Impatiens downy mildew was observed last week on resistant impatiens in a landscape in eastern Massachusetts. Sporulation was light, and plants were not completely defoliated. Plants were also suffering from Rhizoctonia cankers on the lower stems.
Downy mildew has been seen in the Northeast this year on both breeding lines of highly resistant impatiens. The warm, wet weather we have experienced in the region this summer is highly conducive to downy mildew development, and under these conditions even resistant impatiens varieties may show signs and symptoms of the disease. The Rhizoctonia canker may also have weakened the plants, making them more susceptible to the downy mildew.
Plants infected with impatiens downy mildew cannot be cured. The disease is highly specific to species in the genus Impatiens. Replant affected areas with other shade-loving plants such as coleus, begonia, or lobelia. New Guinea impatiens are also highly resistant to downy mildew.