Mildew is a form of fungus. It is distinguished from its closely related counterpart, mold, largely by its color: molds appear in shades of black, blue, red, and green, whereas mildew is white. It appears as a thin, superficial growth consisting of minute hyphae (fungal filaments) produced especially on living plants or organic matter such as wood, paper or leather. Both mold and mildew produce distinct offensive odors, and both have been identified as the cause of certain human ailments.
Horticulturalists classify mildew differently than most other people. 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 requires a microbiologist or mycologist. 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. “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.
Mildew requires certain factors to develop. Without any one of these, it cannot reproduce and grow. The requirements are a food source (any organic material), sufficient ambient moisture (a relative humidity of between 62 and 93 percent), and reasonable warmth (77 °F (25 °C) to 88 °F (31 °C) is optimal, but some grown can occur anywhere between freezing and 95 °F (35 °C)). Slightly acidic conditions are also preferred. At warmer temperatures, air is able to hold a greater volume of water; as air temperatures drop, so does the ability of air to hold moisture, which then tends to condense on cool surfaces. Ironically, this can work to bring moisture onto surfaces where mildew is then likely to grow (such as an exterior wall). Preventing the growth of mildew requires a balance between moisture and temperature either in such a way that minimal moisture is available in the air and the air temperature is low enough to inhibit growth (at or below 70 °F (21 °C) without causing condensation to occur, or by in such a way that warmer air temperatures, without an actual change in the amount of water vapor in that air, is by its nature "drier" (has a lower relative humidity) than cooler air and will tend to inhibit mildew growth in this way. Warm temperatures coupled with high relative humidity set the stage for mildew growth.
Air conditioners are one effective tool for removing moisture and heat from otherwise humid warm air. The coils of an air conditioner cause moisture in the air to condense on them, eventually losing this excess moisture through a drain and placing it back into the environment. They can also inhibit mildew growth by lowering indoor temperatures. In order for them to be effective, air conditioners must recirculate the existing indoor air and not be exposed to warm, humid outside air. Ironically, some energy efficient air conditioners may cool a room so quickly that they do not have an opportunity to also effectively collect and drain significant ambient water vapor.
- mildew: Compact Oxford English Dictionary
- mildew: Merriam-Webster's Online Dictionary, 11th Edition
- The American Heritage Dictionary of the English Language, 1969, entry "melit-" in Appendix
- "Cleaning Mildew from Retractable Awnings". Shade & Privacy. Retrieved 15 September 2014.
- Peart, Virginia (October 2001), How to prevent and remove mildew (PDF), University of Florida IFAS Extension