Entomology and Plant Pathology, Oklahoma State University
127 Noble Research Center, Stillwater, OK74078
Vol. 12, No. 18
May 23, 2013
Downy Mildew of Impatiens
Jen Olson, Assistant Extension Specialist, Plant Disease Diagnostician
Impatiens (Impatiens walleriana) are a shade-loving annual that is widely planted in Oklahoma and throughout the United States. A fungal-like disease called downy mildew (Plasmopara obducens) has been in the news as a concern for home gardeners. Two recent articles have appeared, one in the Tulsa World and another in the New York Times. It is likely that you will receive questions about this disease and possibly plant samples.
Downy mildew thrives under cool, wet conditions. These conditions are found in the spring in Oklahoma, but it is not likely that our environment will support the disease in the summer. Downy mildew of impatiens is primarily a problem in Eastern states which have higher humidity and lower temperatures throughout the growing season.
The most susceptible impatiens to this pathogen is Impatiens walleriana. These impatiens are referred to as standard garden or common impatiens, double impatiens or mini-impatiens. Balsam or garden balsam (Impatiens balsamina) is susceptible, but this plant is less common and symptoms are less severe than on I. walleriana.
Although downy mildew diseases do occur on other plants such as roses and cucurbits, they are caused by different species than the downy mildew that affects impatiens. (For example, Plasmopara sparsa is the downy mildew species found on roses and Pseudoperonospora cubensis is the downy mildew species found on cucurbits). Each downy mildew fungus has a specific and narrow host range, so they are unable to attack other types of plants in the