Prevention is important when it comes to root rot pathogens. Once symptoms occur those plants are generally not treatable and should be disposed of. 

If symptoms are just starting you could apply a fungicide, but many of the products available for use in greenhouses are not available to homeowners. I've looked at what's available and none of what's offered is really appropriate for treating a root rot pathogen like rhizoctonia. Plus, without doing some testing and more investigation, there's no way to be sure that this is rhizoctonia and not another root rot pathogen that would warrant a different fungicide. Impatiens downy mildew can cause similar symptoms to what you're seeing.

You may want to consider some other cultural controls to deal with this issue in the future. Select disease resistant cultivars of Impatiens walleriana and/or New Guinea impatiens. Also, ensure that you are watering and fertilizing appropriately.

Here some further information on growing impatiens.

https://extension.umn.edu/flowers/impatiens

https://yardandgarden.extension.iastate.edu/how-to/growing-impatiens-home-garden

https://www.canr.msu.edu/resources/impatiens_downy_mildew_a_curse_and_opportunity_for_smart_gardeners