White powder on peonies

Peony Leaves Turning White: Fixing A Peony With Powdery Mildew

Are your peony leaves turning white? It’s likely due to powdery mildew. Powdery mildew can affect many plants, including peonies. Although this fungal disease doesn’t usually kill them, it does weaken the plant, leaving them more susceptible to pests or other types of disease. Peony powdery mildew can also disfigure peony blooms, making them quite unsightly. Learning the causes of white powder on peonies and how to prevent this common problem is your best defense.

Powdery Mildew on Peonies

So what does a peony with powdery mildew look like? You can easily recognize this condition by the white, powdery growth that forms on the plant’s leaves. Occasionally, powdery mildew can be seen on the flowers as well.

Any new growth may also appear powdery, exhibiting a stunted or distorted appearance too. In addition to the powdery

growth, infected leaves may drop from the plant and flowers become distorted and unattractive.

Causes of White Powder on Peonies

Powdery mildew is caused by a fungus. There are actually many types of powdery mildew, all having varying growth requirements. However, most species of powdery mildew can germinate with or without water—though humid conditions are quite common for growth. Other ideal conditions for powdery mildew are moderate temperatures and shade, which generally spawns moisture.

Lots of heat and sunlight, on the other hand, can hinder its development. Therefore, these conditions are more suitable for preventing powdery mildew on peonies.

Treating Peony Powdery Mildew

Once powdery mildew appears, it can be difficult to treat, depending on the type and how severe the problem. For this reason, prevention is important. Avoiding susceptible cultivars, locating plants in full sun, providing suitable air circulation, and practicing proper maintenance (i.e. water, fertilizer, etc.) is usually adequate. Watering in the morning hours may also help.

But even with the best of precautions taken, powdery mildew may still strike. Although fungicides can help when applied early on, heavier infections may need to be treated with horticultural oil or neem oil. You can also use a homemade solution—mixing together a tablespoon each of baking soda, horticultural oil (or canola), and liquid dish soap (without bleach) with a gallon of water. Spray on your peonies every 10 to 14 days throughout the summer months. Do not spray the solution during hot and sunny days and always test on a small part of the plant before using it on the whole plant.

Spots on peony leaves or mold on the plants is usually caused by one of two peony fungal diseases, botrytis blight (gray mold) or leaf blotch.

These diseases tend to flare up in wet weather and plants that are infected early in the season may have severe leaf damage by late summer.

Botrytis blight

Peonies with mildew and leaf blotch disease

This peony fungal disease is especially common during a wet spring, and can flare up in cool rainy weather in summer too.

It is especially common when drizzle or rain continues for several days in a row.

This disease appears as a thick velvety gray mold on the plant stems and leaves. Sometimes you’ll see spots on young leaves and stems before they turn black.

Flowers can fail to open, or turn brown, and spread the infection down the stem. In severe cases, the fungus can cause a significant number of stems and leaves to turn brown and begin to die back in mid-season.

What to do: Remove infected plant parts (blighted buds, flowers, leaves and stems) when you notice damage. Wait until conditions are dry; cutting the plants back when they are wet with dew or rain can cause fungal spores to spread.

Prevention: If you have spots on peony leaves, cut stems at ground level in the fall, and thoroughly clean up all peony remains, and discard in the garbage (not in your compost pile), or burn or bury in the ground away from your peonies.

Spots on peony leaves – leaf blotch disease

If your peonies look like they have burn spots on them, the problem is most likely leaf blotch, which is common in warm, wet weather. It is caused by a fungus called Cladosporium paeoniae, which appears as dark reddish purple blotches or spots on peony leaves in mid- to late summer.

Peony leaf spot disease infections are encouraged by spent petals clinging to damp foliage. Prolonged humid conditions at the petal-leaf interface allows fungi to become established. Rainy, cool weather will cause the diseases to spread among the leaves.

None of the leaf-spotting diseases adversely affect the vigor of the plant, but the spots are not pleasant to look at. It is always a good policy to remove peony flowers from the plants before they shatter.

Prevention: Your best defense is to thoroughly deadhead all flower stems and to clean off an old petals on your plants.

