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What to Do About Mildew

July 9, 2014

White and orange impatiens

Credit: Angela Altomare

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Have you noticed yellow leaves and wilting on your impatiens? If watering hasn’t done the trick, you may be looking at a new disease called impatiens downy mildew that is ravaging impatiens across North America! The All-Seasons Garden Guide has the facts on this infectious mildew and offers advice on how to spot it and prevent it from infecting future plantings!

What to Do About Mildew

Impatiens downy mildew, or Plasmopara obducens, was first observed in the United States on native impatiens known as jewelweed or touch-me-nots (Impatiens pallida and I. capensis) in the late 1800s. Recent problems with the mildew first appeared in greenhouses in California in 2004. Now it is seen in the landscape from California to Florida and north to Minnesota, as well as in southern Ontario.
This disease affects only bedding impatiens (I. walleriana). New Guinea impatiens (I. hawkeri) and the SunPatiens series are not susceptible (unlike bedding impatiens, these plants tolerate full sun).

Moisture, high humidity, and high temperature encourage the disease. Once in place, it works quickly. Margery Daughtrey, plant pathologist at Cornell University, says that a planting can be “devastated in 3 weeks under conducive conditions.”

Telltale Signs: The first indication of mildew presence is a few yellowed leaves and wilting that can easily be taken for water stress. If, on the underside of the leaves, a cottony-white growth is visible, your plants are infected.
How It Spreads: Besides being airborne and thus able to float from other plantings, the disease can be spread through splashing from overhead irrigation. Moreover, the fungus can overwinter in the soil and survive temperatures as low as 5°F.
What to Do Now, for Later: If you want to grow impatiens in the future, sanitation is critical. Remove and destroy all fallen impatiens leaves and stems. Do not put them in your compost pile. Do not plant new impatiens in the same bed where diseased plants grew.

For advice on bringing color to shaded beds without impatiens and more gardening advice, check out The 2014 All-Seasons Garden Guide.

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