Garden Detective: What does next year hold for impatiens?
These impatiens are just starting to yellow and curl their leaves downward. There is formation of downy mildew spores on the undersides of the leaves. Credit: Cornell University / M. Daughtrey
There is nothing that can be used to treat soil as an antidote to this disease. If you want to try to grow impatiens next year, plant them in different beds because the downy mildew makes spores that survive in the soil. Only steam-sterilization could eliminate them from the soil -- or time, but we don't know how many years the spores will last. For next year, we know it is unwise to put them back in the same place.
Compost to which infected impatiens have already been added could be used in vegetable or flower beds where you aren't trying to grow impatiens next year. Composting with careful, thorough technique is always best: Make sure all parts in the pile get exposed to the heat by turning the pile over, not just throwing the dead plants into a pile in the corner of the yard.
Different organisms attack the different species of plants; they don't just spread indiscriminately from one kind of plant to another. This particular disease is caused by a downy mildew called Plasmopara obducens, and it cannot attack dusty miller, grass, roses or anything else, except some of the impatiens species, including Impatiens walleriana (the bedding plant impatiens) and Impatiens balsamina (the balsam impatiens). Even the close relative Impatiens hawkeri (New Guinea impatiens) does not get the disease.
The reader with the dusty miller death probably had a situation of overwatering, drought or Rhizoctonia stem rot. The lawn problem is also not connected, except that the grass experienced the same stormy muggy weather as the impatiens did, and those kinds of environmental conditions are very conducive to the development of many different fungal diseases, including downy mildew. There is probably a different fungal disease on the lawn.