What to do if you notice diseased leaves: If you already have leaf blotch, remove all infected plant parts, and, as with botrytis, do a careful clean-up of stems and leaves in the fall.

Fungicides for disease prevention

Fungicides can help prevent peony foliar diseases.

  • Remember that while fungicide sprays can help nip fungal diseases in the bud, they will not cure the problem once it has taken hold. When you see signs of fungal disease, remove affected plant parts, as described above.
  • Start fungicide applications early in the season when new peony growth is two to four inches tall. Be sure to follow product recommendations, and spray so that tops and undersides of leaves and stems are covered.

Make your own fungicide spray:

If you live in Ontario, you won’t be able to buy commercial fungicides, as they are banned under the pesticide law that came into effect in spring 2009.

You can make your own mixture by dissolving two tablespoons of baking soda with a few drops of dish soap (to help the baking soda solution adhere to the plants) in a gallon of water and apply the mixture with a hand sprayer.

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Dear Carol: For the past two years, my peonies have gotten white spots on the leaves. This happens after they have finished blooming. Some of the stems are now covered with what appears to be a white mildew. I cut off the worst of them, and one of the plants I cut back to the ground and removed the foliage from the area. I hate to cut them all back as it will leave a big hole in my garden but will do whatever you recommend. What is the cause of this? I usually don’t water them, although we did have a lot of rain in June. This never happened until last year. — S.G., Syracuse.

Dear S.G.: I also have been seeing more powdery mildew on peonies in recent years. This is a fungus disease that is common on many types of plants. Different kinds of fungus cause powdery mildew on lilacs, phlox, beebalm, summer squash and others, even on bindweed, but the symptoms are similar: a floury white coating on leaves and stems. The disease will vary in severity from year to year.

Good sanitation helps. Cutting off the worst affected stems and plants is a good tactic. The problem is deciding whether the leaves and stems are of any value to the plant. Mildew cuts down on photosynthesis by blocking sunlight from the chlorophyll in the leaves. Leaves that are completely coated with mold aren’t photosynthesizing, so they might as well be removed. Leaves that are still green? When they are looking bad enough to bother you, cut them off. Don’t put the foliage in the compost; bag it and get rid of it.

Plenty of sunshine, good air circulation and keeping the foliage dry will minimize any fungus disease. If the peony plants are crowded, they could be divided in September and spaced farther apart. A large peony may be divided into five to 10 new, vigorous plants, enough to replant and share with friends.

Compost the old, woody parts at the center of the old plant, although the discards will probably grow in the compost pile. Peonies can stay in the same place for years and benefit from a good soil with lots of compost dug into the top eight inches or so.

In the fall, after hard frost, cut all herbaceous peonies back close to the ground and dispose of the plant debris.

Fungicides are available. The sprays have to start as the foliage is emerging and continue regularly all season. Read and follow the label directions.

Varieties differ in their susceptibility to mildew disease. If a plant is disfigured year after year, I would get rid of it and try something else.

Peonies need sunshine. If the mildewed peonies are shaded, plant hydrangeas instead. They do well in shade, and there are many new cultivars to try. Masses of Quickfire look beautiful.

Carol T. Bradford, of Syracuse, has been gardening in Central New York for more than 25 years. Her column also appears in Stars magazine Sundays. Send questions in care of Home & Garden, The Post-Standard, P.O. Box 4915, Syracuse, NY 13221, by fax to 470-2111 or by e-mail to [email protected] Letters might be edited for space and clarity.

Let’s face it… The word “mildew” means different things to people. To the gardener, “powdery mildew” is a fungi on vegetables, especially on cucurbits.

To the homeowner, it may be any discoloration or growth found in the bathroom.

Powdery mildew at a cooperative extensive agency

To growers “powdery mildew fungi” means infected plants and three very different fungus plant diseases:

  • Black mildew disease – a soot-like coating often found on leaves of slow-growing tropical plants
  • Downy mildew disease (white spots on plants) – a delicate, white, frosty coating forming on the undersides of leaves
  • Powdery mildew disease – makes infected leaves and stems (powdery spots) look as though they’ve been sprinkled with flour

The last one – powdery mildew – is perhaps the most common and destructive mildew of all in greenhouses during winter and spring months. A detailed discussion of this mildew, therefore, is appropriate at this growing season.

