What is happening to my Impatiens?
About 1/3 of my Impatiens in my window box are wilting, going yellow and have whitish-grey, powdery looking ends on the leaves. Help!
Likely a Powdery Mildew issue.
Trim away the effected plant material.
Also check watering, make sure the soil is damp but not soggy. Soggy roots can cause yellowing leaves and root rot.
Here is a link with more information.