Microfungi
Microfungi or micromycetes are fungi—eukaryotic organisms such as molds, mildews and rusts—which have microscopic spore-producing structures. They exhibit tube tip-growth and have cell walls composed of chitin, a polymer of "N"-acetylglucosamine. Microfungi are a paraphyletic group, distinguished from macrofungi only by the absence of a large, multicellular fruiting body. They are ubiquitous in all terrestrial and freshwater and marine environments, and grow in plants, soil, water, insects, cattle rumens, hair, and skin. Most of the fungal body consists of microscopic threads, called hyphae, extending through the substrate in which it grows. The mycelia of microfungi  produce spores that are carried by the air, spreading the fungus. Many microfungi species are benign, existing as soil saprotrophs, for example, largely unobserved by humans. Many thousands of microfungal species occur in lichens, forming symbiotic relationships with algae. Other microfungi, such as those of the genera "Penicillium", "Aspergillus" and "Neurospora", were first discovered as molds causing spoilage of fruit and bread. Certain species have commercial value. "Penicillium" species are used in the manufacture of blue cheeses and as the source of the antibiotic penicillin, discovered by Sir Alexander Fleming in 1928, while "Fusarium venenatum" is used to produce Quorn , a mycoprotein food product. Harmful microfungi. Microfungi can be harmful, causing diseases of plants, animals and humans with varying degrees of severity and economic impact. The irritating human skin disease known as athlete's foot or tinea pedis is caused by species of the microfungal genus "Trichophyton". Microfungi may cause diseases of crops and trees which range in severity from mild to disastrous, and in economic importance from beneficial to seriously costly. The mould "Botrytis cinerea" can cause spoilage of crops including grapes, but is also responsible for the "noble rot", which concentrates sugars in the grapes used to make  the intensely sweet and concentrated Sauternes dessert wines from the Bordeaux region of France. Dutch elm disease, which has ravaged elms across Europe and North America in the last 50 years, is caused by the microfungi of the genus "Ophiostoma". Rice blast, a devastating fungal disease of cereals including rice, wheat and millet, is caused by the phytopathogenic Ascomycete fungus "Magnaporthe grisea". In the built environment, the toxic fungus "Stachybotrys chartarum" causes damage to damp walls and furnishings, and may be responsible for sick building syndrome. Types of epidermal microfungal infections are: Diversity. Within the United States, approximately 13,000 species  of microfungi on plants or plant products are 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 the nation's natural heritage. Based on the number of species reported in the literature and those represented in the collections; the number of microfungi known in the United States is estimated at 29,000 species. In areas of the world where fungi have been well studied, the ratio of vascular plants to fungi is about 6 to 1. This suggests that there may be as many  as 120,000 species of fungi within the United States and 1.5 million worldwide.