Q. Why are my Impatiens turning yellow and losing their leaves?
Did you see slugs? Generally, impatiens will do fine in shade with adequate water. They do not like to be in full sun and it's possible that they were OVER watered as plants need both water and oxygen to survive. You should be able to stay away from pesticides which can do more damage than good.
Gardeners in Pennsylvania are being plagued with a downy mildew problem with impatiens this year. According to Annette MaCoy, Penn State horticulture educator "if your impatiens are infected, it’s too late to spray a fungicide to control the disease. Remove infected plants and fallen leaves. Researchers are not yet sure if this fungus overwinters in our region; but if you have had problems this year, it’s recommended to plant or interplant with alternative plants next year, such as New Guinea impatiens, which are highly resistant, begonias, or coleus."