Issue 12, August 1, 2017

Powdery Mildew on Herbaceous Ornamentals

Powdery mildew on phlox. Photo credit: Diane Plewa / University of Illinois Plant Clinic

As the season progresses we start to see powdery mildew appear on herbaceous ornamental plants. Common hosts include phlox, peony, bee balm, zinnia, and many more. There are over a thousand species of fungi that cause powdery mildews on a wide range of hosts. Most are fairly host specific.

Powdery mildews that appear in mid- to late- summer are rarely lethal to their hosts. However, repeated infections over the years can weaken hosts, and the disease reduces the plants’ ornamental value. The severity of the disease is dependent on numerous factors, including the variety or cultivar of the host, the timing of the initial infection, and environmental conditions affecting the host. Powdery mildews are favored by high humidity, low air movement, and warm or hot days followed by cool nights. Powdery mildews are usually more severe on overcrowded plants growing in shade.

Infection is usually quite obvious. Initially, small, white, diffuse patches appear on infected tissue. Chlorotic (yellow) spots may also appear, and young leaves may become distorted. Infection is often seen on leaves, but the pathogen can infect buds, flowers, and stems. If environmental conditions favor disease development, the white patches will expand and coalesce. Heavy infections result in leaves and stems completely covered in a grey/white powdery substance. Near the end of the season small black spots the size of a poppy seed may be visible on infected tissue. These are the structures where spores will survive the winter, only to start the infection cycle again next spring.

Management consists of selecting resistant cultivars or varieties when possible. Plant catalogs will often note if a variety is resistant to powdery mildew. Varieties of some extremely susceptible hosts have been trialed to identify good potential resistance. Two examples include trials performed at the Chicago Botanic Garden observing resistance to powdery mildew on bee balm and phlox, available here: (bee balm) and (phlox).

Sanitation is an important step to controlling powdery mildew. Fallen leaves, spent blossoms, and infected tissue should be pruned during dry weather and removed from the garden. In fall, remove the above-ground portions of plants remaining in the landscape to reduce the amount of the pathogen overwintering in plant residue. While this pathogen can still spread via wind, avoiding early-season infections is important as these tend to be more severe.

Space plants appropriately when they are installed, and try to avoid overly shaded locations. Maintain plant vitality by mulching the soil around the plants, watering regularly during dry periods during the growing season, and fertilizing appropriately.

Fungicides are available to protect new growth from infection. However, these sprays need to begin at the first signs of powdery mildew and be applied every 7-14 days (at labeled interval) throughout the growing season to continue to provide protection. Fungicides are often not warranted unless disease pressure is high and the host is particularly susceptible. Fungicide choice is going to be dependent on host. Always follow the label of any pesticide you choose to apply. (Diane Plewa)

Diane Plewa

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