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Lawn & Garden

Ask Dayle

8/1/2013 - West Side Leader
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By Dayle Davis

White-blooming tall garden phlox
Photo: Dayle Davis
Q: There is a perennial in my garden that is about 24 inches tall with green leaves and clusters of pink flower blossoms at the top. The leaves are turning white with some sort of disease. Do you know what this is? Should I dig out these plants before others get it too? What else should I do?

A: I am pretty sure you are describing the perennial tall garden phlox, Phlox paniculata.

If so, this plant has been well loved in the perennial bed for more than a century.

Tall phlox can grow to 2 to 3 feet or more and is a great specimen plant, as well as an anchor at the back of the garden bed. Newer varieties that grow about 1-foot tall also are available.

Butterflies and hummingbirds love these plants, which can be planted in partial shade to full sun. Tall phlox boom for about six weeks in mid-to late summer, but some will bloom earlier if planted in full sun. The most commonly remembered tall phlox of my youth bloomed pale magenta. There are many more cultivars these days that bloom in a range of hues from pink to lavender, purple, white and bright red. There are some cultivars with bicolor blossoms — one color in the middle and rimmed with a contrasting color at the edges. Many blossoms are sweetly scented.

Tall garden phlox is relatively easy to grow, fast to establish and easy to divide and transplant. It likes to be watered, but only at its feet, not on its leaves. The only fall cleanup required is to cut the stems to the ground and cart off the debris.

Garden phlox is relatively disease and pest free, except for two things: powdery mildew and occasionally spider mites. Powdery mildew is an unsightly but rarely fatal fungus. The spider mites can be controlled with a routine strong water spray from the garden hose and/or an application of insecticidal soap.

Powdery mildew is what I suspect has developed on your phlox. In general, powdery mildew often appears as a superficial white or gray powdery growth of fungus over the surface of leaves, stems, flowers or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. Young foliage and shoots may be particularly susceptible. Most powdery mildew fungi produce airborne spores and infect plants when temperatures are moderate, from 60 to 80 degrees F. Sometimes leaf curling and twisting announce its imminent arrival before the fungus is actually noticed. Severe infection results in yellowed leaves, dried and brown leaves and disfigured shoots and flowers. It can make the plant extremely unattractive and can hasten plant defoliation and fall dormancy.

Usually powdery mildew develops on tall garden phlox when the plant foliage grows full and bushy, which reduces air circulation and causes the leaves and stems to stay wet for long periods of time. The recent long periods of rain we have been experiencing don’t help.

Powdery mildew fungi infect almost all ornamental plants. Some plants are more naturally susceptible to the disease. Susceptible plants include some deciduous azaleas, buckeye, catalpa, cherry, a few of the flowering crabapples, dogwood, English oaks, euonymus, honeysuckle, horse chestnut, lilac, privet, roses, serviceberry, silver maple, sycamore, tulip tree, some viburnums, walnut, willow and wintercreeper. Powdery mildews also are common on garden flowers such as chrysanthemums, dahlias, delphiniums, kalanchoes, phlox, Reiger begonias, snapdragons and zinnias.

Each species of powdery mildew has a very limited host range. Infection of one plant type does not necessarily mean that other types of plants will get the same thing. For example, your roses cannot give their particular powdery mildew to your nearby lilac shrub. And although the fungi that cause powdery mildew are usually different on different plants, all of the powdery mildew diseases are similar in appearance.

In any case, here are a few tips to reduce the possibility of powdery mildew on your plants:

  • Prune out stems at ground level to make for an airier plant with plenty of air circulation.
  • Refrain from getting the leaves and stems wet when watering.
  • Plant varieties of tall phlox, such as ‘David,’ which have been bred to resist mildew.
  • Space plants for good air circulation.
  • Avoid planting highly susceptible plants such as phlox, rose and zinnia in damp, shady locations.
  • Do not handle or work among the plants when the foliage is wet.
  • Avoid overhead watering and sprinkling the foliage, especially in late afternoon or evening. Use a soil soaker hose or root feeder to avoid wetting the foliage.

For more information, call The Ohio State University Summit County Extension Hotline Tuesdays from 9 a.m. to noon at 330-928-4769, ext. 3, and request Fact Sheet HYG-3047-96, “Powdery Mildews on Ornamental Plants.”

Please note fact sheets are sent out free; there is a fee for bulletins. Many bulletins are available online at ohioline.osu.edu and can be printed from home or accessed at the public library.


Dayle Davis is a freelance writer and avid perennial gardener, with a B.A. in communications and course work in botany, geology and wildflowers. Davis is a master gardener emeritus under The Ohio State University’s Horticultural Extension. Readers can send in questions regarding lawn and garden issues, which could be featured in a future edition, as well as about the OSU Master Gardener program. Questions can be emailed to kcollins@akron.com, faxed to 330-665-9590 or sent to Leader Publications, 3075 Smith Road, Suite 204, Akron, OH 44333. Please do not send any leaves, seeds or other organic material. Inquiries about area garden clubs or groups should be sent directly to the particular organization in question.

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