Powdery Mildew- Ornamental plants

Need to know: 

• Pathogen favors moderate temperatures and humid conditions. 
• Powdery mildew occurs most commonly in shady areas with little air circulation. 
• Best preventative measure is planting at a good site for the specific plant.  
• Fungicides are available but mostly only beneficial prior to infection. 

Overview of powdery mildew 

As its name suggests, powdery mildew is visible as a light gray or white dusty coating on leaves, stems, flowers, or fruits. Depending on the plant species, affected leaves may be distorted, and tiny dark dots may appear in the white coating. Powdery mildews can occur on nearly all kinds of plants, including, trees, perennials and bedding plants but in most cases, each plant type  is affected by a different powdery mildew fungus. For example, the fungus that causes powdery mildew on lilac will not infect roses.

Symptoms of powdery mildew

Infected leaves are normally consumed by the fungus and drop to the ground when they die.

Powdery mildew is usually favored by moderate temperatures and humid (but not wet) conditions. It tends to be more severe in the shade and in areas with little airflow. Because of this, putting plants in full sun and cultural practices that promote airflow (such as pruning and appropriate spacing) can help to minimize powdery mildew problems. Keeping plants in good vigor helps them to resist infection.

Signs of powdery mildew

White powdery growth (mycelium) can be seen growing on the surface of infected leaves and shoots. 

Disease Cycle of powdery mildew

Powdery mildew occurs most commonly in shady areas. Slow or non-existent air circulation, shade, high humidity, and temperatures of 60-70° F are the components needed for disease development. 

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us

Management of powdery mildew 

The best strategy for controlling powdery mildew in most plants is pairing the plant with a good site. For example, for lilacs, peony or roses, avoid location with long periods of shade during the day.  On some plants such as lilac and sahde trees, powdery mildew does not cause serious damage to the plant and can be tolerated. On other plants, damage may be severe and chemical controls may be desired. Many fungicides are labeled for use against powdery mildew on ornamentals, but they are most effective when used preventatively (before infection occurs) and good coverage of the plant is essential.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.