Archuleta County Colorado
Most treatments for downy mildew focus on treating the plants. Removing or sterilizing the soil would also be an option however total control is difficult and some of the products which were used for soil sterilization in landscapes are no longer on the market or readily available.
Detailed information on control can be found here:http://www.extension.umn.edu/garden/yard-garden/flowers/managing-impatiens-downy-mildew-in-landscape...
One of the easiest "fixes" is to switch species to one not susceptible to the disease. Options for replacement include Begonias and New Guinea Impatiens.
I hope this help please let me know if you have further questions.