Powdery mildews are obligate parasites – that is, they grow only on living tissues and mainly on the surface of those tissues. Although they are primarily leaf parasites, they may grow upon stems, flower parts or fruits.

On some infected plants the fungal disease causes relatively little apparent injury; others are highly destructive.

A few different species, such as crape myrtle mildew, are highly host-specific as to food preferences and hence attack but a single kind of host plant.

Some powdery mildew species (like Golovinomyces cichoracearum formerly Erysiphe cichoracearum) attack a wide variety of plants.

Others, such as phlox mildew, can attack more than 280 different kinds of ornamentals, including these plants susceptible to powdery mildew:

  • Aster
  • Phlox drummondii (the annual phlox)
  • Begonia (rex)
  • Calendula plants
  • Chrysanthemum
  • Delphinium
  • Gerberas
  • Zinnia

Related Reading:

  • Powdery Mildew on Phlox
  • Black Sooty Mold On Plants – What Is It?

Difference Of Powdery Mildew Fungus

Powdery mildew fungi differ in one important respect from most other fungi. Their spore germination do not require water to take place.

High humidity on the leaf surface is sufficient for fungal growth in which the mildew thrives. Such a situation exists frequently when plants are grown without good air circulation or when cold nights are followed by warm days.

Powdery mildew on roses has been the most widely studied mildew.

Most of our information on the behavior of mildews and their control is based on this particular one, which goes under the botanical name Sphaerotheca pannosa, variety rosae.

At one time old-fashioned syringing was used to control spider mites on roses in greenhouses, before miticides became widely popular for control.

Syringing with water reduces the severity of mildew. But, it cannot be recommended for mildew control, as it brings on other more destructive diseases, for example, black spot.

How To Treat Powdery Mildew On Squash

Powdery mildew affects a variety of vegetable plants like those in the squash, cucumber and pumpkin family.

Fortunately, it can be easily identified and is treatable. Look for white powdery spots on stems and leaves of infected plants.

NOTE: Most of the white powdery growth is the asexual spores which are the primary means of dispersal of the fungi. Young succulent growth is a favorite target of this disease.

Below are several organic ways for preventing powdery mildew on squash.

1. Milk – Diluted cow milk is a common safe and effective organic way to get rid of powdery mildew. Create a powdery mildew spray of 40% milk and 60% water. It was found to be as effective as chemical fungicides in managing powdery. Treat powdery mildew with milk every week making sure to alternate between other methods.

2. Garlic – Garlic extract is an effective home remedy. Not only this powdery mildew killer is organic and easy to make, but it is also safe for plants and family.

To make this home powdery mildew treatment, blend two bulbs of garlic and add a quart of water and few drops of liquid soap. Once the mixture has blended well, strain and refrigerate.

To prevent germination of asexual spores, dilute the concentrate with 1:10 parts water before spraying the squash.

3. Water – Powdery mildew, unlike other types of mildew, thrives well in dry conditions with high humidity. It sounds strange but by watering your plants can help wash the powdery mildew spores away.

4. Baking Soda (sodium bicarbonate) – can be a better option in treating powdery mildew on plants. Here’s the recipe:

  • 1 Tablespoon of baking soda
  • 1 Teaspoon of horticultural oil
  • 1 Teaspoon insecticidal or liquid soap (not detergent) – or try neem oil insecticide
  • 1 gallon of water

Spray on plants every one to two weeks.

Before spraying any plants, test the diluted mixture on several plants to ensure they do not have any issues. Potassium bicarbonate additives also help control powdery mildew problems.

5. Mouthwash – Because of its ability to kill germs, mouthwash can be used to destroy mildew spores

Mouthwash is very powerful and therefore, it’s important to test a small area on squash leaves to make sure plants do not suffer any damage.

Dilute mouthwash with water in the ratio of 1 part mouthwash to 3 parts water

Use a generic, ethanol based mouthwash. Jeff Gillman, Ph.D and Associate Professor at the University of Minnesota, Department of Horticulture had very effective control.

