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How to Prevent and Control Powdery Mildew

powdery mildew plant disease on a yellow summer squash plant and leaves
Image Credit:
AJCespedes/Shutterstock
Written By: Catherine Boeckmann Executive Digital Editor and Master Gardener

Wondering about white spots and splotches on your plants? The fungal plant disease called powdery mildew is a common problem in gardens, infecting a wide variety of plants and reducing the quality and quantity of flowers and fruit. Learn how to identify, prevent, and treat powdery mildew.

What is Powdery Mildew?

Powdery mildew is a fungal disease that affects a wide variety of plants. There are many different species of powdery mildew, and each species attacks a range of different plants. In the garden, commonly affected plants include:

When the fungus begins to take over one of your plants, a layer of mildew made up of many spores forms across the top of the leaves. These spores are then carried to other plants by the wind. Powdery mildew can slow down the growth of your plant and, if the infection is severe enough, will reduce fruit yield and quality.  According to Gary W. Moorman, Ph.D., of Pennsylvania State University, “All the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce.”

How Does Powdery Mildew Spread?

Powdery mildew spores typically drift into your garden with the wind. Still, if you’ve had powdery mildew occur in the past, new outbreaks may also arise from dormant spores in old vegetative material or nearby weeds.

Unlike many other fungal diseases, powdery mildew thrives in warm (60°–80°F / 15°–27°C) and dry climates, though it requires fairly high relative humidity (i.e., humidity around the plant) to spread. In cooler, rainy areas, it does not spread as well, and its growth is also slowed by temperatures higher than 90°F (32°C). It tends to affect plants in shady areas more than those in direct sun.

Identification

How to Identify Powdery Mildew

  • Plants infected with powdery mildew look as if they have been dusted with flour.
  • Powdery mildew usually starts as circular, powdery white spots, which can appear on leaves, stems, and sometimes fruit.
  • Powdery mildew usually covers the upper part of the leaves but may grow on the undersides as well.
  • Young foliage is most susceptible to damage. Leaves turn yellow and dry out.
  • The fungus might cause some leaves to twist, break, or become disfigured.
  • The white spots of powdery mildew will spread to cover most of the leaves or affected areas.
  • The leaves, buds, and growing tips will also become disfigured. These symptoms usually appear late in the growing season. 

Not certain yet? Use the Almanac’s Plant Problem ID tool to understand out what’s wrong with your plant.

Control and Prevention

How to Prevent Powdery Mildew

As with all pests and diseases, the best means of controlling powdery mildew is proactive prevention. 

  • Choose plants for your garden that are resistant to powdery mildew. Many mildew-resistant varieties of cucurbits (melons, cucumbers, squash, etc.) have been developed and are available from major seed suppliers.
  • Plant in sunnier spots, as powdery mildew tends to develop more often in shady areas.
  • Selectively prune overcrowded areas to increase air circulation around your plants; this helps to reduce relative humidity.
  • Watering from overhead can help to wash spores off leaves. However, wet foliage can often contribute to developing other common diseases, so it’s best not to rely on this as a prevention tactic.

Homemade Powdery Mildew Prevention

Effective organic fungicides for treating powdery mildew include sulfur, lime-sulfur, neem oil, and potassium bicarbonate. These are most effective when used before infection or when you first see signs of the disease.

  • Baking soda has been proven by many gardeners to be effective in treating powdery mildew. Mix 1 teaspoon of baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill the fungus that it comes into contact with.
  • Milk spray is another effective home remedy. Dilute the milk with water (typically 1:10) and spray on roses at the first sign of infection or as a preventative measure.

Fungicides

There are many fungicides, especially for rose bushes, that are highly effective with low toxicity, no residue, and long duration. One example is Triadimefon. It can be sprayed with 1000–1200 WP of 15% wettable powder, 1 time intervals of 10 days, and 2–3 times. But check with your local nursery for fungicides approved in your area.

