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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

<span>Annie Herbert</span> Sat, 10/05/2019 - 04:44
Will I have to dig up new plants .honey suckle .
<span>Sian</span> Sat, 06/22/2019 - 14:42
Hey, there! I just noticed a peculiar thing I've never seen before; three leaves of my rose-scented geranium have what appears to be salt in their gullies. I thought at first I was looking at powdery mildew (a familiar foe), until I noticed the loose, sparkling quality of the substance. Are the leaves leaching salt? I do not fertilize, never need to, and the effected leaves still look as healthy as ever (I have still removed them in case the substance is otherwise nefarious). What could this be? For information's sake, this geranium is planted in a half-gallon SmartPot.
<span>Ron Lane</span> Fri, 06/07/2019 - 13:45
This is one of the better PM articles I have seen on the net. One correction is that baking soda does not stop powdery mildew, it only helps to prevent it. You may spray it on and think your PM is gone, but if you come back in a couple of hours your PM has returned. Pristine is a BASF product that actually eliminates PM. Some form of it might be available to homeowners. Of the softer chemistries, Circadian Sunrise is the only one I've seen that consistently stops PM cold. The problem with that product is that the smallest unit presently sold is a 2.5 lb concentrate, which is far more product than most homeowner will use in their gardens over the course of a year or more.
<span>Anna</span> Thu, 11/01/2018 - 20:51
I have white powdery substance on my zinnias. Will this make the seeds in the flowers be bad for next year?
<span>Dave</span> Fri, 11/16/2018 - 15:40
Yes, the mother plant has been infected and will pass along those genetics to the seedlings which will also carry the disease.
<span>Marc</span> Tue, 06/11/2019 - 09:27
That's not how genetics works, Dave.
<span>Fredric Lacarbonara</span> Mon, 10/08/2018 - 13:52
the info I have is to spray with milk/water mix or baking soda/water mix...I am doing that , but is it a one time treatment or multiple treatments?......please advise how many times and how often to spray,,,ty
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 10/09/2018 - 16:13

It will likely require multiple treatments to keep the fungus at bay. Test the mixture on one plant first, waiting at least 24 hours to be sure that there are no negative effects. Then spray all affected plants at least once a week. After three or four applications, wait to see if symptoms return before spraying again.

 <span>Lisa In Texas</span> Fri, 10/05/2018 - 22:39
I have been fighting this in my house for almost 4 years. I have tossed out so many plants I can’t tell you how much money its cost. I am wondering if two giant chinese evergreens are causing it - the have never had it at least not visibly. I am ready to torch the house or at least throw away every plant I have and fumigate / steam clean every nook and cranny including AC ducts. I am at my wits end.
<span>Jack G.</span> Mon, 11/13/2017 - 20:59
I'm just sick, I just got 2 new cherry trees 1 Bing, 1 Black Tartarian cherry tree and I spread hay down first before I mulched around the bottom of the trees. The hay I used has white powdery looking stuff in the middle of it, not knowing I put it down and put mulch on top of it. It is Nov 13th here in Wisconsin and cold out. Does the hay have white powdery mildew and will this now infest the trees next spring? Should I dig up all of the mulch and get rid of it all? What should I do?