This was what ours looked like the past two years.
They started our lush and colorful, but by the middle of the summer about all that was left was stems. We wracked our brains trying to come up with a cause and solution which included the plants being eaten by rabbits, bugs, or getting too dry. But nothing really explained this sudden onset of total ruin of the impatiens.
But today, thanks to KMOX's Garden Hotline expert Mike Miller, we now know what this is. Mike answered a caller's question about impatiens which described our situation to a T and he knew immediately that it was brought on by something called Downy Mildew.
Downy Mildew is a pathogen which can attack plants even when they appear to be growing nicely in the landscape. With the right weather conditions (cool temperatures and plenty of moisture), downy mildew can infect a patch of impatiens seemingly overnight. Not always noticeable are powdery or fuzzy white spores.
Mike Miller suggested not putting impatiens back in the areas where this has occured because the pathogens will survive from one season to the next. Instead consider plants which will not be affected such as New Guinea impatiens, Begonias of all types which come in a variety of flower and leaf colors, Coleus, and Caladium.
Sad, though, there is nothing like a bed of brightly colored impatiens.