Mold, or Mould, the common name of many types of fungi and funguslike protists; the fuzzy growths these organisms form are also called molds. The growths consist of numerous individual molds growing in colonies. Some molds are saprophytes; they live on dead organisms such as decaying plants or animals and on nonliving organic substances such as food, paper, and fabrics. Other molds are parasites; they obtain nourishment from a live host.

Molds have many harmful effects. For example, molds often grow on breads, pastries, jellies, and dairy products. They can damage stored grain, fruit and vegetables, and livestock feed, thereby causing serious financial loss to farmers. They can also cause diseases, such as gray mold, in garden plants. Athlete's foot as well as other types of ringworm are skin diseases caused by parasitic molds. Mold growth is prevented by maintaining dry, airy surroundings; by heat-radiation techniques in the processing of food; and by using fungicides.

Molds, however, also have many beneficial effects. They are instrumental in the decay of dead things, thereby aiding in the elimination of debris. Molds are the source of such antibiotics as penicillin. Molds are used in making such cheeses as Roquefort and Camembert and in the commercial production of such biochemicals as enzymes and hormones.

Structure and Reproduction

Molds are tiny, multicelled organisms made up of branching filaments called hyphae (singular: hypha). Some of the hyphae are embedded in the material on which the molds grow and are called vegetative hyphae. Many molds also have other hyphae, called aerial hyphae, which absorb oxygen from the air. Rhizoids, hairlike filaments resembling roots, grow from the vegetative hyphae of some molds. Fragments of vegetative hyphae that break off can develop into new individual molds. Some molds develop filamentous runners, called stolons, which give rise to new individuals.

A mold normally reproduces by forming spores, either sexually or asexually. (A spore is a one-celled reproductive body.) Sexual spores are formed by the joining of two hyphae of the same or different molds. Asexual spores are produced by fruiting bodies, which, in the case of most molds, form at the tips of special types of aerial hyphae, called fertile hyphae.

The various species of molds differ in the types of asexual spores that they produce. Sporangiospores and conidia are the most commonly produced asexual spores. Sporangiospores are produced within a rounded structure called a sporangium. Conidia are long chains of naked spores, produced and supported by a swollen structure called a sterigma.

Most molds produce spores that are distributed by air currents. Water molds, which live in freshwater, produce spores that propel themselves through the water with whip-like structures called undulipodia. Depending on the species of mold, spores may be of various colors, including blue, green, and black. The edge of a mold growth is whitish while the center—the mature portion containing the spores—is pigmented.

Most molds belong to the divisions Zygomycota, Ascomycota, or Deuteromycota of the kingdom Fungi. Water molds belong to the phylum Oomycota of the kingdom Protista.