Phlox

Plant Health Clinic Disease Note Issue 21

Sherrie Smith and Jason Pavel

Phlox powdery Mildew- Erysiphe cichoracearum

Photo by David L. Clement, University of Maryland, Bugwood.org

Tall garden phlox, (Phlox paniculata and Phlox maculata) are lovely summer blooming perennials. They are available in a range of colors from pure white, orange, blue violet, through all the pink and magenta shades.

They have a long bloom period beginning in late spring to midsummer, depending on cultivar. Some varieties such as Carolina phlox Miss Lingard repeat bloom. Many have a sweet fragrance. They all make lovely long lasting cut flowers. Size ranges from 18 inches to nearly 3 feet tall.

They do best in rich moist soil in full sun. Unfortunately, some cultivars are very susceptible to Powdery Mildew, caused by the fungus Erysiphe cichoracearum. Symptoms are a powdery white-grayish film on the surface of the leaves.

Are there phlox varieties that are powdery mildew resistant?

As a rule, maculata varieties are more resistant than paniculata varieties. Nursery catalogs will often mention whether a given variety has resistance to powdery mildew. Phlox ‘David” has excellent powdery mildew resistance in a lovely white. ‘Franz Shubert’ in a lilac blue has good resistance also.

How do I treat powdery mildew?

For Powdery mildew, homeowners may use Fertilome Broad Spectrum Lawn and Garden Fungicide, (chlorothalonil), or Hi-Yield Vegetable, Flower, Fruit, and Ornamental Fungicide,(chlorothalonil) or Ortho Maxx Garden Disease Control, (chlorothalonil), or Garden Tech Daconil Fungicide, (chlorothalonil), or Bonide Fung-onil Multipurpose Fungicide, (chlorothalonil), or Spectracide Immunox Plus, (myclobutanil & permethrin), or Bonide Rose Rx Systemic Drench, (tebuconazole), or Bio Advanced Garden-Disease Control for Roses, Flowers, Shrubs, (tebuconazole), or Bio Advanced Garden-All-in-One Fungicide Insecticide Fertilizer, (tebuconazole & imidacloprid), or Fertilome 2-N-1 Systemic Fungicide, (tebuconazole & imidacloprid), or Bonide Infuse Systemic for Turf and Ornamentals, (thiophanate-methyl).

Take Aways:

• Choose resistant varieties.
• Begin fungicides at the first sign of powdery mildew on susceptible varieties. 

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This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture.