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Mildew is defined as a thin, superficial, usually whitish growth consisting of minute hyphae (fungal filaments) produced especially on living plants or organic matter such as wood, paper or leather. Molds are similar superficial, often "woolly", downy, or furry growths of unspecified color, typically on food or suggesting decay.
In horticulture, mildew is either species of fungus in the order Erysiphales, or fungus-like organisms in the family Peronosporaceae. It is also used more generally to mean mold growth. In Old English, mildew meant honeydew (a substance secreted by aphids on leaves, formerly thought to distill from the air like dew), and later came to mean mold or fungus.
What horticulturalists and gardeners often refer to as mildew is more precisely powdery mildew. It is caused by many different species of fungi in the order Erysiphales. Most species are specific to a narrow range of hosts, and all are obligate parasites of flowering plants. The species that affects roses is Sphaerotheca pannosa var. rosae.
Another plant-associated type of mildew is downy mildew, caused by fungus-like organisms in the family Peronosporaceae (Oomycota). They are obligate plant pathogens, and the many species are each parasitic on a narrow range of hosts. In agriculture, downy mildews are a particular problem for potato, grape, tobacco and cucurbits farmers.
The term mildew is often used generically to refer to mold growth, usually with a flat growth habit. Molds can thrive on many organic materials, including clothing, leather, paper, and the ceilings, walls and floors of homes or offices with poor moisture control. Mildew can be cleaned using specialized mildew remover, or substances such as bleach (though they may discolor the surface).
There are many species of mold. The black mold which grows in attics, on window sills, and other places where moisture levels are moderate often is Cladosporium. Color alone is not always a reliable indicator of the species of mold. Proper identification should be done by a microbiologist. Mold growth found on cellulose-based substrates or materials where moisture levels are high (90 percent or greater) is often Stachybotrys chartarum and is linked with sick building syndrome.[dead link] “Black mold,” also known as “toxic black mold,” properly refers to S. chartarum. This species commonly is found indoors on wet materials containing cellulose, such as wallboard (drywall), jute, wicker, straw baskets, and other paper materials. S. chartarum does not grow on plastic, vinyl, concrete, glass, ceramic tile, or metals. A variety of other mold species, such as Penicillium or Aspergillus, do. In places with stagnant air, such as basements, molds can produce a strong musty odor.