For daily wit & wisdom, get the Almanac newsletter.

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

More Like This

Comments

<span>Rocky</span> Thu, 02/14/2019 - 20:24
Jack everything you're describing to me sounds like the hay possibly has mycelium growing on it and while PWM is a fungal culture Mycelium in and of itself can be very very beneficial to plants/trees. The Mycelium will grow throughout the same substrate the roots also colonize and create a symbiotic relationship with the roots themselves, while mycelium itself is started with other mediums hay is a much loved substrate to colonize after the mycelium has been birthed. By using a layer of hay beneath your trees you've given the mycelium that was either spores lying dormant in wait or living mycelium beneath the hay previously a nice secondary substrate to grow through before expanding into the substrate your roots have begun colonizing. I'm not going to pretend to know all the names of the different ways the relationship is symbiotic ;but I have recently been researching this topic a lot and can say for sure it's not just beneficial for one or 2 little things. I can't say that every strain of mycelium is going to be equally as beneficial obviously or even beneficial at all, some fungus (mycelium fruits fungus/mushrooms) like PWM use the plants themselves for hosts as mycelium itself does actually consume food rather than absorb nutrients through roots like plants ;but only certain types are threats to plants/trees. Chances are the Mycelium you seen on the hay would take a very very long time (many seasons) to reach the top layer ;but if it does it could very well start fruiting some mushrooms annually if that's the type of mycelium you seen (not all types produce mushrooms of course). Many strains of mycelium have a natural fruiting time in Oct./ Nov. and also really thrives at that time of year so it makes sense for the time of year you seen it, which is another part of what you said that makes me feel it wasn't PWM at all. "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" PWM wouldn't be living on hay at that time of year outside ;but mycelium definitely could! Laying down mulch on top of the hay is an interesting touch as many beneficial mycelium species really love wood as well! Anyways I know my reply is very late ;but hope you still find some useful information here and if you enjoy gardening at any level I highly recommend further investigating yourself as mycology is a very interesting hobby to get into.
<span>Linda Schwin</span> Fri, 07/28/2017 - 08:14
I have powdery mildew on my peonies. I sprayed with neem oil but now the plants look dry and damaged. Should I cut the plants all the way down? Will they come back next year if I do? Is there anything I can add to the soil? This is the second year this has happened to these plants. Last year I did not treat them with anything.
<span>Simon Viola</span> Mon, 06/18/2018 - 20:06
Every year after the season I start getting fungus on my peonies what can I use to prevent this.
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 08/27/2018 - 11:29

Are your peonies crowded? You might try thinning them out, per above to improve circulation: Selectively prune overcrowded areas to increase air circulation; this also helps to reduce humidity around your plants.

You could also try the spray solution listed above:

  • 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>Ken Diehn</span> Thu, 07/20/2017 - 22:10
You mention to destroy plants and not compost. What method is best? Will burning cause the mildew spread to other plant? How about other diseases on plants, will burning spread disease?
<span>Cindy</span> Sun, 10/22/2017 - 14:33
Did you ever find if powdery mildew is spread by burning infected plants?
<span>Oliver</span> Thu, 10/18/2018 - 19:59
I have been doing a lot of research on powdery mildew. Temperatures above 90 degrees kill it.
<span>Peter</span> Thu, 07/20/2017 - 11:11
Our concord grape vine seems to have "expired", after 5 years of fairly reliable fruity growth. How can I tell if I need to start over, or find some "life" in what remains? Sad. The neighboring vines are doing well.
<span>Dev Prashad</span> Mon, 06/12/2017 - 21:17
Would like to know what some white fuzzy looking stuff on the rosebuds might be and what can be done to eliminate it. Looks like the fuzz n peaches but is a bright white in colour
<span>Cass</span> Wed, 06/05/2019 - 09:25
Sounds more like aphids to me. Roses are certainly prone to white mealy bugs. The “bright white” is what convinced me. Mildew isn’t typically bright white but the infuriating little critters definitely are. If it’s them, alcohol will kill them only if it actually touches them. Neem will repel them. Most commercial pesticides are useless thanks to the typical resistance. I knock them off using water pressure and then use Neem to keep them off. Repeat as needed, at least 3 times since they’ll also dig into the soil plus you’re dealing with the juveniles and eggs. If it’s a small houseplant I’ll pick them off or kill them individually with isopropyl alcohol. Again, retreat as necessary and keep in quarantine. The awful things can lie in wait without a plant to feed on for a long time so be careful about the site and disinfect.