How to Distinguish Between Mildew and Mold
Products labeled "mildewcide" and findings of "mildew" in buildings are speaking a bit loosely InspectAPedia® -
What's the difference between "mold" and "mildew" - does it matter?
Photographs of mildew - real mildew, growing on living plants
How to recognize mildew on plants
Why you won't find mildew on building surfaces (except living plants)
Questions & answers about mildew reported in buildings and found on plants.
This article explains the difference between mildew and mold (or other forms of mold).
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As we discuss in this article, mildew, a sub-class of molds, is an obligate parasite that grows only on living plants, and is generally white in appearance. Our page top photograph shows mildew growing on a jasmine plant at a Vassar College home in Poughkeepsie, New York. More photographs of mildew are included in this article.
Readers concerned with building mold contamination should see MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD. At this website, other photos of mold on indoor building surfaces may help you recognize
mold in buildings, recognize probably-cosmetic mold, and recognize stuff that is not mold and does not need to be tested. See MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for photographs that help recognize mold growth in buildings. See MOLD APPEARANCE - STUFF THAT IS NOT MOLD for other substances sometimes mistaken for mold in or on buildings. Also see Mold spores in the Home - a Photo ID Library for detection and identification of mold allergens
on indoor building surfaces.
What's the Difference between "Mold" and "Mildew"?
Our closeup photographs above show powdery mildew growing on the leaves of a jasmine plant. Mildew is a specific type of mold that grows on living plants. Just on plants. Notice that this mold (mildew) is white? Mildew is always white or perhaps light gray in color.
Mildew: Mildew is a proper subset of the broader mold family. It is a plant parasite.
Among the major groups of molds is a very large mold family, Basidiomycota (or basidiomycetes); this family in turn includes, among the many Basidiomycota members, we find a relatively small sub-group, the Ustilaginales, also familiarly called Smuts and Mildews.
Both smuts and mildews are parasites of living plants (in mycological terms these are obligate biotrophic pathogens) that cause serious crop damage as well as damage to ornamental shrubs, flowers, etc. See Kiss et als. for an example of mildew damage to tomatoes.
At left, a lab photo of mildew spores collected from the jasmine plant at page top. To understand our comment that mildew does not grow in or on buildings, notice our use of the words obligatebiotrophic pathogens - meaning that these fungi are pathogens that grow only on living plants and that they cause a disease in the plant.
Mold: In general, mold is a term encompassing a very wide family of organisms (the Fifth Kingdom) that includes more than a million and a half species. Lots of molds grow on lots of different organic substances, under a variety of conditions of light and temperature, but all molds require moisture and something organic on which to grow (paper, wood, paint, cloth, leather, plastic, etc.).
Building mold contamination: although a very large number of molds may grow on various building surfaces and building contents, there are about 200 "bad boys" of mold commonly found contaminating buildings. None of these include the mildews. See MOLD FREQUENCY in buildings for a table of the most common molds found in buildings, and also see MOLD ATLAS & PARTICLES INDEX.
Our photograph (left) shows black, brown, and gray mold growing on drywall in a basement that was flooded. This is mold, but it is not mildew. Different mold genera/species might be growing on the wood framing or insulation materials inside this moldy wall.
There are mold genera or species that can grow on a remarkably wide range of organic materials that are found both outdoors and inside, and that can appear in an wide range of colors (black, brown, red, green, gray, white, orange, tan, yellow, for example) and textures. See MOLD APPEARANCE - WHAT MOLD LOOKS LIKE and see MILDEW ERRORS - MOLD PHOTOS for examples of indoor or outdoor building mold mistaken for mildew.
Often specific mold genera or species prefer to grow on particular materials, so not all molds grow on all materials. (Most molds will not grow in mold culture media, for example.) Some molds even grow on top of (and eat) other molds - parasitic molds. Molds are capable of breaking down cellulose, for which we should be thankful. Even though we don't like to find mold breaking down paper covered drywall (paper is made of cellulose), if we didn't have a mold kingdom on earth, we'd all be buried in tons of un-decayed dead plant matter.
