"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.