Your impatiens died of impatiens downy mildew. It is a plant specific disease, meaning it only attacks impatiens walleriana, so your myrtle or hostas are safe. Unfortunately there is no known product that will kill the spores and they may live for 6 years but the folks at MSU are working on it. Best and Happy Yardening, Nancy
Treatment Of Diseased Impatiens And Affect On Other Plants
Question From: MICHIGAN
Q: My impatiens were diseased this year and have completely died. I haven't done anything to the area they were planted in. Should I treat the area where they were and with what? Also will the disease hurt myrtle or hostas?