Last year the impatiens were attacked by the exceptionally damaging and rapidly spreading downy mildew. This disease first appeared in the USA in 2004 and by 2011 had spread through much of the Midwest. Last summer reports of downy mildew were confirmed in the Asheville area.
Downy mildew can seem to appear overnight. This is because the early stages of the disease are microscopic allowing the pathogen to advance unnoticed until it explodes with a white to light gray mildew on the underside of leaves. Within a couple of days, infected leaves may appear yellowish, small and cupped under. You could easily think the plants need water or maybe fertilizer.
As the disease progresses, infected leaves fall off until only bare stems remain with perhaps one or two small yellow leaves clinging to them. The cooler, humid weather we experienced is conducive to spreading the spores of this pathogen since they can be carried by air or water. The spores will not thrive in hot, dry conditions.
Once downy mildew is detected nothing can be done to save the plants, as the disease does not respond to traditional fungicides. Carefully remove the infected plants making sure you collect all the associated debris. Do not shake the soil out of the roots. Place everything in a bag and dispose in the trash. Do not compost infected material as it will contain abundant long lived spores.
Since the pathogen can survive the winter, do not plant impatiens in the affected area this year. Fortunately, this disease is only known to affect the Impatiens walleriana varieties. So this year you might try New Guinea impatiens, coleus, wax, tuberous or dragon wing begonias, Torenia or wishbone flower. You might want to plant perennials also. For shady beds under trees one might plant hostas, ferns, and heucheras (fancy coral bells). Cordyalis is a perennial that prefers dry partial shade with its soft green lacy leaves and small yellow flowers.