His tests used one part mouthwash to three parts water. Be careful when mixing and applying mouthwash as new foliage can be damaged.

6. Vinegar – Vinegar with its acetic acid, when sprayed on powdery mildew, will change the fungus pH thereby killing them effectively. Mix one gallon of water with four tablespoons of vinegar. Spray after every three days until the mildew has been totally wiped out.

How To Get Rid Of Powdery Mildew on Rose Bushes

The powdery meldew cure!

The first powdery mildew symptoms appear on young leaves. The leaves hold their color but begin to crinkle. Small patches of mold develop with spore-bearing fungal filaments.

These spread to the stems and other parts of the rose including the buds. As the fungal disease spreads it anchors itself to the foliage. From there, the fungus will draw on the nutrients and moisture within the leaves.

NOTE: Enzymes structures produced by powdery mildew help break up the plant cells by forming haustoria in its host. “Haustoria are specialized organs formed inside living plant cells, which absorb nutrients and anchor the fungus.”

This destructive disease can kill the plant within a short duration. Take appropriate measures to prevent powdery mildew or put out the disease in its early stages.

The drawing below shows how powdery mildew attacks a rose leaf.

  • A – exterior “conidia”
  • B – “mycelium” on the leaf surface
  • C – “haustoria” of the mildew growing within the epidermal cells of the leaf
  • D – the interior cells of the leaf

How powdery mildew attacks a rose leaf. image: PlantCareToday.com

Inspection – Inspecting your plants for any signs associated with the disease. The signs are likely to be found on new leaves although it can occur on older parts as well. Carefully inspect the foliage and the blooms.

Prune – Like most fungal diseases, powdery mildew tends to affect crowded plants, which receive little sunlight and reduced air flow around and through them.

Pruning areas affected, this reduces the treatment area and allows more air movement If the infection is not very serious, you can simply pluck off the affected buds and leaves.

Water Spray – Just like the above water spray on squash, the same can be used on roses. Spraying with water washes off the spores before they have time to embed.

Apply Fungicide Spray – You can apply a copper fungicide spray once a week until all the signs of the mildew are gone. Apply early in the morning or late evening to avoid leave burn.

Powdery Mildew Baking Soda Spray Recipe from Rose Magazine:

  • 1 tbsp vegetable oil
  • 1 gallon unchlorinated water
  • 1 tbsp apple cider vinegar
  • 1 tsp Listerine (yes, the famous mouthwash, not mint flavor, just regular)
  • 1 tbsp liquid soap
  • 1 ½ tbsp baking soda
  • Pump sprayer (large)

Before the mildew has affected your roses or to avoid re-infection you can use these tips

  • Plant roses in locations that receive plenty of sunlight
  • Improve air circulation around bushes by pruning
  • Leave enough space between plants to facilitate good airflow
  • Avoid overwatering creating conducive environments for mildew development. Consider drip irrigation for regular watering.

General Recommendations For Controlling Powdery Mildew

A high humidity condition allows and causes powdery mildew to spread and thrive on plant parts including:

  • Plant surfaces
  • Plant tissues
  • and even plant debris

The usual preventative measure recommendations are to avoid excessive high humidity, drafts and sudden changes in temperature to prevent outbreaks of mildew.

However, exhaustive state university studies on the effect of temperature and humidity on powdery mildew of roses concluded that this disease…

“cannot be effectively controlled under greenhouse conditions through regulation of temperature and humidity, but that it may he held somewhat in check by keeping both temperature and relative humidity as low as possible for the cultivation of roses and by avoiding drafts and extreme changes in temperature. Since these conditions are difficult to meet, except possibly in winter, the application of fungicides seems to remain an indispensable measure in the control of the disease.”

How to Kill Powdery Mildew With Chemical Controls

Over the years many fungicide applications have been tested to control or kill powdery mildew.

Some have proved to be effective eradicants even after the mildew is quite abundant.

NOTE: According to the Journal of Plant Pathology Vol. 92, No. 3 (November 2010), pp. 775-779 – Powdery mildew on raspberry is genetically different from strawberry powdery mildew. This is why using and applying a chemical labeled for the disease is important.

