Powdery Mildew

Published On: January 10th, 2023


powdery mildew on Paeonia leaf Heydens Gardens

One of the most common plant problems we see at our diagnostic center (see below) is Powdery Mildew. Powdery Mildew is a fungal disease that causes unsightly powdery white growth on the leaves of susceptible plants. It most often becomes evident during humid summer weather and, once damage has started, can become progressively more pronounced as the season advances. If the infection is only affecting a small percentage of the leaf surface of the total plant, then powdery mildew can be considered merely an aesthetic issue. However, severe infection can result in the entire plant looking as though it has been coated in ash. When infection become this extensive the plant’s ability to photosynthesize is compromised and the plant will be weakened.

Avoiding Powdery Mildew

There are certain plants that are especially susceptible to powdery mildew.
These include:

  • Tall Garden Phlox (Phlox paniculata)
  • Beebalm (Monarda)
  • Zinnia (Zinnia)
  • Peony (Paeonia)
  • Roses (Rosa)
  • Summer Squash & Winter Squash
  • Old Fashioned Lilacs (Syringa vulgaris)
  • Ninebark (Physocarpus)

When selecting plants from this list, seek out mildew resistant varieties. Plant breeders have worked hard to develop disease resistant varieties, with very good success. Many newer varieties of these plants offer good to excellent resistance to powdery mildew.

Cultural practices can also help reduce powdery mildew.
These include:

  • If your plant has been infected during the current season, remove and destroy (burn or bag – do not compost) fallen leaves and stems at the end of the growing season.
  • Avoiding overhead watering. One way that fungal spores are spread is by water droplets, so avoiding splashing water on the foliage may decrease spread.
  • Create good air circulation. Leave space between plants in your garden. You may also want to thin out stems on plants such as Phlox and Beebalm to allow good air flow around remaining foliage.
  • Watch closely for evidence of powdery mildew throughout the growing season and remove affected leaves to prevent spread of disease.
  • Keep plants healthy. Healthy plants are less susceptible to damage from diseases and insects. Make sure sun loving plants have at least 5 hours of sun a day. Fertilize annually, in spring, with a slow-release organic fertilizer.
powdery mildew on Paeonia leaf Heyden's Gardens
powdery mildew on squash leaf
powdery mildew on Rudbeckia leaf Heyden's Gardens
Bonide organic preventative treatments for powdery mildew Heyden's Gardens
Bonide conventional preventative treatments for powdery mildew Heyden's Gardens

Treatment for Powdery Mildew

Treatment for fungal diseases on plants is preventative only, not curative. In other words, you can stop the spread, but you can’t make current infection disappear. For this reason, it is important to watch your susceptible plants closely so that – if treatment is desired – it can begin early. Another option is to begin a spray program early in the season, before damage begins, to prevent powdery mildew from getting established at all.

Many fungicides are effective in preventing the spread of powdery mildew. For organic gardening we recommend Bonide’s Sulfur, Copper, or Neem Oil. Another option for organic gardeners is  biofungicides such as Bonide’s Revitalize,  which trigger plant immune response. Conventional/chemical treatments include Bonide’s Fung-onil® and Infuse™ fungicides.

For roses and fruit trees there are Bonide products that combine disease and insect control and are meant to be used preventatively. These include Bonide’s Orchard Spray (organic), Fruit Tree & Plant Guard®, and Rose Shield™.

Our Diagnostic Center

If you suspect insect or disease damage on your plant, bring in a sample to our diagnostic center. Our horticulturist will look at your sample under our powerful digital microscope and assist you in identifying the problem and selecting the best treatment.

powdery mildew on Rudbeckia leaf under digital microscope Heyden's Gardens diagnostic center

By Zannah Crowe