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Extension > Garden > Yard and Garden > Flowers > Powdery mildew in the flower garden

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Powdery mildew in the flower garden

M. Grabowski

Top five things to know about powdery mildew

Zinnia plant with low levels of powdery mildew

M. Grabowski, UMN Extension

Low level of powdery mildew on zinnia will not reduce flowering

Importance

Powdery mildew is a common fungal disease that occurs on many different plants in the flower garden. Although powdery mildew can cause plants to look less attractive, it rarely results in significant damage to the plant. When severe, the disease may reduce plant growth and flowering.

Bottom leaves of a sunflower plant infected with powdery mildew

M. Grabowski, UMN Extension

Powdery mildew on lower leaves of a sunflower

Susceptible plants

Powdery mildew occurs on over 10,000 plants. Many commonly grown annual and perennial flowering plants, as well as ornamental grasses, can be infected by powdery mildew. Zinnia, phlox, bee balm, and peony are a few of the plants regularly infected by powdery mildew in the flower garden.

Plant stem with white cobweb looking spots

M. Grabowski, UMN Extension

Black, round fungal resting structures of powdery mildew on a phlox stem

Identification

Biology

Powdery mildew is caused by a group of related fungi in the Erysiphaceae family.

Powdery mildew spores are easily carried by the wind to neighboring plants or to plants hundreds of miles away. Once a spore lands on a host plant, it will quickly germinate and start a new infection. Unlike other leaf spot fungi, powdery mildew fungi do not need moisture on the leaf from rain or dew to infect. Some powdery mildew fungi require high humidity but others can germinate even when humidity is low.

peony plant with powdery white leaves infected by powdery mildew

M. Grabowski, UMN Extension

Powdery mildew on a peony

Powdery mildew fungi produce a mat of fungal growth on the surface of the plant. Specialized structures penetrate the plant tissue to take up nutrients. Spores are produced in long chains rising up in a vertical column perpendicular to the leaf surface. These spores break off and are spread by the wind. Powdery mildew fungi thrive with cool humid nights that stimulate spore production and warm (70 to 80° F), dry days that allow for spore spread.

Powdery mildew fungi survive winter in several ways. Some infect buds of woody plants. Powdery mildew fungi can also create a dark, round, hard resting structure known as a chasmothecia. These resting structures contain and protect spores during harsh weather. Spores are later released when the weather becomes favorable for disease development. Some powdery mildew fungi survive on leaves that remain green throughout the winter. In Minnesota, this may mean that the powdery mildew survives on greenhouse plants or new spores are carried by the wind into the state after surviving the winter on plants in southern states.

Management

Caution: Read all label directions completely before buying and applying fungicides. Follow all instructions. The label is the final authority for use of the product.

2017

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