One reason for this is that the powdery mildews do not penetrate deeply into leaf tissue as do some diseases such as black spot.

Sulfur has long been a favorite way of combating mildew.

Bayer Advanced Disease Control for Roses contains the active ingredient Tebuconazole, providing three-way action: killing existing fungi while forming a protective barrier on the outside of the plant and is absorbed into the plant to keep on protecting regardless of weather conditions. More Here…

Natural Controls

The best “natural control” is to grow mildew-resistant varieties or mildew-tolerant varieties of vegetables, crape myrtle, and roses.

However, here are a couple recommended natural controls (aka homemade organic fungicides) for powdery mildew homeowners can try courtesy Organic Gardening:

  • To try this at home, mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution. Reapply after rain.
  • Spraying leaves with baking soda (1 teaspoon in 1-quart water) raises the pH, creating an inhospitable environment for powdery mildew.

Correct Timing Essential

As with disease control on outdoor plants, the correct timing of spray applications is most essential. Mildew is more easily controlled if the sprays are applied when the disease first appears. It is far more difficult to control on plants already heavily infected.

Although, control can be a challenge if your plants experience problems with powdery mildew look to improve air circulation and reduced humidity does seem to be a good start in reducing outbreaks.

If there’s one disease that always seems to cause Northwest gardeners headaches, it’s powdery mildew. With all the humid weather this year, we’re seeing quite a bit of it. What’s a gardener to do? While there is no guaranteed cure for this ubiquitous disease, there are quite a few all-natural ways to help prevent and suppress it.

Identifying powdery mildew

Powdery mildew is a fungus parasite that attacks a wide variety of plants. It starts with a few small white spots on your plants’ leaves. If you look closely, you’ll see that they are made up of fine white filaments almost like a tiny spider web.

If the fungus gets the better of a plant, then the spots will multiply and the filaments will eventually spread until the whole leaf is covered in these fine white threads. In later stages, the leaves may turn yellow. Although powdery mildew is rarely fatal, it can stunt growth, inhibit flowering, and decrease crop yields. Besides, many gardeners just don’t like the looks of it.

Different species of powdery mildew attack different plant species. That’s good news because it means a single outbreak can’t spread, for example, from your cucumbers to your roses. However, you may have more than one species of the fungus in your garden, and the symptoms look the same regardless.

Treating outbreaks

By this time of the year, most gardeners who are going to get powdery mildew probably already have it. It can affect any kind of plant, including vegetables, annuals, perennials, shrubs, and trees.

Although most treatments work better as preventative measures, there’s still hope of eradicating or at least slowing down the disease once you have it. The earlier you catch the outbreak, the better your chances of victory.

Horticultural oils are usually the most effective at eradication. This category includes neem oil, an all-natural plant extract that is allowed in organic gardening. Apply oils in overcast weather when temperatures are below 80°F, and always water your plants thoroughly before applying. If you previously used a sulfur-based fungicide, wait at least two weeks before applying any kind of horticultural oil.

The preventative fungicidal bacterium in Serenade, Bacillus subtilis, may also help slow down an established infection.

Preventing outbreaks before they start

Once you have a handle on eradicating this year’s powdery mildew outbreaks, it’s time to think about putting yourself in a better position for next year. Especially now that you know which of your plants are most susceptible, you can prevent problems before they start.

General best practices

As with any problem in the garden, the first step in management is providing the right conditions for your plants to thrive. Here are a few tips for supporting your plants’ natural defenses and limiting powdery mildew habitat in your garden:

  • Choose varieties that are naturally resistant to the disease, such as ‘Diva’ cucumbers, ‘Blue Stocking’ bee balm, and ‘Zahara’ series zinnias. If you have certain plants that seem to cause problems year after year, consider removing them.
  • Create good airflow by pruning shrubs and providing adequate space between annuals and vegetables.
  • As always, follow the principle of “right plant, right place.” Sun-loving plants mistakenly placed in the shade are especially susceptible to powdery mildew.
  • Prevent drought stress by watering deeply and consistently. Check out these tips for effective summer watering.
  • Water in the morning rather than the evening. Powdery mildew loves to take hold in humid nighttime conditions.
  • Feed your plants consistently with a slow-release fertilizer. Nutrient deficiencies can make plants more vulnerable to disease, while fast-acting synthetic fertilizers can encourage too much susceptible new growth all at once.
  • Rotate your annuals and vegetables. Since different species of powdery mildew affect different plants, you can disrupt the fungus’s lifecycle by changing the locations of your plants from year to year.
  • Always remove and discard infected material, especially as you clean up your garden before winter.

