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Mildew is a form of
fungus A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
. It is distinguished from its closely related counterpart,
mould A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. No ...
, largely by its colour: moulds 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 mould and mildew produce distinct offensive odours, and both have been identified as the cause of certain human ailments. In horticulture, mildew is either species of
fungus A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
in the order Erysiphales, or fungus-like organisms in the family '' Peronosporaceae''. It is also used more generally to mean mould 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 mould or fungus. Mildew grows on damp cloth, leather, or on plants, and growing on leaves can damage the plant.


Household varieties

The term mildew is often used generically to refer to mould growth, usually with a flat growth habit. Moulds 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 discolour the surface). There are many species of mould. The black mould which grows in attics, on window sills, and other places where moisture levels are moderate often is '' Cladosporium''. Colour alone is not always a reliable indicator of the species of mould. Proper identification requires a microbiologist or mycologist. Mould growth found on cellulose-based substrates or materials where moisture levels are high (90 percent or greater) is often ''
Stachybotrys chartarum ''Stachybotrys chartarum'' (, ), also known as black mold or toxic black mold, is a species of microfungus that produces its conidia in slime heads. It is sometimes found in soil and grain, but the mold is most often detected in cellulose-rich ...
.'' "Black mould," also known as "toxic black mould", properly refers to ''S. chartarum''. This species is commonly found indoors on wet materials containing cellulose, such as wallboard (drywall), jute, wicker, straw baskets, and other paper materials. ''S. chartarum'' does not, however, grow on plastic, vinyl, concrete, glass, ceramic tile, or metals. A variety of other mould species, such as ''
Penicillium ''Penicillium'' () is a genus of ascomycetous fungi that is part of the mycobiome of many species and is of major importance in the natural environment, in food spoilage, and in food and drug production. Some members of the genus produce ...
'' or '' Aspergillus'', may appear to grow on non-cellulosic surfaces but are actually growing on the
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
that adheres to these surfaces. Glass, plastic, and concrete provide no food for organic growth and as such cannot support mould or mildew growth alone without biofilm present. In places with stagnant air, such as basements, moulds can produce a strong musty odour. The pink "mildew" often found on plastic shower curtains and bathroom tile is actually a red yeast, '' Rhodotorula''.


Environmental conditions

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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
of between 62 and 93 percent), and reasonable warmth ( to is optimal, but some growth can occur anywhere between freezing and ). 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. 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 therefore requires a balance between moisture and temperature. This can be achieved by minimizing the moisture available in the air. Air temperatures at or below will inhibit growth, but only if the
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
is low enough to prevent water condensation (i.e., the
dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will c ...
is not reached). With warmer temperatures, the water holding capacity of the air increases. This means that if the amount of water vapour in the warming air remains the same, the air will become drier (i.e. it has a lower
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
). This again inhibits fungal growth. Warm, growth-favoring temperatures coupled with ''high'' relative humidity, however, will 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 Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
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. 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 vapour.


See also

*
Downy mildew Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of c ...
* Phase I environmental site assessment * Powdery mildew * Obligate parasite


References

{{Reflist Building defects Plant pathogens and diseases