Mildews are divided into two sub-groups, with quite a few sub-species
Oidium-Erysiphe, familiarly named Powdery Mildew, and
Peronosporaceae, familiarly named Downy Mildew
What Does Mildew Look Like
Though both of these groups may also include other species, they all look similar on plants - white or gray powdery or splotchy deposits on plant leaves and stems. Mildew may appear on the plant's buds, flowers, fruits, in sum, just about on any live plant tissue, though not so likely on heavy dry bark such as on tree bark or on woody stems. See our plant photographs earlier in this article.
Our lab microphotograph (above-left) shows the hyaline (colorless) spores of Oidium-Erysiphe that we collected from the jasmine plant shown in the photographs in this article. In the microscope, mildew spores (Oidium-Erysiphe) are colorless; on a plant this mold species appears as a white powdery substance.
In sum, mildew is a white powdery fungus that is an obligate parasite found on living plants.
Does Mildew Grow on or in buildings?
No. Mildew grows on living plants.
But lots of people (incorrectly) call various building molds "mildew", and lots of mold cleanup products use the words mildew or mildewcide in their name and instructions.
A mycologist or a good text will tell you what substrates a particular mold has been
known to grow on and which types of material it prefers.
For example, mildew is unlikely to be found ever growing on an indoor surface (except for a house
plant), since mildew is a pair of sub-group of molds (powdery mildew and downy mildew) which grow only on living plants.
Our photograph of a white mold found growing on an indoor surface (left) is not mildew. It's a mold, but it is not mildew.
So you can see that indoor mold that your home inspector or "mold test expert" has called "mildew"
might be white but it is not mildew unless it is growing on a plant
might be other colors than white or gray, which means that it's certainly not mildew
Does it Matter Whether We Call Mold Found on or in a Building Mildew or Mold?
In a practical sense, making the mistake of calling an indoor or outdoor mold found on a building "mildew" is not a big deal. With the exception of avoiding spending on costly cleanup of harmless cosmetic molds or stains, the mold remediation and mold prevention procedures in a building are about the same regardless of mistakes in naming the mold.
You won't find mildew on or in a building on anything but a living plant, despite the names found on indoor or outdoor cleaning products that use the term "mildewcide" and "mildew-resistant product", or found in "mold reports" or in home inspections notwithstanding.
We think that mildew just a less scary name that some folks like to use to describe mold found in buildings, especially in real estate sales. We think that mildew is used on mold cleanup products or mold-resistant products also because of the widespread use of that word by consumers faced with an indoor mold problem.
What Does it Mean if Your Inspector or Mold Expert Calls it Mildew?
See MOLD EXPERT, WHEN TO HIRE for help in deciding if you need a mold expert or not. But if your "mold expert" or home inspector tells you that s/he found mildew growing in or on the building, that person may be less of an expert on finding, identifying, and removing problem mold than you hoped, and therefore
The mold test consultant or mold inspector may not be adequately skilled in finding the most important reservoirs of mold contamination in the building. If the mold inspector does not understand the basic mycology of what kinds of mold are likely to grow where in a building under what conditions, the inspector is unlikely to know where to look for problem mold and is at risk of reporting superficial mold contamination while completely missing the more serious problem mold reservoir in the structure. We discuss this problem further at MOLD APPEARANCE - WHAT MOLD LOOKS LIKE, at MOLD FREQUENCY in buildings, and at FIND MOLD in buildings, HOW TO.
If the mold test or mold consultant fails to identify the important mold reservoir in the building, advice given to a building occupant's doctor about building mold exposure may be incorrect.
The home inspector, mold inspector, or mold remediator's advice on what to do about building mold might also be less than fully accurate. See MOLD KILLING GUIDE for an example of this problem, and also see MOLD CLEANUP - MISTAKES to AVOID.
If your "mildew" is found on building surfaces, it is mold, but it is not mildew, and you need to follow normal mold cleanup procedures. See BASICS YOU NEED to FIND, TEST, REMOVE MOLD for a detailed step by step guide to removing problem mold and for identifying and fixing the cause of mold growth in the first place.
If you are having trouble tracking down a moldy smell in your building, try the suggestions we describe at ODORS & SMELLS DIAGNOSIS & CURE.
