A lot of the wholesale growers have eliminated impatiens from production or drastically cut back on the numbers they grow due to impatiens downy mildew (IDM). Breeders are surely working on developing new regular impatiens that resist IDM.
Just pull the plants out and burn them or bag them up in plastic for the landfill. The spores travel far, so if there's a problem at your place, there's a problem throughout the entire neighborhood, if not for miles around.
You can grow New Guinea Impatiens, which are tolerant of the disease, tend to be larger, and can also take more sun.