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. 

Management of powdery mildew 

The best strategy for controlling powdery mildew in most plants is to pair the plant with a suitable site. For example, for lilacs, peonies, or roses, avoid locations with long periods of shade during the day.  

On some plants, such as lilac and shade trees, powdery mildew typically appears late in the season. Because of this, the disease does not cause significant harm to the plant, as the leaves have almost completed their job of producing food for the year and will soon drop off. No management is typically recommended in this instance. 

Planting resistant varieties is an effective way to minimize this disease in certain ornamentals, such as garden phlox and bee balm, where powdery mildew is particularly common. 

Other important cultural practices include increasing air circulation and avoiding excess fertilizer use. Overhead irrigation is not recommended as it can contribute to the development of other diseases.  

The fungal pathogens that cause powdery mildew are capable of surviving the winter.  Therefore, late fall sanitation is recommended to help reduce the amount of the pathogen present in the garden next year.  When removing diseased material, do not place it in the compost bin, as most home compost piles do not reach high enough temperatures to destroy the disease-causing pathogens.

For susceptible varieties, fungicides are available but are mainly protectant, not curative.  Fungicides can only help when applied prior to the infection. Fungicides available for the home garden include:

• chlorothalonil products (such daconil or Echo)
• biofungicide (like Serenade)
• sulfur products (such Safer Garden Fungicide; may burn plants if label directions are not followed carefully)
• some oil sprays

All sprays will have little to no effect if applied in late summer after symptoms appear. 

If applied preventatively, 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.

Additional Resources

• Integrated Pest Management for Home Gardens and Landscapes
• Ornamental varieties with resistance to powdery mildew (Purdue Univ)

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