Commercially available products

There are several easy-to-use products that can help prevent powdery mildew available at Sky Nursery. Gardeners who prefer the DIY approach may also want to look into preventive home remedies.

Using sulfur-based fungicides is one effective method. Although sulfur is a natural control allowed in organic gardening, it can still be harmful to your garden if over-used, so be sure to carefully follow all label directions.

The bacteria-based organic fungicide Serenade is also effective at preventing powdery mildew. When sprayed on plant leaves, the bacteria destroy fungal spores before they have a chance to take hold.

With either of these products, applying a liquid solution all over the leaves about once a week is likely to be the most effective method.

Have a plant disease problem? We’re happy to help.

Summer is the season of garden abundance. It’s also the time of year that all sorts of plant diseases tend to pop up. Sky Nursery staff can help identify disease problems and recommend solutions that fit your gardening style. Bring in some pictures, or better yet a leaf sample. We’ll be happy to help you diagnose the problem and decide what to do. Then you can get back to enjoying your summer garden.

Powdery Mildew

Caused by various fungal species.

Infected plants develop a distinct white powdery film that covers their leaves. Commonly found on a large range of plants, and is very common on pumpkins, cucumbers, courgettes, and dahlias – particularly towards the end of their growing season in autumn.

Mainly a problem when it affects plants early in their growing season, as it can significantly reduce the plants’ yield of fruit, attractiveness and the length of life expectancy.

Sometimes confused with downy mildew, however it can easily be distinguished as the powdery film only forms on top of leaves and doesn’t penetrate the leaves or form on the underside.


Keep plants healthy, well fed and watered. When planting, leave enough space between plants to ensure that air can flow freely around the plants. When watering, avoid wetting the foliage and if possible water in the morning.

Organic Treatment options

Spray affected plants with Freeflo Sulphur. It’s an effective organic solution suitable for edibles and ornamentals. On cucurbits (pumpkins, courgettes, etc) treatment is only necessary if they are affected badly, or affected early in the season, as some powdery mildew is inevitable towards the end of the growing season.

Note: don’t confuse this with Lime Sulphur as this can cause your plants to defoliate.

Other Options

For severe cases on ornamentals, use Yates Fungus Fighter. Apply at most 4 times a year and don’t use on edibles.

How to Get Rid of Powdery Mildew

Learn ways to treat and prevent powdery mildew By Linda Hagen


Photo by AJ Cespedes / .com

Powdery mildew is a common fungus that affects a wide variety of plants. It is easily identified and appears as white or light grey powdery spots usually found on the topside of leaves, but can also be found underneath, or on stems, flowers, fruit or vegetables. The spots spread and will eventually cover most of the leaves on the plant, with new plant growth being most susceptible.

Powdery mildew thrives in warm, dry climates; however, it also needs fairly high relative humidity — like the warm days and cool nights in late spring to early summer. Not enough sunlight and poor air circulation also contribute to conditions that encourage powdery mildew.

Although rarely fatal, if left unchecked it can eventually cause serious harm to your plants by robbing it of water and nutrients. Most infections cause minor damage such as yellow, withered or distorted leaves, but plants can also become weak, bloom less, and grow slower.


Here are some things you can do to ward off an infection before it occurs:

  • Thin out existing susceptible plants to improve airflow within the plant.
  • Maintain adequate spacing between plants and keep them far enough away from walls and fences to ensure good air circulation and help reduce relative humidity.
  • Locate plants in proper sunlight according to their needs.
  • Maintain healthy plants by removing dead or diseased foliage.
  • Disinfect pruners or shears after use on infected plants. (See Tool Care & Maintenance)
  • Because new growth tends to be more susceptible, be careful to not over-fertilize and cause a rush of new foliage.
  • Treat regularly with an organic fungicide that contains sulfur. This can be used as a preventative measure as well as treatment for existing powdery mildew.
  • When shopping for plants, choose varieties with increased resistance to powdery mildew.


