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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
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Identify the Powdery Mildew Plant Disease

Catherine Boeckmann
Pest Type
Plant Diseases

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 , of Pennsylvania State University, “All the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce.â€

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 come from dormant spores in old vegetative material or weeds nearby.

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

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Identification

How to Identify Powdery Mildew

  • Plants infected with powdery mildew look like 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. 
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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 can be bought 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 loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s ÃÛÌÒÁµÈË found each other. She leads digital content for the ÃÛÌÒÁµÈË website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann
 

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Comments

Anna (not verified)

6 years ago

I have white powdery substance on my zinnias. Will this make the seeds in the flowers be bad for next year?

Yes, the mother plant has been infected and will pass along those genetics to the seedlings which will also carry the disease.

fredric lacarbonara (not verified)

6 years 1 month ago

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

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.

Lisa in Texas (not verified)

6 years 1 month ago

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.

Jack G. (not verified)

7 years ago

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?

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.

Linda Schwin (not verified)

7 years 3 months ago

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.

Simon viola (not verified)

6 years 5 months ago

Every year after the season I start getting fungus on my peonies what can I use to prevent this.