Questions & answers about mildew (really mold) in buildings, what's the difference between mildew and mold? Does mildew grow in buildings? How do we get rid of mildew on plants or other surfaces?
Mildew growth & contamination: How do we get rid of, clean off, and prevent future growth of mildew?
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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. This article explains the clean-up or clean off of mildew, how to get rid of mildew, and how to prevent mildew growth on house plants and in home gardens.
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.
Mildew: Mildew is a proper subset of the broader mold family. It is a plant parasite.
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.
If you see mildew growth on dead plant leaves, it surely developed while the plant leaf was alive (and damp or wet and perhaps shaded).
How do I Get Rid of Mildew?
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.
You will probably need to move the plant outside when weather permits, so that the plant can be cleaned and treated with a plant mildewcide or fungicide. Treat the plant with a garden mildewcide intended for that purpose. If your plant is producing edible fruits or vegetables, be sure to choose and follow the instructions on a mildewcide approved for use on edible plants. More detail on curing mildew follows.
How to Prevent Mildew From Growing On Plants
Keep your plants healthy; a weakened plant is more vulnerable to mildew.
Use a mildew resistant potting soil or cultivar.
If your plant is one prone to mildew infection, plant it in the sun, not the shade.
Crop rotation is used for agricultural plant maintenance including prevention of mildew
Curing Mildew On Plants
Most experts advise cutting away all mildew-infected parts of the plant; If the plant is almost entirely covered with mildew, it may be lost, and/or it may not survive extreme pruning. We had success with the mildew-infected jasmine shown at the top of this page by the following few steps. (Our focus in this article is on house and home garden plants, and on building mold and mold problems, not on agricultural applications).
Moving the mildew-infected plant outdoors (if it is a potted house plant)
Washing the plant with plain water spray to remove the heavy mildew (but see our warning below about watering mildew-infected plants)
Pruning away the worst-infected mildew-coated sections of the plant - this removes infected material and improves air circulation around and through the plant, keeping it more dry
Keeping the plant in the sun
Other Steps to Cure or Prevent Mildew on Plants
Spray the plant with a mildewcide intended for use on that plant type; mildewcides that contain neem oil, sulfur, copper, or baking soda (one tablespoon of baking soda and 1/2 teaspoon of liquid detergent to a gallon of water) (potassium bicarbonate) may be effective; If the plant is an ornamental (you won't be eating it), chemical fungicides such as triforine may be effective.
Water the plant from below, around its roots; don't keep wetting the plant leaves by watering it from above
If your plant is potted such as our Jasmine shown here, moving it into the sun is easy and weeds are not a problem;
If your plant is in the ground, keep weeds away from the plant as they may be a source of problem molds including mildews, and because they compete for nutrients, sun, air, etc.
Don't leave leaves and plant debris around your mildew-sensitive plantings
Disinfect garden tools used to maintain your plants
Monitor soil fertility and, when the plant is not suffering from mildew, fertilize it properly
Our references include additional advice from expert sources on mildew prevention on crops, flowers, shrubs, such as the USDA
Questions & Answers regarding this article
Questions & answers about mildew (really mold) in buildings, what's the difference between mildew and mold? Does mildew grow in buildings? How do we get rid of mildew on plants or other surfaces?
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Atlas of Clinical Fungi, 2nd Ed., GS deHoog, J Guarro, J Gene, & MJ Figueras, Centraalbureau voor Schimmelcultures, Universitat Rovira I Virgili, 2000, ISBN 90-70351-43-9
Mycology, Fundamentals of Diagnostic, Fran Fisher, Norma B. Cook, W.B. Saunders Co. 1998, ISBN 0-7216-5006-6
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.
Atlas of Indoor Mold, Online Clinical Mold Atlas, Toxins, Pathogens, Allergens and Other Indoor Particles - Medical Health Effects of Mold (separate online document)
Building Floods: quick steps after a building flood or plumbing leak can prevent costly mold contamination
Classes of Mold: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
"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.
Fungi, Identifying Filamentous, A Clinical Laboratory Handbook, Guy St-Germain, Richard Summerbell, Star Publishing, 1996, ISBN 0-89863-177-7 (English)
Mold Action Guide: Step-by-Step Instructions, What to do about mold, mildew, and other indoor allergens
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