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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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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, 鈥淎ll the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce.鈥

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掳F / 15-27掳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掳F (32掳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. 
Gardening Products
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鈥1200 WP of 15% wettable powder, 1 time intervals of 10 days, and 2鈥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鈥檚 not surprising that she and The Old Farmer鈥檚 蜜桃恋人 found each other. She leads digital content for the 蜜桃恋人 website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann
 

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Comments

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.

Ken Diehn (not verified)

7 years 4 months ago

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?

Cindy (not verified)

7 years 1 month ago

Did you ever find if powdery mildew is spread by burning infected plants?

I have been doing a lot of research on powdery mildew. Temperatures above 90 degrees kill it.

Peter (not verified)

7 years 4 months ago

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.

Dev Prashad (not verified)

7 years 5 months ago

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

Cass (not verified)

5 years 5 months ago

Sounds more like aphids to me. Roses are certainly prone to white mealy bugs. The 鈥渂right white鈥 is what convinced me. Mildew isn鈥檛 typically bright white but the infuriating little critters definitely are.
If it鈥檚 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鈥檒l also dig into the soil plus you鈥檙e dealing with the juveniles and eggs.
If it鈥檚 a small houseplant I鈥檒l 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.

It sounds like powdery mildew. See guidance above and

鈥 plant roses in full sunlight.

鈥 space plants to allow ample air circulation

鈥 fertilize to maintain vigor but do not overfertilize

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

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

鈥 use fungicide with care

鈥 in future, select rose varieties that are resistant

Pawar Bhalchandra (not verified)

8 years ago

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.

Alison (not verified)

8 years 1 month ago

Can you tell me what to spray to rid the plant from this pest. It has spread to other plants in the area.