If your mildew is indeed growing on a living plant (tomatoes, grapes, crops, house plants, other plants) it can weaken or even kill the plant, at least by interfering with photosynthesis. Mildews on plants may be a species of mildew fungus that is plant specific. For example a mildew that grows on grapes may affect only those plants and may not infect nearby plants of other types, roses for example. But the conditions that cause powdery mildew to grow on plants invite infection of many plants in a given area. These include crowding (poor air circulation within or around the plant), and dampness or high humidity. If a plant is already stressed or weak from other conditions, it may be more susceptible to mildew infection as well.
While lots of "mildew cleaning" products are sold with the intention of removing mold from building surfaces (bathtub tile grout, for example), do not use such products on plants - you will probably kill the plant.
See MILDEW REMOVAL & PREVENTION for details on what to do about cleaning off or removing mildew, and how to both cure and prevent mildew.
Mold on or In buildings - Molds that are Mistaken for Mildew
See MILDEW ERRORS - MOLD PHOTOS for examples and details about indoor or outdoor building mold mistaken for mildew such as the moldy building exterior paint shown just below.
Questions & Answers regarding this article
Questions & answers about mildew reported in buildings and found on plants.
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Fifth Kingdom, Bryce Kendrick, ISBN13: 9781585100224, is available from the InspectAPedia online bookstore - we recommend the CD-ROM version of this book. This 3rd/edition is a compact but comprehensive encyclopedia of all things mycological. Every aspect of the fungi, from aflatoxin to zppspores, with an accessible blend of verve and wit. The 24 chapters are filled with up-to-date information of classification, yeast, lichens, spore dispersal, allergies, ecology, genetics, plant pathology, predatory fungi, biological control, mutualistic symbioses with animals and plants, fungi as food, food spoilage and mycotoxins.
Fungi, Identifying Filamentous, A Clinical Laboratory Handbook, Guy St-Germain, Richard Summerbell, Star Publishing, 1996, ISBN 0-89863-177-7 (English)
"Identification of two powdery mildew fungi, Oidium neolycopersici sp. nov. and O. lycopersici, infecting tomato in different parts of the world",
Levente KISS, Roger T. A. COOK, Gregory S. SAENZ, James H. CUNNINGTON, Susumu TAKAMATSU, Ian PASCOE, Marc BARDIN, Philippe C. NICOT, Y. SATO and Amy Y. ROSSMAN, Mycological Research (2001), 105:6:684-697 Cambridge University Press, The British Mycological Society 2001 provides an example of the problem of mildew damage to crops, from which we quote the abstract: A world-wide study of the Oidium species causing economic damage on tomato has identified two taxa using classical morphological, scanning electron microscope (SEM) and molecular phylogenetic analyses. The material consisted of a total of 25 tomato powdery mildew isolates and 29 herbarium specimens coming from all continents where tomatoes are grown. A taxon with non-catenate conidia widespread in Europe, Africa, North and South America and Asia was identified as an O. subgen. Pseudoidium species (teleomorph: Erysiphe sect. Erysiphe). Formerly mistaken for O. lycopersicum (or O. lycopersici), it is now recognised as a distinct species, O. neolycopersici sp. nov. A phylogenetic analysis of the internal transcribed spacer (ITS) sequences of the ribosomal DNA (rDNA) indicated that O. neolycopersici is closely related to Erysiphe macleayae, E. aquilegiae and other Pseudoidium species. Only a taxon with catenate conidia was found on Australian specimens. This was identified as a species of O. subgen. Reticuloidium (teleomorph: Golovinomyces sp.). Phylogenetic analysis of the rDNA ITS sequences showed that this species is closely related to O. longipes infecting eggplant. Because it is most likely to be the same species as the original O. lycopersicum, which was actually first described in Australia, this is here neotypified as O. lycopersici.
"Greenhouse Flower Crop Disease Prevention," USDA, originally from the Cornell University Extension, see www.reeis.usda.gov/web/crisprojectpages/209797.html Abstract is quoted in part just below: Greenhouse flower growers are annually faced with the need to manage new diseases caused by powdery mildews and downy mildews that disfigure crops and make them unsaleable. Information is not available to help them determine the source of the epidemics, to control the environment so it is not conducive to disease, to identify less-susceptible crops, or to use the most environmentally responsible chemical protectants.
