Q. Last summer, something wiped out our entire bed of impatiens. We started with flats of healthy-looking impatiens and wound up with nearly bare stems. What happened? —Christopher Jones, Lafayette, Indiana
A. Pale or yellowing leaves with whitish fuzz on the undersides, curling and dropping foliage, spindly stems, and few or no flowers are symptoms of impatiens downy mildew (Plasmopara obducens), a fungus that infects common impatiens (Impatiens walleriana) and garden balsam (I. balsamina). Plants may appear healthy and then decline rapidly when cool (59° to 73°F), moist conditions favor the development and spread of the disease. It is not the same downy mildew that affects other bedding flowers or vegetable crops. New Guinea impatiens (I. hawkeri) are highly resistant to the disease and are among the plants recommended as replacements for common impatiens.
"Anyone who has had impatiens with the disease in the past should not plant impatiens in that same landscape bed again this year (or for many years to come)," says Kristin Getter, floriculture outreach specialist in the department of horticulture at Michigan State University. "We believe the disease spores can survive for many years in the soil." This year's impatiens still may fall prey to downy mildew if spores blow in from nearby plants, she warns, but planting in a different location or in fresh growing medium at least eliminates the potential of infection from spores in the soil. Remove and discard any impatiens that develop downy mildew symptoms, Getter advises. "Do not compost, as we do not have enough evidence to know if the composting process will kill the spores."
In response to widespread outbreaks of impatiens downy mildew in 2011 and 2012, many commercial growers have significantly reduced the size of their impatiens crops. Getter notes that most commercial growers will use a strict, frequent fungicide program in their greenhouses to grow impatiens that are free of disease, which means that garden-center transplants are more likely to carry chemical residues. But impatiens downy mildew is not believed to be seedborne, so it is possible for home gardeners to grow disease-free plants from seed.
Originally published in Organic Gardening Magazine August/September 2013.