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How to Prevent and Control Powdery Mildew

powdery mildew plant disease on a yellow summer squash plant and leaves
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AJCespedes/Shutterstock

Identify the Powdery Mildew Plant Disease

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.

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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.

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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 website companion of The Old Farmer's Almanac. She covers gardening, plants, pest control, soil composition, seasonal and moon c...
Read More from Catherine Boeckmann

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Comments

<a title="View user profile." href="/author/editors">The Editors</a> Sun, 09/20/2015 - 09:24

Powdery mildew should not affect your pumpkins now that the skins are hardened and cured. It is a fungus that grows on the surface of leaves and some fruit.

 <span>Isabelle Lachance</span> Tue, 09/08/2015 - 15:42
I have powdery mildew on my pumpkin plants (took me a while to realize what it was) and I have 8 pretty big pumpkins on them at the moment. half the leaves have died and we still have 1.5 months to go before halloween comes along. Can I save my pumpkins? if yes how? do I need to cut them off right away or can I leave them to mature a bit more? I dont want to cut them off now and they rot before halloween comes! Thank you!!
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 09/09/2015 - 15:37

Test to see if your pumpkins are mature: press the end of yout thumbnail into the flesh of the fruit; if little indentation is left in the fruit, the pumpkin is mature.

You could target the powdery mildew with spray, if it is not covering most of the plant. Consult a nursery for a recommendation.

When the plants are done, remove all of the decayed plant material to reduce overwintering fungus. Avoid fertilzing with too much nitrogen. Next season, avoid crowding plants.

 <span>Jesse Camilla</span> Sun, 09/06/2015 - 17:59
my pumpkin plant is severely infested with white powdery mildew after a week or two of rain. With more rain to come, is there a chance that spraying the leaves will help save the vines, or should I tear them out before it spreads?
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 09/09/2015 - 15:20

Several sources we consulted, Jesse, say that your pumpkins will be ok, esp this late in season and esp if they are strong and healthy otherwise. Dampness is not the best thing. Spraying, esp  with rain coming that may wash off the spray, may not be the best solution. If you do want to spray, consult a nursery about horticultural oil of a biological fungicide.

In future, consider planting resistant varietes, give the plants plenty of room to grow, and certainly full sun.

Here's hoping you get this before the rains!

 <span>Sandra Surface</span> Thu, 07/23/2015 - 16:05
I to am having trouble with black spots and powdery patches, thanks for all the information...My question can the water from a water softner harm roses or any plants?I planted 20 roses lost 10 and other annuals.......
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 07/29/2015 - 08:54

It's OK for a little while but not advised for the long-term due to the build-up of sodium. Water softeners take away calcium and magnesium and then leave sodium in the water. It is better to use the outside tap runs off the mains. Rain water, tap water, purified water, and boiled water are better options.

 <span>Jessica Martin</span> Mon, 06/01/2015 - 12:31
I have powdery mildew on my sunflowers and it killed them off and I would like to know if it will go away or do I have to remove my dead sunflowers and plant new ones next year? I really hope next year they will come back or can I do something now before the summer ends? I live in Tampa Fl where it is the start of the rain season every afternoon.
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 06/03/2015 - 14:26
Hi Jessica, You need to pull up and destroy the dead sunflowers. You can try to plant new seeds in a different area of your garden. Plant the seeds in full sun and space them so that they will have good air circulation. For prevention spray with a neem oil spray or a mixture of 1 teaspoon of baking soda blended into a quart of water.
<span>Kittygirl44</span> Wed, 11/19/2014 - 23:23
I bought a lilac bush (a young plant) a few months ago and a few days later I noticed it had powdery patches on it. I suspect it is powdery mildew. I planted it in my backyard a month ago. I didn't know what to do about the powdery patches so I haven't done anything about it yet. My lilac bush hasn't grown at all since I bought it and this concerns me. It is in partial sun--it gets the sun from the morning, and is shaded by trees in the afternoon. Any suggestions? Does it just take forever to grow? Will the powdery mildew kill it if I don't do anything? Please reply with suggestions. Thanks.