"Disease Prevention Program for Certain Vegetable Crops," David B. Langston, Jr., Extension Plant Pathologist - Vegetables, University of Georgia (PDF document) original source: www.reeis.usda.gov/web/crisprojectpages/209797.html
"Disease Prevention in Home Vegetable Gardens,"
Patricia Donald,
Department of Plant Microbiology and Pathology,
Lewis Jett
Department of Horticulture, University of Missouri Extension - extension.missouri.edu/publications/DisplayPub.aspx?P=G6202
"Management of Powdery Mildew, Leveillula taurica, in Greenhouse Peppers," Ministry of Agriculture and Lands, British Columbia - Original source: www.agf.gov.bc.ca/cropprot/peppermildew.htm Below we provide a partial quote from the introduction to this excellent document: Powdery mildew, Leveillula taurica, became a new and serious threat to British Columbia’s greenhouse bell pepper industry early February, 2003. The disease was found simultaneously in two separate locations near Langley in southern British Columbia. Trace amounts of mildew developed on the foliage of cultivar Zamboni in one operation and 10-80% of the foliage of cultivar Triple Four in an organic operation. Reported pepper fruit yield losses have been 2 to 4 kg/m2. By summer, pepper powdery mildew was reported throughout British Columbia’s pepper greenhouses. Dutch research has shown a direct relationship between per cent mildew infection of the leaves and yield loss. One per cent mildew infection on the leaves results in a one percent yield loss. Studies show that the higher the level of powdery mildew infection the higher the loss of production. An early, heavy infection with mildew had about 30% loss of production compared to a later, lighter infection. Powdery mildew generally has caused 10-15% yield loss in NA greenhouse pepper crops. Greenhouse pepper growers need to follow an intensive disease prevention plan because it is very important that powdery mildew never gets out of hand. Once pepper leaves are infected with powdery mildew it is difficult to control; if left unchecked the crop can be entirely destroyed. Monitor the crop right from the start. Apply preventive fungicide sprays once the disease is detected. Carry out strict hygiene throughout the year and practice a thorough year-end clean up. There are no powdery mildew resistant pepper varieties currently available.
Also see:
First report of powdery mildew of greenhouse pepper caused by Leveillula taurica in British Columbia, Canada. Cerkauskas, R.F. and A. Buonassisi. Plant Disease 87: 1151. 2003.
First report of powdery mildew of greenhouse pepper caused by Leveillula taurica in Canada. Cerkauskas, R.F. and J. Brown. Plant Disease 83:781. 1999.
Suppression of powdery mildew on greenhouse-grown cucumber by addition of silicon to hydroponic nutrient solution is inhibited at high temperature. Schuerger, Andrew and W. Hammer. Plant Disease 87:177-185. 2003.
Thanks to reader Debra H. for discussing this topic and whose questions suggested this clarifying article 11/28/2009
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
"A Brief Guide to Mold, Moisture, and Your Home",
U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
US EPA - Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
US EPA - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol
"A Brief Guide to Mold, Moisture, and Your Home", U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
"Disease Prevention Program for Certain Vegetable Crops," David B. Langston, Jr., Extension Plant Pathologist - Vegetables, University of Georgia (PDF document) original source: www.reeis.usda.gov/web/crisprojectpages/209797.html
"Disease Prevention in Home Vegetable Gardens,"
Patricia Donald,
Department of Plant Microbiology and Pathology,
Lewis Jett
Department of Horticulture, University of Missouri Extension - extension.missouri.edu/publications/DisplayPub.aspx?P=G6202
Fifth Kingdom, Bryce Kendrick, ISBN13: 9781585100224, is available from the InspectAPedia online bookstore - we recommend the CD-ROM version of this book. This 3rd/edition is a compact but comprehensive encyclopedia of all things mycological. Every aspect of the fungi, from aflatoxin to zppspores, with an accessible blend of verve and wit. The 24 chapters are filled with up-to-date information of classification, yeast, lichens, spore dispersal, allergies, ecology, genetics, plant pathology, predatory fungi, biological control, mutualistic symbioses with animals and plants, fungi as food, food spoilage and mycotoxins.
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