Microfungi: Molds, Mildews, Rusts, and Smuts | ||
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 by Amy Y. Rossman U.S. Department of Agriculture |
| Fungi are a group of organisms that exist as a vast network of tiny threads growing in and out of all kinds of organic matter. As they grow, the threads secrete enzymes that break down the substances around them, releasing nutrients into the environment. Without fungi, the world would be completely covered with organic debris that would not rot, and nutrients would not be available for plant growth. All plants would die. | ||
| Microfungi include the organisms that are called molds and mildews as well as rusts and smuts, which cause plant diseases. They grow in all substrates, including plants, soil, water, insects, cows' rumen (see glossary), hair, and skin. Microfungi are said to be small because only part of the fungus is visible at one time, if at all. The visible parts produce thousands of tiny spores that are carried by the air, spreading the fungus. Most of the fungal body consists of microscopic threads extending through the substrate in which it grows. The invisible fungal structure may be extremely large, often extending for miles as, for example, the "humongous fungus" occurring in the north-central United States (Rensberger 1992). | ||
| Among the multitudinous molds are humble servants such as Penicillium notatum, the source of penicillin, and Tolyposporium niveum, a producer of cyclosporin, the immune-system suppressant used for organ transplant operations. In sustainable agriculture the fungal performers are agents of biological control and crop nutrition, helping the environment through the reduced use of chemical pesticides and fertilizers. Fungi can stop a hoard of locusts by attacking the chitinous insect exoskeleton or control nematodes that destroy the roots of crop plants (CAB 1993). Although strains of fungi can degrade plastics and break down hazardous wastes such as dioxin (Jong and Edwards 1991), only a fraction of these fungi have been screened as beneficial organisms. | ||
| Microfungi can also be harmful, causing the irritating human affliction known as athlete's foot as well as disastrous diseases of crops and trees. The potato famine in Ireland during the mid- to late 1800's was caused by a fungus called Phytophthora infestans that rotted the potato crops for several years (Large 1962). Because of this disease, many Irish immigrated to the United States. Once the nature of the disease was determined, a solution based on fungus control was found. Knowing what fungi exist, where they occur, and what they do is essential. | ||
Diversity of Microfungi |
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| The microfungi are the most diverse group of all the fungi but the least understood or documented. Only about 5%-10% of all fungal species have been described, much less characterized and put to use or controlled. Investigations to explore the diversity of microfungi have shown that they are much more diverse than previously thought. Very small samples of tropical rainforest leaf litter yielded up to 145 different species of microfungi (Bills and Polishook 1994). About 200,000 fungal species have been described worldwide (Reed and Farr 1993), yet an estimated 1-1.5 million species may exist (Hawksworth 1991; Rossman 1994). | ||
| Within the United States, information has been published about 13,000 species of microfungi on plants or plant products (Farr et al. 1989), probably only a fraction of the species thought to exist. Specimens of microfungi are housed in the U.S. National Fungus Collections and other institutions that serve as reservoirs of information and documentation about our nation's natural heritage. By comparing the species reported in the literature with those represented in the collections, one can estimate the number of microfungi known in the United States at 29,000 species (Farr et al. 1989). In areas of the world where fungi have been well studied, the ratio of vascular plants to fungi is about 6 to 1, suggesting that there may actually be 120,000 species of fungi within the United States. | ||
Internet Information |
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| Although the numbers and kinds of fungi in the United States are not known, information about the microfungi associated with plants and plant products in the United States is available over Internet at this telnet address: FUNGI.ARS-GRIN.GOV. After the word OK appears on the screen, type login user; when prompted for a password, type user. By doing this, anyone can find out what fungi might occur on the flowers in his or her own backyard. Data can also be consulted on accurate scientific names of microfungi, recent literature on plant-associated fungi, specimens in the U.S. National Fungus Collections, and records of microfungi on plants throughout the world. In an instant, reports of fungi can be consulted by those making land-management decisions or helping a farmer control a disease. | ||
Survey and Inventory Needs |
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| Knowing which microfungi occur within the United States provides information upon which plant quarantine decisions are made. A wrong decision allowing entry of a harmful pathogen can profoundly affect this nation's biological resources. In the eastern United States, a devastating disease called chestnut blight, caused by Cryphonectria parasitica and unknowingly imported from Europe on logs, killed virtually all the towering chestnut trees that once dominated our forests in the last century (Anagnostakis 1987). Now on the forest floor only skeletons of the trees can be seen with decay fungi rotting the bleached "bones" of these fallen giants. | ||
| Another disease, dogwood anthracnose, occurs on flowering dogwood trees in both the eastern and western United States. The causal fungus, Discula destructiva, was unknown until 1991 (Redlin 1991). Still unknown is whether this fungus was imported or was already present in the United States before its appearance as dogwood anthracnose. Because microfungi are small, their existence may not be noticed until they cause serious diseases. | ||
| A program to inventory and monitor microfungi in the United States does not exist at present; thus it is impossible to determine if species of microfungi are increasing or declining. Efforts to document the biodiversity of microfungi in the United States are limited to reports by plant pathologists who encounter disease-causing organisms or search for useful biological-control organisms. Information about the occurrence and biology of microfungi will increase the ability to make accurate decisions about the importation of agricultural products, to control microfungi already present, and to determine if beneficial microfungi are being lost because of habitat destruction. With increased knowledge the unexplored world of microfungi can be put to work to solve our most pressing environmental and agricultural problems. | ||
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U.S. Department of Agriculture U.S. National Fungus Collections Beltsville, MD 20705 |
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| References | |
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Anagnostakis, S. 1987. Chestnut blight: the classical problem of an introduced pathogen. Mycologia 79(1):23-37. CAB. 1993. Locust project enters phase two. Commonwealth Agricultural Bureau (CAB) International News. June. p. 4. Bills, G.F., and J.D. Polishook. 1994. Abundance and diversity of microfungi in leaf litter of a lowland rain forest in Costa Rica. Mycologia 86:187-198. Farr, D., G. Bills, G. Chamuris, and A. Rossman 1989. Fungi on plants and plant products in the United States. American Phytopathological Society Press, St. Paul, MN. 1,152 pp. Hawksworth, D.L. 1991. The fungal dimension of biodiversity: magnitude, significance, and conservation. Mycological Res. 95:641-655. |
Jong, S.C., and M.J. Edwards. 1991. American type culture collection catalogue of filamentous fungi, 18th ed. Rockville, MD. 667 pp. Large, E.C. 1962. Advance of the fungi. Dover, New York. 488 pp. Redlin, S.C. 1991. Discula destructiva sp. nov., cause of dogwood anthracnose. Mycologia 83:633-642. Reed, C.F., and D.F. Farr. 1993. Index to Saccardo's Sylloge Fungorum. Volumes I-XXVI IN XXIX 1882-1972. Reed Herbarium, Darlington, MD. 884 pp. Rensberger, B. 1992. Underground goliath. Michigan mushroom over 1,500 years old. Washington Post, April 2. Rossman, A.Y. 1994. A strategy for an all-taxa inventory of fungal biodiversity. In C.I. Peng, ed. Biodiversity and terrestrial ecosystems. Bull. of the Academy Sinica Institute Botany. In press. |