Powdery Mildew on Ornamental Plants

Posted: August 16, 2016

At first glance it may appear as if someone has whitewashed your landscape ornamentals. But upon closer examination with a hand lens you should notice the white hyphal growth that confirms the presence of the disease known as powdery mildew. Powdery mildew is a common disease that begins to appear in mid-to-late summer in Pennsylvania on a wide array of woody ornamentals.
Powdery mildew on monarda. Photo: Tom Ford, Penn State Extension

Powdery mildew on monarda. Photo: Tom Ford, Penn State Extension

As a rule, this organism tends to attack young foliage, stems, flowers, and/or fruit. On some hosts the organism may prefer to colonize the older lower hanging leaves. In some cases, powdery mildew is best described as an aesthetic nuisance since it does not seemingly compromise the overall health and viability of the plant. In the most extreme cases the pathogen can however, cause tissue death as well as flower and fruit distortion.

Plant nutrition often plays a role in the susceptibility of a plant to infection by powdery mildew. Vigorous plants grown under higher fertility regimens tend to have more leaves with thinner cuticles or “softer” foliage. Plants grown under high levels of fertility tend to be more susceptible to powdery mildew than plants that are faced with nutritional stresses. While I am not advocating that you nutritionally starve susceptible plants, I do encourage that all landscape managers be mindful of the impact that certain nitrogen sources will have on plant growth.

Water insoluble nitrogen sources and organic fertilizers tend to have slower nitrogen release rates resulting in a more toned growth. Water soluble nitrogen sources are readily available which frequently translates into soft succulent plant growth that is more readily colonized by pathogens that cause diseases like powdery mildew.

powdery mildew outbreak on lilac
Photo 1. Lilac foliage with powdery mildew. Photo: Tom Ford

Lilacs are one of the most commonly infected hosts in PA landscapes. While the pathogen’s presence is easily noted by the white film that covers the foliage, it does not usually compromise the long term health, or blooming potential of the plant. Peonies are a common herbaceous perennial that can be found in many landscapes. While we typically do not value the aesthetic beauty of a peony leaf, the pathogen does mar any ornamental value that it may have.

powdery mildew infection on a peony
Photo 2. Peony foliage with powdery mildew. Photo: Tom Ford

Most fungicides are protectant in nature and will not eradicate the pathogen once it has infected the foliage. Protectant fungicides place a protective coating and/or layer on the plants parts in an effort to prevent infection by fungal pathogens. Their usage post-infection is negligible except for the ongoing protection that they may impart to newly emerging foliage that had not been previously infected.

Potassium bicarbonate based fungicides like Arbicarb 100, Milstop, and Kaligreen are frequently used by gardeners and growers to prevent powdery mildew in their gardens and landscapes. Some of the Potassium bicarbonate fungicides are also registered for use in organic production systems.

Conventional fungicides which can be used to manage powdery mildew in the landscape include: azoxystrobin (Heritage), chlorothalonil (Daconil), myclobutanil (Immunox), triforine (Funginex), and triadimefon (Bayleton).

When considering the use of any fungicide or pesticide product, please read the label before its purchase so that you can ensure that you may use it legally for the pest and/or disease that you wish to manage.

Contact Information

Thomas Ford
  • Senior Extension Educator
Phone: 814-472-7986