Common Disease Spotlight: Powdery Mildew

Each year, I wait in anticipation for peonies to turn my small St. Paul backyard into a beautiful array of pinks, whites and yellows amidst lush, tender foliage. I know I’m not alone, as they are a popular June garden favorite, and their heyday never lasts as long as we’d like it to. 

After the peonies have dropped their last petals, summer in Minnesota becomes increasingly warm and humid. With those changes, many gardeners will notice some of their favorite flowering plants look like they’ve been dusted with flour, and some will look as if they are covered in cobwebs. This is often due to a fungal disease called powdery mildew.

Peony plant with powdery mildew disease, creating a silver look to the leaves and blotchy white spots

Powdery mildew can affect over 10,000 plant species in flower gardens, including peonies, roses, bee balm, phlox and zinnias. It can also affect vegetables in the cucurbit (e.g., squash, pumpkins, cucumbers, melon), nightshade (e.g., tomato, eggplant, pepper) and legume (e.g., beans, peas) families. It is caused by a fungus that can be passed on to plants through the wind from neighboring plants, or even plants hundreds of miles away. When a spore, a single-celled reproductive body, lands on a host plant, it germinates and begins infecting the plant. Powdery mildew fungi penetrate the plant tissue to take up nutrients. As spores are produced, they build up on leaves until they eventually break off,  and the fungus is further spread by wind. While powdery mildew is a common garden nuisance, knowing how to identify and prevent it can help you manage it. 

Identifying Powdery Mildew

Powdery mildew is often recognized by its appearance—plants may look like they’ve been dusted with a fine white powder or covered in cobwebs. There may be white or gray spots on the leaves, stems or buds. Some leaves may turn yellow and fall off, and young leaves may look disfigured. The most severely diseased plants are usually found in shaded or densely planted areas with little air movement. 

Photo credit: Scot Nelson on Flickr.

Preventing and Managing Powdery Mildew

Since powdery mildew is transmitted from plant to plant through the wind, it’s nearly impossible to fully prevent it from entering your garden, but it does prefer ideal conditions. Powdery mildew fungi thrive with cooler, humid nights that stimulate spore production and warm (70-80 degrees Fahrenheit), dry days that allow for spores to spread in the wind. In cooler, rainy areas, it does not spread well, and its growth is slower when temperatures rise above 90 degrees.

While it’s impossible to know if powdery mildew spores will enter your garden, you can take some preventative measures to reduce the risk, including:

• Inspect greenhouse plants carefully before bringing them into your garden. If you visit a greenhouse, check out plants susceptible to powdery mildew, even if you are not buying those specific plants. This can help minimize the risk of bringing spores into your garden.

• Catch powdery mildew early by inspecting susceptible plants weekly.

• Water from overhead to wash spores off leaves. Leaving foliage wet, however, could contribute to other diseases.

• Plant in sunny spots, as mildew prefers shadier areas. 

• Choose powdery mildew–resistant varieties or replace infected plants with plants from a different family.

• Reduce humidity in the garden by leaving enough space between plants for good airflow.

• Prune to thin out foliage in crowded areas to improve airflow and increase light.

• Keep the garden free of unnecessary plant litter. 

• Avoid over-fertilizing plants, as too much nitrogen will encourage more new growth, which is where the fungus does best.

Once a plant is infected by powdery mildew, it’s very difficult to get rid of it, so if you discover it in your garden, it’s best to focus on preventing the spread to other plants. First, remove infected foliage and stems. Depending on where you live, you may not be able to throw garden material in the trash. Do not compost infected material, as it can spread to other plants. Burning the material is ideal if local ordinances allow it, and the conditions are safe for a fire. After pruning infected plants, be sure to clean shears or pruners thoroughly.

Fungicides are rarely needed, as the conditions for powdery mildew do not usually stay favorable in Minnesota. That said, if you must use them, know that they will not cure or remove existing powdery mildew infections. Organic fungicides for treating the disease include sulfur, lime-sulfur, neem oil, and potassium bicarbonate. They must be applied to green foliage early in the season or at the first signs of the fungus, with repeat applications necessary. As always, be sure to follow instructions on the product’s label.

If your garden gets hit with powdery mildew this season, you are not doomed forever. Next year, the conditions may not be as favorable, and there are ways to prevent the disease from doing too much damage. 

Sources:

Boeckmann, Catherine. “How to Prevent and Control Powdery Mildew.” Almanac.com, 2025, www.almanac.com/pest/powdery-mildew.

Grabowski, Michelle. “Powdery Mildew in the Flower Garden.” UMN Extension, 2026, extension.umn.edu/plant-diseases/powdery-mildew-flower-garden. 

Writer Biography:

Melissa (Mel) Andersen (she/her/hers) lives on Dakota land in St. Paul. She is an educator, a writer, and a gardener, particularly interested in native plants and pollinator-friendly garden practices. She has been with the Ramsey County Garden Education Volunteer program since 2019.