Fungi typically follow a reproductive cycle that involves the production of sexual spores. These spores contain one or more nuclei and are usually haploid—that is, their nuclei contain one set of chromosomes. When environmental conditions are favorable, the spores germinate and develop into a mycelium that produces fruiting bodies with enormous numbers of sexual spores, which repeat the reproductive cycle. Some fungi produce asexual spores directly from hyphae, which then germinate to produce additional mycelium. The mycelium spreads rapidly, aiding the fungus in dispersal and colonization.

Many fungi can reproduce by the fragmentation of their hyphae. Each fragment develops into a new individual. Yeast, a type of unicellular fungus, reproduce by budding, in which a bump forms on the yeast cell, eventually partitioning from the cell and growing into a new yeast cell.

Fungi lack chlorophyll, the green pigment that enables plants to make their own food. Consequently, fungi cannot synthesize their own food the way plants do. In order to feed, fungi release digestive enzymes that break down food outside their bodies. The fungus then absorbs the dissolved food through its cell walls.

Depending as they do on outside sources for food, fungi have developed various living arrangements that enhance their opportunities for food absorption. Some fungi live as parasites, feeding on living plants, animals, and even other fungi. Certain fungus parasites injure plants and animals, causing millions of dollars of damage to farm animals, crops, and trees each year. For example, the fungus Ophiostoma ulmi, which causes Dutch elm disease, has killed more than half of the elm trees in North America since it was accidentally introduced there in the 1930s.

Fungi that obtain their food by breaking down dead organisms or substances that contain organic compounds, such as starch and cellulose, are called saprobes or saprophytes. While they are invaluable decomposers of organic material, saprobes can also cause food spoilage and destroy wood products. During the American Revolution 1775-1783, more British ships were destroyed by wood-digesting saprobes than by enemy attack. Some saprobes even grow in aviation fuels, where they breakdown the fuels, destroying their usefulness.

Some fungi also form highly specialized relationships with other organisms. For example, the roots of most plants develop a mutually beneficial association with fungi to form mycorrhizae. Mycorrhizae greatly increase the nutrient-absorbing capacity of the plant root—the fungus absorbs minerals from the soil and exchanges them for organic nutrients synthesized by the plant. Fungi also form mutualistic associations with various animals. For example, leaf-cutting ants cut pieces of leaves and bring them into their underground nests, where they feed them to certain fungi. These fungi primarily live in ant nests, and the ants eat nothing but the fungi. Some termites and wood-boring beetles use fungi to break down the cellulose in wood, making the wood easier for the insects to digest. Plant parasites such as rusts invade plant cells via specialized structures called haustoria that absorb nutrients from the cell.

Fungus is a diverse group of organisms that—unlike plants and animals—obtain food by absorbing nutrients from an external source. The fossil record suggests that fungi were present 550 million years ago and may have evolved even earlier. Today thousands of different types of fungi grow on and absorb food from substances such as soil, wood, decaying organic matter, or living plants and other organisms. They range from tiny, single-celled organisms invisible to the naked eye to some of the largest living multicellular organisms. In Michigan for example, the underground portion of an individual Armillaria mushroom, a type of fungus, extends more than 12 hectares. Other fungi are among the longest-lived organisms on Earth—some lichens, a living partnership of a fungus and an alga, are thought to be more than 4,500 years old.

A large and widely distributed group of organisms, fungi perform activities essential to the functioning of all natural ecosystems. They are among the foremost decomposers of organic matter, breaking down plant and animal remains and wastes into their chemical components. As such, fungi play a critical role in the recycling of minerals and carbon. Fungi’s value to humankind is inestimable. Certain types of fungi, including several types of mold, have proven extremely valuable in the synthesis of antibiotics and hormones used in medicine and of enzymes used in certain manufacturing processes. Some fungi, such as mushrooms and truffles, are considered tasty delicacies that enhance a wide variety of recipes. Not all fungi are beneficial—some damage agricultural crops, cause disease in animals and humans, and form poisonous toxins in food.

Common fungi include mushrooms, puffballs, truffles, yeasts, and most mildews, as well as various plant and animal pathogens (disease agents), such as plant rusts and smuts. Some experts estimate that there are 1.5 million fungus species, of which approximately 100,000 have been identified. The unique characteristics of fungi led scientists to classify these important organisms into a separate kingdom, Kingdom Fungi.