Although most products on the market are targeted more toward the prevention of powdery mildew, there are many home remedies to treat an existing infection. Spray mixtures will only kill what they come in contact with, so be sure to coat all affected areas thoroughly. It may take multiple applications for complete treatment. Apply once a week for three to four weeks, then wait to see results. Reapply as needed.

Pruning apple leaves damaged by powdery mildew. Photo by: agrofruti / .com

  • Baking soda solution: Mix 1 tablespoon baking soda and ½ teaspoon liquid soap such as Castile soap (not detergent) in 1 gallon of water. Spray liberally, getting tops and bottoms of leaves and any affected areas. This method may work better as a preventative measure, although it does have some effect on existing powdery mildew as well.
  • Potassium bicarbonate: Mix 1 tablespoon potassium bicarbonate and ½ teaspoon liquid soap (not detergent) in 1 gallon of water. Spray liberally to all affected areas. This mixture may work better than baking soda as a treatment for existing infections.
  • Milk: Mix 1 part milk to 2 to 3 parts water and spray liberally. While the science behind this solution isn’t fully understood, it seems to work rather well, especially on zucchini, melons and cucumbers. It is believed that naturally-occurring compounds in the milk not only combat the disease, but also boost the plant’s immune system.
  • Neem oil: By itself, neem oil has mixed reviews on its effectiveness to treat powdery mildew, but it can be added to the above mixtures for an extra boost.
  • Powdery mildew fungicide: Use sulfur-containing organic fungicides as both preventive and treatment for existing infections.
  • Trim or prune: Remove the affected leaves, stems, buds, fruit or vegetables from the plant and discard. Some perennials can be cut down to the ground and new growth will emerge. Do not compost any damaged or diseased foliage as the spores can spread and persist in the composted material. Disinfect pruners and all tools after using on infected plants.


Mildew spores are spread by the wind in warm, dry weather, but don’t spread well when conditions are rainy and cool. Powdery mildew strains are specialized to certain groups of plants and generally don’t spread to other plant families. Spores can survive over winter in leaf piles and on plants, so it’s important to discard and not compost any material removed or dropped from infected plants to prevent spreading or allow it to resurface the following spring.


Essentially, there aren’t any plants that are completely immune to some form of powdery mildew, including vegetables, roses, trees and shrubs.

There are some plants that are more susceptible than others, such as:

  • Begonias
  • Sunflowers
  • Chrysanthemums
  • Dahlias
  • Roses
  • Zinnias
  • Melons
  • Zucchini
  • Squash
  • Lettuce
  • Cucumbers
  • Potatoes
  • Parsley
  • Pumpkins
  • Grapes
  • Peppers
  • Tomatoes

If you have recurring problems with powdery mildew, look for varieties that are noted to have improved disease resistance — this should be noted on the plant tag.

How to treat powdery mildew on roses? Remove and discard any affected leaves, as well as any that have dropped to the ground, and treat the rest of the plant preventatively. If you see powdery mildew on buds, clip and discard them as well. Thoroughly clean and disinfect any cutting tools that were used in the process. Apply one of the treatments above, such as a fungicide, baking soda, potassium bicarbonate or milk mixture once a week for 3 to 4 weeks and wait to see the results. Prevent further outbreaks with regular applications every couple of weeks or follow directions on product labels. Ultimately, results will be much better if the infection is caught at the first signs and treated quickly.

Try these roses that boast increased resistance to powdery mildew:

  • Oso Easy Double Red™
  • Oso Easy Double Pink™
  • Suñorita™

What is the best treatment for powdery mildew on squash? The milk mixture mentioned above seems to have better results that the other methods. Again, the science behind why it works is still being discovered, but it does appear to not only prevent a powdery mildew infection, but also boost the plant’s immunity.

Black Spot on Roses
Eco-Friendly Pest Control

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