Of all the cheery garden subjects you may be excited to learn about, powdery mildew on peonies is probably not one of them. This topic is important, though, because the infection can quickly turn your prize-winning peonies into a patch of fungal fuzz. Luckily it’s possible to take measures to prevent it from happening in the first place, or, if it does afflict your plant, you can try to prevent it the following growing season.

Powdery mildew is common in humid climates. It’s a worry on thousands of plants, including squash, cucumbers, lilac bushes, phlox, roses, beebalm, basil, and other garden favorites. Until recent years, finding unsightly powdery mildew on peonies has been rare. Unfortunately, it’s now more common, so anyone growing peonies should understand what to do about it.

In this article I’ll share the possible the causes, how to tell if your peonies have powdery mildew, the potential impact on your peonies’ health, and how to prevent and control the issue.

Conditions that cause powdery mildew on peonies

The primary culprits behind powdery mildew on peonies are too much shade and poor air circulation.

When we think of fungal infections on our garden plants, we think of rain splashing infected soil onto tender leaves and subsequent cool, wet conditions allowing the fungus to spread. That’s not exactly the case with powdery mildew. You’ll most often see powdery mildew from late June to September—periods of cool, humid nights and hot, dry days. The powdery mildew fungus is one of few that takes hold without water.

What is powdery mildew?

Erysiphe polygoni is the fungus that causes powdery mildew on peonies. The Erysiphe genus has many species, and each is host-specific, meaning they infect only certain plants. When you find powdery mildew on your grapevine, for example, you don’t have to worry about it spreading to your peonies. The grape’s powdery mildew is not caused by the same fungus as the peony’s powdery mildew.

Symptoms of powdery mildew infection on peonies

Powdery mildew on peonies starts out looking like a splotchy dusting of white powder across the foliage, like someone shook a flour sack over your plant. The splotches expand until the “powdery” coating covers the whole leaf. Look closely, and you’ll see a web of slender mycelial filaments spreading over the leaves.

If you’re seeing groupings of brown fuzzy spores, rather than white ones, you’re more likely dealing with the fungus Botrytus, which is less common but poses a more serious concern to your peonies’ health.

How powdery mildew impacts peony plant health

As bad as a powdery mildew infection looks, it doesn’t typically kill peony plants. The spores puncture the plant cells and take nutrients it, but the worst impact is from the fungal mat spreading across the leaf surface. As the mat becomes thicker, it blocks sunlight from reaching the cells of the leaf surface, reducing the plant’s ability to photosynthesize. A lack of photosynthesis means a lack of energy production in the plant.

Here’s how that looks:

• The plant’s growth overall may be stunted for the season.
• The leaves may yellow and drop.
• Because powdery mildew typically sets in after peonies bloom—the flowers emerge in the spring—the fungus shouldn’t impact flower production.

Preventing powdery mildew on peonies

The best prevention for powdery mildew on peonies relates to your management of these plants. Good management includes:

• Planting them in a full sun location. You’re most likely to see this fungal disease creeping up in the shade. (A lack of sunlight may also prevent peonies from blooming.)
• Allowing good air movement. Peonies need space. Don’t plant them too close together. Every few years, divide your peonies in the fall to keep the plant size small and allow good air circulation around the leaves.
• Washing your hands and sanitizing your tools after working with infected plants. Don’t handle healthy plants after you’ve touched infected plants.
• Planting powdery mildew-resistant peony varieties. Kansas State University found Red Charm, Lillian Wild, and Pink Luau to be most resistant in their trials.

Control methods

The best powdery mildew control is prevention, as once the fungus sets in, you’re probably going to have it for the rest of the season. Resolve to pay closer attention to your plants’ growing conditions next year, following the advice above.

Treatments for powdery mildew on peonies aren’t highly effective, and you must preemptively apply topical treatments before the fungus attaches to the leaf. If you’re grasping for a control solution, there are a few you can try that won’t hurt your plants:

• You may already have neem oil on hand to control pests. Neem oil is also a fungicide. More research is needed, but scientists have had some success slowing or preventing the spread of powdery mildew by spraying infected leaves with the product.
• Make your own topical at home with 1 gallon of water and 1 tablespoon each of canola oil, baking soda, and dish soap. Mix this in a spray bottle, and coat the leaves with the spray.

What to do at the end of the growing season

At the end of the growing season, you have the opportunity to rid your garden of powdery mildew spores that would otherwise overwinter there.

If you saw powdery mildew on your peonies, use these tips for good sanitation in the fall:

• Use hand pruners to cut back infected peony plants to the ground. Cutting back peonies after the frost is a best practice, regardless of powdery mildew infection.
• Remove all of the diseased plant materials from the area.
• Remove the top 2 to 3 inches of mulch from around your peony plants and replace it with fresh mulch.
• Don’t put any infected materials in the compost! Put them in the trash to remove them from your property altogether.

Even though powdery mildew on peonies is becoming more common, you now know what to do if you find it. It may be an unattractive blight on your ornamental garden, but it won’t kill the plants, and you can rest assured it won’t spread to other plants.

More tips for caring for your peonies

• What to do if your peonies aren’t blooming
• How to fertilize peonies
• When to cut back peonies
• Learn to propagate peonies