How to Control Powdery Mildew 

Once plants are heavily infected, it’s very difficult to get rid of the disease, so focus on preventing it from spreading to other plants. Remove all infected foliage, stems, and fruit and destroy them, either by throwing them in the trash or by burning them. Do not compost any infected plant, as the wind can still spread the disease and persist in the composted materials.

After pruning off infected parts, do not allow pruning shears to touch healthy leaves. First, sterilize your pruners with rubbing alcohol.

Plant Disease

About The Author

Catherine Boeckmann

Catherine Boeckmann

Executive Digital Editor and Master Gardener

Catherine Boeckmann is the Executive Digital Editor of Almanac.com, the digital home of The Old Farmer’s Almanac. She leads digital editorial strategy and publishing across a wide range of Almanac sub...

Read more from Catherine Boeckmann

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Comments

<a title="View user profile." href="/author/editors">The Editors</a> Tue, 06/13/2017 - 13:48

It sounds like powdery mildew. See guidance above and

• plant roses in full sunlight.

• space plants to allow ample air circulation

• fertilize to maintain vigor but do not overfertilize

• avoid wetting leaves when watering; use drip irrigation, if possible

• prune affected canes and collect and destroy cuttings (do not compost)

• use fungicide with care

• in future, select rose varieties that are resistant

 <span>Pawar Bhalchandra</span> Thu, 10/27/2016 - 12:04
Would like know about relationship between disease powdery mildew and environmental factors specifically Temperature, Relative Humidity, Rain Fall and Wind speed. Is there any quantitative MODEL which defines relationship amongs them in control of disease.
<span>Alison</span> Sun, 10/16/2016 - 02:42
Can you tell me what to spray to rid the plant from this pest. It has spread to other plants in the area.
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 10/18/2016 - 17:11

Mite infestations often occur when plants are stressed from insufficient water, so be sure you are watering enough. General purpose insecticides can exacerbate the problem. Instead use an insecticidal soap according to the directions on the label to knock the population back.

 <span>Deirdre Madgwick</span> Sat, 07/16/2016 - 17:35
ILLEUS GALBULA: little black beetle with with yellow spots (looks like a ladybird, but with louse-like larvae) eats mould, but unfortunately I took a couple of seasons to realise that they also spread mould spores in their vicinity to provide themselves with a food source. :( There isn't enough Baking Soda, the plant dies .......
<span>Kurt Kellerall</span> Sat, 07/16/2016 - 13:59
Can this white mildew become solid, and fall to the ground like crystal: When stepped on crushes into white powder?
<span>Elrene Cowan</span> Sat, 08/25/2018 - 17:40
We have clumps of white mildew on the dirt around some of our plants and on the trunk of one of our ferns, which is virtually impossible to remove other than spraying it with something. What to spray the fern with is the question, and can we use the same solution on the ground? Thank you, Elrene
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 08/27/2018 - 11:24

The fern should be safe if sprayed with the solution described above and sure, spray it on the ground:

If you don’t want to use fungicides, try spraying your plants with a bicarbonate solution:

  • Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.
 <span>Gregorio Cruz</span> Thu, 07/14/2016 - 19:31
My pear tree when I moved in to my house last year had some leaves, by this time last year it had small fruit growing, now it seems that it is dying. No fruit has grown or there hasn't been any leaves. I don't know what has gone wrong with my pear tree. The pear tree is in my back yard. I also have a tree, it grows small green pea shape balls, but this year it hasn't grown anything either, and seems to dying only partial of the tree has leaves. but, I feel it end is at hand. What could have come on my property to kill my tree?
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 07/15/2016 - 16:16

Hi Gregorio,

Since both trees are failing, I suspect it is a soil-related issue--there are many pathogens that live in the soil that can kill different tree species. There is really only one way to know for sure, and that is to have your soil tested by a cooperative extension agent, who will be able to identify any problems and provide recommendations.