Prevent Powdery Mildew in Gardens & Landscape 

By C. Rae Hozer, Cumberland County Master Gardener

The best weapon against powdery mildew is disease prevention through careful site and plant selection along with good growing practices, instead of fungicide use.

1. Buy/grow resistant cultivars. (Remember that seedlings from hybrid stock typically donít have the same resistance as the mother plant.) 

2. Select a sunny site (minimum 6 hours of sunlight). Highly susceptible plants (phlox, roses, etc.) should not be located in damp, shady spots. Maintain enough distance between plants for good air circulation. 

3. Avoid overhead sprinkling that wets foliage in late afternoon or evening. Do not let plants be stressed by lack of water during a drought.

4. Donít handle plants or work among them when foliage is wet.  

5. Clean up and destroy any diseased leaves and other plant litter.     

Powdery mildew shows up as a white or grayish haze on leaf surfaces, stems, flowers or fruit of affected plants. (Plant parts coated with this fungus look like theyíve been powdered, hence the name.) Sometimes young, infected shoots and leaves appear twisted or curled. As the disease progresses, leaves often turn yellow or brown and dry out. These fungi send infectious spores into the air when temperatures are 60 to 80 degrees Fahrenheit.

Susceptible plants growing in damp, shady areas are easy targets for powdery mildew. When it is above 80 in sunlight, air temperatures in shade are cooler. High humidity favors spread of this disease. The wet leaves many fungal diseases require arenít necessary because powdery mildew spores germinate without a film of water on plant surfaces. Plants growing too close shade one another and keep humidity higher than in beds with wider plant separation.

Scientists tell us that, though symptoms look the same, powdery mildew disease on different varieties of plants are typically caused by different fungus strains. Each powdery mildew species has just a small number of host plants. That means airborne powdery mildew spores from a sick dogwood tree in your yard might infect another dogwood but wouldnít spread this disease to a nearby lilac or rose bush, to euonymus vines climbing the enclosure for your propane tank, to phlox and zinnias in your flower garden or to green bean and cucumber plants in your vegetable patch. However, all of the individual plants just named are naturally susceptible to some kind of powdery mildew. Other susceptible woody ornamentals are deciduous azaleas, buckeye, cherry, some flowering crabapples, honeysuckle, privet, serviceberry, tulip tree, some viburnum, walnut and willow trees. Fleshy-stemmed ornamentals prone to powdery mildew include chrysanthemums, dahlias, delphiniums, some begonias and snapdragons. Some wild-growing trees, shrubs and weeds serve as host plants, too.

Spring applications of fertilizer, lots of rain, and warm temperatures combined to give a big boost to perennials (and weeds) in my flower gardens. Unfortunately, my schedule kept me elsewhere instead of in those beds pulling weeds and thinning the desirable plants. My phlox have big, bright pink and purple blooms reminiscent of flowers in Motherís gardens when I was a child, but their lower leaves are totally covered with powdery mildew. Newer, disease resistant phlox cultivars are available but those arenít the varieties I have.

Photo caption: Abundant flowers and foliage in a bed by front walk look lush from the street but plants are overcrowded. Phlox leaves (foreground) are covered in powdery mildew.

Plateau Gardening is written by Master Gardeners for those tending home landscapes and gardens in Tennesseeís Upper Cumberland Region. Contact UT Extension Cumberland County, P.O. Box 483, Crossville, TN 38557, (phone 931-484-6743) for quick answers to specific questions, free publications, or to learn about becoming a Master Gardener. Email comments or yard and garden inquiries to Master Gardener Rae, mgardenerrae@frontiernet.net.