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Addressing powdery mildew on phlox

Q: I love the beautiful show put on by my Phlox paniculata each year, but can you recommend some powdery-mildew-resistant varieties? What can I do to prevent and treat powdery mildew?

Jennifer Stock, Glen Burnie, MD

Lavender-and-white flowered 'Kathrine' is among the Phlox selections found to be fairly resistant to powdery mildew. Lavender-and-white flowered 'Kathrine' is among the Phlox selections found to be fairly resistant to powdery mildew. Photo/Illustration: Jennifer Blume

A: Richard Hawke, manager of plant evaluation programs at the Chicago Botanic Garden, in Glencoe, responds:  Powdery mildew is a fungal disease appearing as white patches on the leaves, although stems and flowers can also be affected. This condition rarely kills the plant, but it can reduce its vigor and resistance.

Symptoms of powdery mildew appear in summer and autumn, rather than in the rainy springtime when many other fungal diseases develop. Powdery mildew spores require high humidity rather than wet leaves to germinate.

A gardener’s first line of defense against powdery mildew is to choose a mildew-resistant cultivar. Garden phlox that have been resistant in one or more studies include white-flowered ‘David’, lavender-and-white-flowered ‘Katherine’, pink-flowered ‘Shortwood’, deep-magenta ‘Nicky’, salmon-orange-flowered ‘Orange Perfection’, dark-pink ‘Robert Poore’, and white-and-red-flowered ‘Prime Minister’.

Careful site selection and following a few simple cultural practices will help to limit fungal infection. Start by planting garden phlox in full sun. Next, improve the air circulation by thinning out stems and increasing the spacing between plants. Avoid overhead irrigation, and water phlox early in the day to allow the leaves to dry off before nightfall. Finally, remove infected leaves and stems each autumn to eliminate overwintering spores.

For those gardeners inclined toward battling powdery mildew, there are a couple of control options. Systemic fungicides work well. Begin monitoring plants in early June and treat at the first sign of fungus; subsequent treatments may be needed during the season.

An alternative treatment is a mixture of 1 tablespoon baking soda and 2 table­spoons horticultural oil in 1 gallon of water applied every two weeks beginning in early June, before mildew starts.

The prevalence of powdery mildew can vary widely by climate and location and from year to year. Contact garden centers, botanical gardens, or extension offices for recommendations on cultivars that have proven to be resistant in your area.

From Fine Gardening 82, pp. 80